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Plant hormone

Plant hormones, also known as phytohormones, are naturally occurring organic compounds produced by plants in low concentrations that act as chemical messengers to regulate essential processes such as growth, development, reproduction, and responses to environmental stresses.^[1] These signaling molecules are synthesized in specific tissues and transported via vascular systems or cell-to-cell diffusion to target sites, where they influence gene expression, enzyme activity, and cellular responses at very low levels, often in parts per million or billion.^[2] The five classical classes of plant hormones are auxins, cytokinins, gibberellins, abscisic acid, and ethylene, each with distinct chemical structures and primary functions. In addition to these five classical hormones, other classes including brassinosteroids, jasmonates, salicylic acid, and strigolactones are also recognized as plant hormones.^[3] Auxins, such as indole-3-acetic acid, primarily promote cell elongation, apical dominance, and tropisms like phototropism and gravitropism, while also stimulating root formation.^[2] Cytokinins, derived from adenine, stimulate cell division and delay leaf senescence, often working in balance with auxins to control shoot and root development in tissue culture.^[4] Gibberellins, a large family of over 100 diterpenoids, drive stem elongation, seed germination, and fruit growth by breaking dormancy and promoting enzyme production.^[1] Abscisic acid functions mainly as a stress hormone, inducing seed and bud dormancy, promoting leaf abscission, and closing stomata during drought to conserve water.^[2] Ethylene, a unique gaseous hormone, accelerates fruit ripening, flower senescence, and organ abscission, particularly in climacteric fruits like tomatoes and bananas.^[4] These hormones often interact synergistically or antagonistically to fine-tune plant responses; for instance, auxins and cytokinins ratios determine whether cells differentiate into roots or shoots, while abscisic acid counteracts growth-promoting effects of gibberellins under adverse conditions.^[1] Discovered through pioneering experiments beginning in the late 19th century—such as Charles Darwin's observations on phototropism in 1880 leading to auxin identification in the 1920s and Japanese studies on rice "foolish seedling" disease revealing gibberellins in the 1930s—plant hormones have since been extensively studied for their roles in agriculture, including synthetic applications for rooting cuttings, delaying senescence in harvested produce, and enhancing crop yields.^[2]

Introduction and Characteristics

Definition and Overview

Plant hormones, also known as phytohormones, are small organic compounds produced by plants that function as chemical messengers, regulating a wide array of physiological processes such as growth, development, reproduction, and responses to environmental stimuli.^[5] These molecules are synthesized in one part of the plant and typically transported to another site of action, where they elicit specific responses at very low concentrations, often in the range of nanomolar to micromolar levels.^[5] Unlike larger signaling peptides or proteins, plant hormones are simple in structure and versatile in their effects, coordinating the plant's sessile lifestyle by integrating internal cues with external conditions.^[6] For a substance to qualify as a plant hormone, it must meet several key criteria: it is endogenously produced, acts at a distant site from its synthesis location, operates effectively at trace concentrations (generally below 10^{-6} M), and triggers defined physiological changes.^[5] Experimental validation includes observing responses to exogenous application, inhibition of responses when endogenous levels are reduced, restoration of responses upon reapplication, and correlation between endogenous concentrations and the occurrence of the response.^[5] These criteria distinguish true hormones from other bioactive compounds, ensuring their role as active regulators rather than mere metabolic byproducts.^[5] In contrast to nutrients or vitamins, which are essential for general metabolism and structural integrity and are required in higher quantities, plant hormones exert an active, signaling role that modulates and coordinates developmental and adaptive processes without serving as primary energy sources or building blocks. Nutrients like macronutrients support basic cellular functions, whereas hormones fine-tune these activities through targeted interactions with cellular machinery. From an evolutionary perspective, plant hormones derive from ancient signaling systems with shared ancestry to some animal hormones, particularly in pathways like steroid signaling, but have diverged to accommodate the immobile nature of plants, enabling integrated responses to gravity, light, and stress without locomotion.^[7] The major classes—auxins, cytokinins, gibberellins, abscisic acid, and ethylene—exemplify this diversity, each contributing uniquely to plant coordination.^[5]

Historical Development

The concept of plant hormones emerged from early observations of directed plant growth responses, particularly phototropism. In 1880, Charles Darwin and his son Francis investigated the bending of canary grass (Phalaris canariensis) coleoptiles toward unilateral light, concluding that a transmissible "influence" from the tip mediated this response, as covering the tip prevented bending while decapitation abolished it.^[8] This work laid foundational groundwork for recognizing diffusible chemical signals in plants, though the substances remained unidentified. The discovery of auxins marked a pivotal advancement in plant hormone research. In 1928, Dutch scientist Frits Went developed the Avena curvature bioassay using oat (Avena sativa) coleoptiles to demonstrate that a growth-promoting substance diffused from tips into agar blocks, inducing curvature when asymmetrically applied.^[9] This bioassay quantified the substance, later termed auxin. By 1934, Went and Kenneth Thimann, along with Fritz Kögl's group, isolated and identified indole-3-acetic acid (IAA) as the primary natural auxin, confirming its role in cell elongation and tropisms.^[10] Subsequent decades revealed additional hormones through targeted investigations of abnormal growth phenomena. In 1926, Eiichi Kurosawa identified a fungal secretion from Gibberella fujikuroi causing "foolish seedling" disease in rice, which promoted excessive stem elongation; this led to the isolation of gibberellins as plant growth regulators by the 1930s, with their endogenous production in plants confirmed later.^[11] Ethylene's status as a gaseous hormone solidified in the 1930s when researchers demonstrated its endogenous production by plants, linking it to fruit ripening and senescence after earlier observations of gas effects on foliage.^[12] Cytokinins were identified in the 1950s through research by Folke Skoog and colleagues, who isolated kinetin from degraded herring sperm DNA, demonstrating its ability to promote cell division in plant tissue cultures.^[13] Abscisic acid (ABA) was named in the 1960s following independent isolations of dormin and abscisin II, which proved identical and were shown to regulate dormancy and stress responses.^[14] Advances in molecular biology transformed hormone research from the 1980s onward. Cloning of auxin-related genes, such as auxin-binding proteins in the late 1980s and Aux/IAA repressors in the 1990s, enabled dissection of signaling pathways, with auxin response factors (ARFs) identified in 1998 as key transcriptional regulators.^[10] Post-2000, high-throughput genomics accelerated discoveries, including strigolactones in 2008 as inhibitors of shoot branching, confirmed through grafting experiments and mutant analyses in pea and rice.^[15] In the 2020s, CRISPR/Cas9 editing of hormone pathway mutants has provided precise functional insights, such as targeting ABA biosynthesis genes to enhance drought tolerance in crops like tomato and wheat, revealing redundancies in signaling networks.^[16]

General Properties and Functions

Plant hormones, also known as phytohormones, exhibit pleiotropy, meaning a single hormone can influence multiple physiological processes simultaneously. For instance, auxin regulates not only cell elongation and apical dominance but also vascular tissue differentiation, coordinating the formation of xylem and phloem in response to developmental cues.^[17]^[18] This multifaceted action allows hormones to integrate diverse signals, ensuring coordinated growth and adaptation across plant tissues.^[19] Hormone interactions further amplify their regulatory complexity through antagonism and synergism. Antagonistic effects, such as those between auxin and cytokinin, maintain the balance between shoot and root development; high auxin levels promote root formation while suppressing shoots, whereas cytokinin favors shoot proliferation at the expense of roots.^[20] In contrast, synergistic interactions occur when gibberellins enhance auxin's role in stem elongation, where combined application leads to greater internode expansion than either hormone alone.^[21] These interactions form a dynamic network, often described in systems biology models as interconnected pathways that fine-tune responses to environmental and internal signals, with crosstalk occurring at levels of biosynthesis, transport, and gene expression.^[22] Recent computational approaches since 2015 have revealed hormone networks as robust systems with feedback loops and modular regulation, enabling predictive modeling of plant responses.^[23] The effects of plant hormones are highly concentration-dependent, often following biphasic dose-response curves where low concentrations promote growth and high concentrations inhibit it, a phenomenon akin to hormesis. For auxins, optimal low doses stimulate hypocotyl and root elongation by enhancing cell wall loosening, while excessive levels trigger inhibition through overactivation of signaling pathways or toxicity.^[24] This principle underlies the Arndt-Schulz rule observed in early auxin studies, where minimal stimuli accelerate processes and maximal ones suppress them.^[25] Feedback regulation ensures hormonal homeostasis, with many hormones autoregulating their own biosynthesis. Ethylene, for example, undergoes negative feedback where elevated levels suppress key biosynthetic enzymes like 1-aminocyclopropane-1-carboxylic acid oxidase, preventing overaccumulation during ripening or stress responses.^[26] Such mechanisms maintain precise control, integrating with broader crosstalk to sustain balanced physiological outputs.^[27]

Biosynthesis and Molecular Mechanisms

Sites of Synthesis and Metabolism

Plant hormones are synthesized in specific tissues and organelles, with metabolism occurring through enzymatic pathways that regulate their active levels. Synthesis primarily takes place in actively growing regions such as meristems and young organs, while degradation involves conjugation, oxidation, and hydroxylation to maintain homeostasis. Compartmentalization varies, with most hormones synthesized in the cytosol or plastids, ensuring targeted accumulation and response.^[28] Auxins, particularly indole-3-acetic acid (IAA), are mainly synthesized in shoot apical meristems and young leaves via the indole-3-pyruvic acid (IPyA) pathway, starting from tryptophan and catalyzed by tryptophan aminotransferases (TAA1/TARs) and YUCCA flavin monooxygenases. This pathway predominates in Arabidopsis and other species, with synthesis occurring in the cytoplasm. Metabolism includes conjugation to amino acids (e.g., IAA-Asp, IAA-Glu) by GH3 acyltransferases and oxidation to 2-oxoindole-3-acetic acid by dioxygenase for auxin oxidation 1 (DAO1), both processes helping to inactivate and store IAA. Glucose ester conjugates also form, contributing to auxin homeostasis.^[29]^[28]^[30] Cytokinins are primarily produced in root tips and developing seeds through the isopentenyl transferase (IPT) pathway, utilizing dimethylallyl pyrophosphate (DMAPP) and ATP/ADP to form isopentenyladenine-type cytokinins in the cytoplasm and plastids. Degradation occurs via oxidation by cytokinin oxidase/dehydrogenase (CKX) enzymes, which cleave the N6 side chain, and conjugation to glucosides for storage and inactivation. These mechanisms ensure cytokinin gradients essential for shoot-root balance.^[28] Gibberellins (GAs) are synthesized in young shoot tissues and developing seeds via the terpenoid pathway, initiating in plastids with the formation of ent-kaurene by copalyl diphosphate synthase (CPS) and kaurene synthase (KS), followed by oxidation steps in the endoplasmic reticulum and cytosol involving ent-kaurene oxidase (KO) and kaurenoic acid oxidase (KAO). Active GAs like GA1 and GA4 are produced through these sequential oxidations. Degradation primarily involves 2β-hydroxylation by cytochrome P450 monooxygenases (e.g., CYP88A, CYP714B) to form inactive GA-catabolites, alongside conjugation to glucosides. Plastidial compartmentalization of early steps allows integration with isoprenoid metabolism.^[28] Abscisic acid (ABA) is synthesized in roots, mature leaves, and notably in guard cells under stress conditions such as drought, where the entire pathway is upregulated for rapid stomatal closure. The pathway derives from carotenoids in plastids via the methylerythritol phosphate (MEP) route, with the rate-limiting step being the cleavage of 9-cis-neoxanthin or 9'-cis-violaxanthin by 9-cis-epoxycarotenoid dioxygenase (NCED3 in guard cells), followed by conversion to abscisic aldehyde by short-chain dehydrogenase/reductase (ABA2) and oxidation to ABA by aldehyde oxidase 3 (AAO3) in the cytosol. Degradation occurs through 8'-hydroxylation by CYP707A cytochrome P450s to form phaseic acid, or conjugation to ABA-glucose ester (ABA-GE) by β-glucosyltransferases for vacuolar storage and reversible release. Guard cell-autonomous synthesis under reduced humidity exemplifies stress-specific compartmentalization in plastids and cytosol.^[28]^[31]^[32] Ethylene biosynthesis occurs in ripening fruits, roots, and shoots, derived from methionine via S-adenosylmethionine (SAM) and 1-aminocyclopropane-1-carboxylic acid (ACC), with ACC synthase (ACS) and ACC oxidase (ACO) as key enzymes in the cytosol. This pathway is tightly regulated, with ACC serving as the direct precursor. Degradation involves oxidation to ethylene glycol or conjugation of ACC to malonyl-ACC by malonyl-CoA:ACC transferase, preventing excessive accumulation. Cytosolic localization facilitates rapid response to developmental cues.^[28] Brassinosteroids (BRs) are synthesized in shoots, leaves, and roots through the mevalonate (MVA) or methylerythritol phosphate (MEP) pathways, starting from campesterol and involving multiple cytochrome P450 oxidations (e.g., CYP90B1/C-22 hydroxylase, CPD/C-3 oxidase, DWF4/C-22 hydroxylase) primarily at the endoplasmic reticulum to yield active forms like brassinolide. Early steps may occur in plastids via MEP. Catabolism features C-26 hydroxylation by CYP734A1 (BAS1 in Arabidopsis) and related CYP734As, inactivating BRs like castasterone and brassinolide, with additional hydroxylation by CYP72C1/SOB7 on precursors; these CYP450-mediated oxidations form tyrosol-like or estriol-like metabolites for homeostasis. Conjugation to glucosides further modulates levels. Endoplasmic reticulum compartmentalization supports membrane-associated functions.^[33]^[28]^[34] Jasmonates, including jasmonic acid (JA), are produced in leaves, roots, and flowers from α-linolenic acid in plastids via the octadecanoid pathway, with lipoxygenase (LOX), allene oxide synthase (AOS), and allene oxide cyclase (AOC) generating 12-oxo-phytodienoic acid (OPDA), followed by peroxisomal β-oxidation to JA in the cytosol. The bioactive form, JA-isoleucine, forms by conjugation via JAR1. Degradation involves β-oxidation to hydroxylated or carboxylated forms and oxidation by CYP94 enzymes, alongside conjugation to amino acids or glucosides for inactivation. Peroxisomal and plastidial steps integrate jasmonate signaling with lipid metabolism. These processes link to phloem transport for systemic defense responses.^[28] Salicylic acid (SA) is synthesized primarily in leaves and other tissues via two main pathways: the isochorismate synthase (ICS) pathway in chloroplasts and the phenylalanine ammonia lyase (PAL) pathway. The ICS pathway, predominant in Arabidopsis, involves isochorismate production from chorismate by ICS1, followed by conversion to prephenate and then SA. The PAL pathway, now fully elucidated as of 2025, starts from phenylalanine in the cytosol and proceeds through peroxisomal β-oxidation: phenylalanine is converted to trans-cinnamic acid by PAL, then to cinnamoyl-CoA by CNL, benzoylacetyl-CoA by CHD, and benzoyl-CoA by KAT. Benzoyl-CoA is then transformed to benzyl benzoate by BEBT in peroxisomes, hydroxylated to benzyl salicylate by a cytochrome P450 (BB2H) in the endoplasmic reticulum, and hydrolyzed to SA by a carboxylesterase (BSH) in the cytosol. This pathway is conserved in seed plants and contributes significantly to SA accumulation during defense responses. Degradation occurs via glycosylation or conjugation to form inactive stores, maintaining hormonal balance.^[35] Strigolactones (SLs) are carotenoid-derived hormones synthesized primarily in roots, with some production in shoots, starting from β-carotene in plastids. The pathway involves sequential cleavage: β-carotene is cleaved by carotenoid cleavage dioxygenase 7 (CCD7/MAX3/D17) to 9-cis-β-apo-10'-carotenal, then by CCD8/MAX4/D10 to 2'-epi-5-deoxystrigol precursor, followed by cytochrome P450 MAX1/CYP711A1 in the cytosol to form carlactone, which is further modified by additional CYPs (e.g., CYP722C1) to canonical SLs like strigol. These steps ensure SL gradients for branching and symbiosis regulation. Degradation involves hydroxylation and glycosylation for inactivation, with plastid-to-cytosol progression integrating with carotenoid metabolism. Recent studies highlight species-specific variations, such as in rice.^[36]

Transport and Signal Transduction

Plant hormones are transported through diverse mechanisms that enable their distribution from synthesis sites to target tissues, ensuring precise spatiotemporal control of developmental and stress responses. Auxin, primarily indole-3-acetic acid (IAA), undergoes polar transport directed by the PIN-FORMED (PIN) family of efflux carriers, which are asymmetrically localized on the plasma membrane to create concentration gradients essential for processes like embryogenesis and tropisms.^[37] Recent cryo-electron microscopy (cryo-EM) structures of PIN proteins, resolved at near-atomic resolution, reveal a dimeric architecture with a translocation pathway that accommodates IAA, highlighting how proton gradients drive efflux and polar localization is regulated by phosphorylation.^[37] In contrast, ethylene, a volatile gaseous hormone, diffuses passively across cell membranes without requiring dedicated transporters, allowing rapid intercellular and long-distance signaling.^[38] Strigolactones, carotenoid-derived hormones, are actively exported via ATP-binding cassette (ABC) transporters of the G subfamily, such as PDR1 in petunia and ABCG28 in Medicago, which facilitate root exudation to influence mycorrhizal symbiosis and shoot branching.^[39]^[40] Hormone perception occurs through specific receptors that initiate signal transduction cascades, often involving ubiquitin-mediated proteolysis or kinase activity modulation. For auxin, perception is mediated by TIR1 and related AFB F-box proteins, which function as substrate adaptors in SCF ubiquitin ligase complexes; auxin binding induces a conformational change that enhances TIR1 affinity for Aux/IAA repressor proteins, targeting them for 26S proteasome degradation and thereby derepressing AUXIN RESPONSE FACTOR (ARF) transcription factors to activate gene expression. Crystal structures of the TIR1-auxin-Aux/IAA complex demonstrate how the hormone stabilizes a composite binding surface on TIR1, promoting repressor recruitment with high specificity.^[41] Ethylene perception involves membrane-bound receptors like ETR1, which belong to a two-component histidine kinase family; in the absence of ethylene, receptors activate CTR1 kinase to phosphorylate and stabilize EIN2, inhibiting downstream responses, whereas ethylene binding inhibits this kinase activity, leading to EIN2 dephosphorylation and translocation to the nucleus for EIN3/EIL1 activation.^[42]^[38] Structures of ETR1 domains, including the histidine kinase and receiver modules, illustrate dimerization and autophosphorylation sites critical for signal relay.^[43] Downstream signaling amplifies perception into cellular responses, frequently converging on transcriptional reprogramming or ion dynamics. In auxin signaling, freed ARF proteins dimerize and bind auxin response elements (AuxREs) in promoters of primary response genes, such as SAUR and GH3 families, to orchestrate rapid transcriptome changes; ARF DNA-binding domains recognize TGTCTC motifs, while their middle domains mediate homo- or heterodimerization for cooperative regulation. For abscisic acid (ABA), perception by PYR/PYL/RCAR receptors inhibits PP2C phosphatases, activating SnRK2 kinases that phosphorylate targets including ion channels; this triggers cytosolic calcium fluxes via ABA-induced oscillations, decoded by calcium sensors like CPK6 to fine-tune guard cell closure and stress adaptation.^[44]^[45] Crystal structures of ABA-bound PYR1-PP2C complexes reveal a "lid" closure mechanism that buries the hormone and blocks phosphatase activity.^[46] Feedback loops maintain signaling homeostasis and enable adaptation to sustained stimuli. In ethylene pathways, prolonged exposure leads to desensitization through receptor resynthesis and negative regulators like EBF1/2 F-box proteins, which target EIN3 for degradation, allowing recovery from the triple response (hypocotyl shortening, swelling, and exaggerated apical hook) and preventing overstimulation.^[47] This adaptation involves transcriptional feedback where ethylene induces receptor genes (e.g., ETR1), restoring repression and modulating sensitivity over time.^[38] Similarly, auxin signaling features autoregulatory loops via Aux/IAA feedback repression of ARFs, ensuring transient responses. These mechanisms underscore the dynamic integration of transport, perception, and transduction in plant hormone action.

Regulation by Environmental Factors

Environmental factors profoundly influence the synthesis, transport, and activity of plant hormones, enabling plants to adapt to changing conditions. Abiotic stresses such as drought trigger rapid accumulation of abscisic acid (ABA) through the upregulation of 9-cis-epoxycarotenoid dioxygenase (NCED) enzymes, which catalyze a rate-limiting step in ABA biosynthesis from carotenoids.^[48] This induction is particularly evident in roots during water deficit, where NCED gene expression drives local ABA production to promote stress tolerance without relying heavily on long-distance transport from shoots.^[49] Light quality and intensity also modulate hormone levels via phytochrome photoreceptors, which interact with auxin signaling to regulate root architecture and phototropism; for instance, red light activation of phytochromes stabilizes auxin transporters like PIN-FORMED proteins, enhancing polar auxin flow in response to directional cues.^[50] Biotic interactions, including pathogen attacks, elicit hormone responses through dedicated pathways. Necrotrophic pathogens and herbivore feeding stimulate jasmonate biosynthesis via the lipoxygenase (LOX) pathway, where LOX enzymes initiate the oxygenation of α-linolenic acid in chloroplasts to produce jasmonic acid precursors.^[51] This rapid upregulation, often within minutes of attack, activates defense gene expression and reinforces physical barriers, illustrating how biotic cues fine-tune jasmonate levels for targeted immunity.^[52] Hormone homeostasis is further maintained by environmental modulation of apoplastic conditions, such as pH and ion gradients, which directly impact transport efficiency. An acidic apoplast (pH around 5-6) facilitates auxin influx through protonated forms that diffuse across membranes, promoting cell expansion in growing tissues; disruptions in ion balance, like those from salinity, can alkalinize the apoplast and inhibit this process.^[53] These pH shifts integrate multiple cues, ensuring hormone distribution aligns with local environmental demands. Circadian rhythms impose daily oscillations on hormone profiles to synchronize growth with predictable light-dark cycles. Gibberellin levels in Arabidopsis exhibit rhythmic fluctuations, peaking at dawn to drive hypocotyl elongation, mediated by clock-regulated transcription factors that stabilize gibberellin signaling components like DELLA proteins.^[54] This temporal patterning optimizes resource allocation, linking internal clocks to external photoperiods for enhanced fitness. Emerging research highlights how global climate shifts alter hormone dynamics; elevated CO₂ concentrations, projected to rise further, suppress ethylene production in many species during vegetative growth, potentially delaying senescence but impairing fruit ripening in crops like tomato.^[55] These changes underscore the need for updated models on CO₂-hormone interactions under warming scenarios. Such environmental regulations often culminate in adaptive stress responses, such as ABA-mediated stomatal closure to conserve water during drought.^[48]

Major Classes of Plant Hormones

Auxins

Auxins are a class of plant hormones primarily responsible for regulating cell elongation, apical dominance, and tropistic responses in plants. The most abundant and bioactive natural auxin is indole-3-acetic acid (IAA), characterized by an indole ring fused to a carboxylic acid group, which enables its role in diverse developmental processes.^[56] Synthetic auxins, such as 1-naphthaleneacetic acid (NAA) and 2,4-dichlorophenoxyacetic acid (2,4-D), mimic IAA's structure but exhibit greater chemical stability, allowing their use in experimental and applied contexts.^[57] IAA biosynthesis in plants predominantly occurs through tryptophan-dependent pathways, with the indole-3-pyruvic acid (IPA) route serving as the major pathway in species like Arabidopsis thaliana. In this pathway, tryptophan is first converted to IPA by tryptophan aminotransferases such as TAA1 and its homologs, followed by decarboxylation and oxidation to IAA catalyzed by YUCCA family flavin monooxygenases (e.g., YUC1, YUC4, YUC6).^[58] An alternative tryptophan-dependent route, the indole-3-acetamide (IAM) pathway, involves conversion of tryptophan to IAM by tryptophan-2-monooxygenase, then hydrolysis to IAA by indole-3-acetamide hydrolase (e.g., AMI1).^[58] These pathways are localized in young leaves, shoot apices, and developing seeds, ensuring auxin gradients essential for growth regulation.^[57] Key physiological functions of auxins include maintaining apical dominance, where IAA from the shoot apex inhibits lateral bud outgrowth, thereby promoting vertical growth.^[56] In phototropism, auxins mediate stem bending toward light sources through asymmetric redistribution, as explained by the Cholodny-Went theory, which posits that light-induced auxin transport via PIN-FORMED efflux carriers (e.g., PIN3, PIN4) creates growth differentials on shaded and illuminated sides.^[56] Auxins also drive root initiation by stimulating adventitious and lateral root formation at optimal concentrations, enhancing plant anchorage and nutrient uptake.^[57] At the molecular level, auxin signaling involves the TIR1/AFB receptor complex, which, upon binding IAA, recruits Aux/IAA repressor proteins for ubiquitination and subsequent degradation by the 26S proteasome.^[56] This degradation releases auxin response factors (ARFs) to activate target gene expression, enabling rapid cellular responses to auxin gradients.^[57] Auxins interact briefly with cytokinins during organogenesis, where their ratio influences shoot versus root development.^[57] Recent research highlights the influence of the plant microbiome on auxin dynamics, particularly through bacterial production of IAA in the rhizosphere. Over 80% of rhizosphere bacteria, including genera like Pseudomonas, Bacillus, and Azospirillum, synthesize IAA from tryptophan via pathways analogous to plant mechanisms, contributing to elevated local auxin levels that promote root elongation and proliferation.^[59] This microbial IAA modulates plant auxin homeostasis, enhancing growth under stress conditions and illustrating the interconnectedness of plant-bacterial signaling in the rhizosphere.^[60]

Cytokinins

Cytokinins are a class of adenine-derived plant hormones essential for regulating cell division, differentiation, and growth processes. The core structure of cytokinins consists of an N^6-substituted adenine ring, with naturally occurring forms primarily belonging to the isoprenoid group. Isopentenyladenine (iP) serves as a key precursor, formed by the attachment of a dimethylallyl group to the N^6 position of adenine, while trans-zeatin (tZ), featuring an isoprenoid side chain with a trans-hydroxyl group at the 9' position, represents the predominant active form due to its high biological potency and prevalence in higher plants.^[61]^[62]^[63] Biosynthesis of cytokinins initiates in plastids through the action of isopentenyltransferases (IPTs), rate-limiting enzymes encoded by multigene families. The primary de novo pathway involves ATP/ADP-IPT enzymes that catalyze the prenylation of ATP or ADP with dimethylallyl pyrophosphate to produce iP nucleotides, which are subsequently converted to active free bases via phosphoribohydrolases like LONELY GUY (LOG). A secondary tRNA-IPT pathway contributes by modifying tRNA-bound adenine during protein synthesis, releasing cytokinin precursors upon tRNA degradation. In Arabidopsis, IPT genes are expressed predominantly in root vascular tissues, with trans-zeatin-type cytokinins synthesized there and transported acropetally to shoots via the xylem sap, ensuring long-distance signaling for shoot apical meristem maintenance.^[64]^[62]^[65] Cytokinin signaling follows a multistep phosphorelay cascade resembling bacterial two-component systems. Perception occurs at the endoplasmic reticulum via histidine kinases AHK2, AHK3, and AHK4 (CRE1), which bind cytokinins and undergo autophosphorylation at a conserved histidine residue. The phosphate group is then transferred to histidine-containing phosphotransfer proteins (AHP1–AHP5), which shuttle the signal to the nucleus, where it activates B-type Arabidopsis response regulators (ARRs), such as ARR1, ARR2, ARR10, and ARR12. These B-type ARRs, functioning as transcription factors, bind to cytokinin response motifs in promoter regions to upregulate target genes involved in proliferation and development.^[66]^[67]^[68] Key physiological roles of cytokinins include promoting mitosis and countering auxin-mediated apical dominance to stimulate axillary bud outgrowth. In shoots, elevated cytokinin levels redirect auxin transport away from buds, enabling lateral branch expansion, as demonstrated in decapitated plants where exogenous tZ application induces multiple shoots. Cytokinins also delay leaf senescence by sustaining photosynthetic capacity; through B-type ARRs, they suppress senescence-associated genes, enhance chlorophyll retention, and limit sugar remobilization, thereby extending leaf longevity by weeks in model species like Arabidopsis. In tissue culture, cytokinins interact with auxins in specific ratios to induce callus formation and organogenesis, with high cytokinin-to-auxin ratios favoring shoot development.^[69]^[70]^[71] Recent advances in synthetic cytokinins address limitations of traditional analogs like 6-benzyladenine (BA), which can cause undesirable side effects such as abnormal shoot morphology. Meta-topolin, a hydroxylated BA derivative, exhibits superior efficacy in micropropagation, inducing 3.28 shoots per apple explant with improved quality and reduced hyperhydricity compared to BA. Similarly, chiral N^6-benzyladenine derivatives, developed since 2022, demonstrate enhanced cytokinin activity and receptor affinity in in planta assays, offering potential for precise biotechnological applications in crop improvement and in vitro regeneration.^[72]^[73]

Gibberellins

Gibberellins are a class of plant hormones belonging to the terpenoid family, characterized by over 130 distinct structures derived from the ent-gibberellane skeleton, a tetracyclic diterpenoid framework.^[74] Among these, the bioactive forms primarily include GA1, GA3, and GA4, which exert physiological effects through specific molecular interactions.^[74] These compounds are carboxylic acids that vary in hydroxylation and lactone ring configurations, with non-bioactive precursors serving as intermediates in their formation.^[75] Biosynthesis of gibberellins begins in the plastids via the methylerythritol 4-phosphate (MEP) pathway, though contributions from the mevalonate pathway in the cytosol can occur, leading to the formation of geranylgeranyl diphosphate (GGPP).^[74] GGPP is then cyclized to ent-copalyl diphosphate and further to ent-kaurene by ent-kaurene synthase (KS), followed by oxidation to ent-kaurenoic acid by ent-kaurene oxidase (KO).^[74] Subsequent steps involve cytochrome P450 monooxygenases such as ent-kaurenoic acid oxidase (KAO) to produce GA12-aldehyde, which is converted to GA12 or GA53.^[75] The key rate-limiting oxidations occur through gibberellin 20-oxidases (GA20ox), which transform GA12 and GA53 into GA9 and GA20, respectively, and gibberellin 3-oxidases (GA3ox), which hydroxylate these to the bioactive GA4 and GA1.^[74] In plant growth, gibberellins promote internode elongation by inducing the degradation of DELLA proteins, which are nuclear-localized transcriptional repressors that inhibit cell expansion in the absence of hormone.^[76] Bioactive GAs bind to the soluble receptor GID1, inducing a conformational change that facilitates interaction with DELLA proteins, leading to their ubiquitination by the SCF^SLY1/GID2 E3 ligase complex and subsequent proteasomal degradation.^[76] This relieves repression on growth-promoting factors, such as phytochrome-interacting factors (PIFs), enabling rapid stem elongation observed in many species.^[76] Gibberellins also play a central role in seed germination by stimulating the synthesis of hydrolytic enzymes, notably alpha-amylase, in the aleurone layer of cereal seeds like barley.^[77] Upon imbibition, embryo-derived GAs diffuse to the aleurone, triggering alpha-amylase production via the same DELLA degradation pathway, which breaks down stored reserves to provide energy and sugars for radicle emergence.^[76] Specific DELLA isoforms, such as RGL2 in Arabidopsis, act as key repressors during germination, and their removal is essential for this process.^[76] Mutations disrupting gibberellin biosynthesis or signaling often result in dwarfism, as seen in seminal genetic studies of rice and pea, where defects in GA20ox or DELLA genes lead to shortened internodes and reduced yields.^[78] Recent advances since 2019 have highlighted fine-tuning of GA signaling to enhance crop yield, including targeted overexpression of GA2ox for semi-dwarfism in wheat, which improves lodging resistance and grain production without compromising germination.^[79] In maize, modulating GID1-DELLA interactions has been shown to optimize tillering and biomass allocation, contributing to higher harvest indices in breeding programs.^[78]

Abscisic Acid

Abscisic acid (ABA) is a key plant hormone classified as a sesquiterpenoid with a 15-carbon structure (C₁₅H₂₀O₄), featuring a cyclohexadienone ring and a side chain with a carboxylic acid group.^[80] The biologically active form is the cis-isomer, specifically S-(+)-cis-ABA, which exhibits optical activity at the C-1' carbon and is the predominant enantiomer in plants.^[80] This structure enables ABA to interact with specific receptors, facilitating its role in stress responses. ABA levels increase rapidly under abiotic stresses like drought, positioning it as a central regulator of plant adaptation. Biosynthesis of ABA occurs primarily through the carotenoid pathway in plastids and cytosol. It begins with the oxidative cleavage of 9-cis-epoxycarotenoids, such as 9-cis-violaxanthin or 9'-cis-neoxanthin, by the enzyme 9-cis-epoxycarotenoid dioxygenase (NCED), yielding xanthoxin as the first committed intermediate.^[81] Xanthoxin is then transported to the cytosol, where the short-chain dehydrogenase/reductase ABA2 converts it to abscisaldehyde in an NAD-dependent reaction.^[82] Subsequently, ABA3, an aldehyde oxidase, oxidizes abscisaldehyde to ABA, completing the pathway; this step is molybdenum cofactor-dependent and rate-limiting under stress conditions.^[82] NCED activity is tightly regulated by environmental cues, ensuring ABA accumulation correlates with stress intensity. ABA exerts inhibitory effects on plant growth and development, notably by promoting stomatal closure to conserve water during drought. This process is mediated by PYR/PYL/RCAR receptors, which bind ABA and inhibit protein phosphatase 2C (PP2C) enzymes, thereby releasing SnRK2 kinases from suppression.^[83] Activated SnRK2 kinases phosphorylate ion channels like SLAC1, leading to anion efflux, membrane depolarization, and guard cell turgor loss for closure.^[84] In seed dormancy, ABA maintains quiescence by repressing germination genes; it stabilizes dormancy through PYR/PYL-PP2C interactions that activate downstream effectors, preventing embryo expansion until favorable conditions arise.^[85] The ABA signaling cascade involves SnRK2 kinases as core activators, which autophosphorylate upon PP2C inhibition and target bZIP transcription factors such as ABI5.^[86] Phosphorylated ABI5 binds ABA-responsive elements (ABREs) in promoters, upregulating stress-responsive genes like those for late embryogenesis abundant proteins, thereby enforcing dormancy and closure.^[86] This pathway integrates with other hormones, as ABA antagonizes gibberellins during seed germination to balance dormancy release.^[87] Recent advances highlight ABA's potential in engineering stress tolerance, including 2023 studies on transgenic tomato plants overexpressing ABA biosynthesis genes, which enhanced drought resistance by elevating endogenous ABA levels and improving water-use efficiency.^[88] Similarly, ABA analogs, such as quinabactin derivatives, mimic cis-ABA to activate PYR/PYL receptors without rapid degradation, sustaining stomatal closure and stress responses in crops like Arabidopsis and tomato.^[89] These synthetic ligands offer promise for targeted applications in abiotic stress mitigation.

Ethylene

Ethylene is a simple gaseous plant hormone with the molecular formula C₂H₄, consisting of two carbon atoms connected by a double bond and each bearing two hydrogen atoms, making it the smallest olefin and a non-conjugated hydrocarbon. Unlike other plant hormones, its volatility allows it to diffuse readily through plant tissues and the atmosphere, facilitating rapid intercellular and interplant signaling. The biosynthesis of ethylene begins with the amino acid methionine, which is first converted to S-adenosylmethionine (SAM) by SAM synthetase. SAM is then transformed into 1-aminocyclopropane-1-carboxylic acid (ACC), the immediate precursor to ethylene, by the pyridoxal phosphate-dependent enzyme ACC synthase (ACS), which catalyzes the rate-limiting step of the pathway. ACC is subsequently oxidized to ethylene by ACC oxidase (ACO), a non-heme iron-dependent enzyme that requires ascorbate, ferrous iron, and oxygen as cofactors. The pathway is tightly regulated through feedback inhibition, where ethylene itself represses ACS transcription and activity to prevent overproduction, ensuring precise control in response to developmental cues or stresses.^[90] Ethylene exerts diverse physiological effects, prominently including the "triple response" observed in etiolated seedlings grown in the dark. This response comprises three distinct morphological changes: shortening and radial swelling of the hypocotyl, exaggeration of the apical hook, and inhibition of hypocotyl and root elongation, which collectively enhance seedling survival by reducing extension growth in confined or dark environments.^[91] In reproductive tissues, ethylene drives fruit ripening in climacteric species such as tomato and apple by inducing the expression of genes encoding cell-wall-modifying enzymes, including cellulase, which degrades pectin and facilitates fruit softening and flavor development.^[92] Additionally, ethylene promotes senescence in leaves and flowers, accelerating chlorophyll breakdown and tissue degradation as part of programmed aging processes.^[93] Ethylene signaling initiates at the endoplasmic reticulum membrane, where it is perceived by a family of five receptors, including ETR1, which function as histidine kinases similar to bacterial two-component systems.^[94] In the absence of ethylene, these receptors activate the downstream Raf-like kinase CTR1, which phosphorylates the central positive regulator EIN2 at its C-terminal domain, leading to its ubiquitination and proteasomal degradation.^[95] Upon ethylene binding to the receptors' copper-containing ethylene-binding domain, the receptors are inactivated, preventing CTR1 activation; this allows the unphosphorylated C-terminus of EIN2 to be cleaved by proteases and translocated to the nucleus.^[95] There, EIN2 stabilizes the EIN3/EIL1 family of transcription factors by inhibiting their degradation via the F-box proteins EBF1 and EBF2, enabling EIN3 to bind to ethylene response elements in target gene promoters and activate downstream responses.^[96] Recent advancements have introduced nanotechnology-based sensors for real-time ethylene detection in plants, such as chemiresistive nanosensors using metal oxide semiconductors or carbon dot nanofluorescent probes, which offer high sensitivity (down to parts per billion) and enable in vivo monitoring of ethylene bursts during stress or ripening without invasive sampling.^[97] These tools are facilitating research into ethylene dynamics and support precision agriculture by detecting early stress signals.^[98] In plant breeding, ethylene signaling is increasingly targeted for developing climate-adaptive varieties, as modulating ACS or receptor genes enhances tolerance to abiotic stresses like drought and flooding, improving yield stability under variable environmental conditions.^[93] Ethylene also interacts briefly with auxin to promote epinastic movements, such as petiole bending in response to flooding.

Brassinosteroids

Brassinosteroids (BRs) are a class of polyhydroxylated steroidal phytohormones that play essential roles in plant growth, development, and environmental adaptation. First identified in the 1970s from extracts of rapeseed pollen that promoted stem elongation and cell division in bean plants, BRs were later characterized with the isolation and structural elucidation of brassinolide (BL) in 1979 as the most potent member of this hormone family. Over 70 naturally occurring BRs have been documented across the plant kingdom, with varying levels of bioactivity, but BL exhibits the highest potency in promoting hypocotyl elongation and overall vegetative growth. These hormones are ubiquitously distributed in vascular plants, algae, and even some non-vascular species, underscoring their ancient evolutionary origin. The core structure of BRs is derived from plant sterols, featuring a tetracyclic steroidal backbone with hydroxyl groups at specific positions and a characteristic side chain at C-17. Brassinolide, the prototypical and most active BR, is synthesized from campestanol and includes a unique B-ring lactone (7-oxa-B-homo configuration) along with hydroxylations at C-2α, C-3α, C-22R, and C-23R, contributing to its high affinity for the BR receptor. This structural motif distinguishes BRs from animal steroids and enables their specific perception by plant cells. Other bioactive BRs, such as castasterone and 6-deoxocastasterone, share similar features but lack the full lactone or certain hydroxyl groups, resulting in lower activity. Biosynthesis of BRs proceeds via multiple parallel pathways starting from the sterol precursor campesterol, which undergoes a series of reductions, hydroxylations, and oxidations primarily catalyzed by cytochrome P450 enzymes and reductases. The initial committed step involves the 5α-reduction of campesterol to campestanol by the enzyme DET2 (a steroid 5α-reductase), a process essential for downstream conversions. Subsequent steps include C-6 oxidation (via CPD/CYP90A1), C-22 and C-23 hydroxylations (via DWF4/CYP90B1), and multiple Baeyer-Villiger oxidations to form the B-ring lactone in BL, involving enzymes like SOB7/CYP72C1 and SHOT1/CYP90C1. These pathways, often referred to as the campesterol-dependent route, are tightly regulated by feedback mechanisms and environmental cues, with mutations in key genes like DET2 leading to dwarf phenotypes due to BR deficiency. BRs exert their effects primarily through promotion of cell elongation and division, as evidenced by their role in hypocotyl elongation in dark-grown Arabidopsis seedlings, where exogenous BL application rescues the short-hypocotyl phenotype of BR-deficient mutants via activation of the BRI1 receptor. They also drive vascular differentiation by influencing procambial cell fate and xylem formation, often in coordination with auxins to specify vascular tissues during embryogenesis and organ development. Additionally, BRs enhance plant thermotolerance by modulating heat shock responses; for instance, elevated BR levels alleviate heat-induced damage in Arabidopsis by promoting the nuclear localization of heat shock transcription factors like HsfA1d through inhibition of the kinase BIN2. The BR signaling pathway is initiated at the plasma membrane when BL binds to the leucine-rich repeat receptor-like kinase BRI1, forming a ligand-induced complex with the co-receptor BAK1 (SOMATIC EMBRYOGENESIS RECEPTOR KINASE 4). This association leads to transphosphorylation and activation of downstream components, including the phosphorylation of BR-SIGNALING KINASES (BSKs), which in turn recruit the phosphatase BSU1 to dephosphorylate and inactivate the glycogen synthase kinase 3-like kinase BIN2. Inactive BIN2 allows the accumulation of dephosphorylated transcription factors BES1 and BZR1, which translocate to the nucleus to activate BR-responsive genes involved in cell wall modification, photosynthesis, and growth regulation. This core pathway, first delineated through genetic screens identifying BRI1 mutants, integrates with other hormonal signals to fine-tune developmental processes. In agricultural contexts, BRs have shown promise in enhancing crop yields, particularly in wheat, where modulating BR signaling via targeted mutations in BRI1 homologs produces semi-dwarf varieties with improved lodging resistance and grain output under field conditions. For example, reducing BR sensitivity in elite wheat lines increased yield by up to 13% in multi-year trials without compromising plant height excessively, offering a sustainable alternative to traditional dwarfing genes. Such applications highlight BRs' potential for precision breeding to boost productivity amid climate challenges.

Jasmonates

Jasmonates constitute a family of lipid-derived signaling molecules in plants, with jasmonic acid (JA) serving as the central bioactive form and methyl jasmonate (MeJA) as its volatile methyl ester derivative that facilitates airborne communication.^[99] These compounds originate from the oxidation of α-linolenic acid, a polyunsaturated fatty acid abundant in chloroplast membranes, and play pivotal roles in coordinating developmental processes and stress responses.^[100] The biosynthesis of jasmonates occurs primarily through the octadecanoid pathway, initiating in the chloroplasts where lipoxygenase (LOX) enzymes, such as 13-LOX, catalyze the dioxygenation of α-linolenic acid to form 13-hydroperoxylinolenic acid (13-HPOT). This intermediate is then converted by allene oxide synthase (AOS) to an unstable allene oxide, which allene oxide cyclase (AOC) cyclizes into cis-(+)-12-oxo-phytodienoic acid (OPDA), a key precursor retained in the chloroplast. OPDA is subsequently reduced and undergoes β-oxidation in peroxisomes to yield JA, with the pathway tightly regulated by environmental cues like wounding.^[100]^[101] In plant defense, jasmonates mediate rapid wound responses by inducing the expression of proteinase inhibitors and other defensive proteins; for instance, mechanical damage triggers JA accumulation, which activates genes like the tomato proteinase inhibitor II (pin2) through binding of the basic helix-loop-helix transcription factor MYC2 to G-box motifs in their promoters.^[102] Jasmonates also regulate male fertility, where MYC2 and related factors promote stamen development and pollen viability in Arabidopsis; disruptions in JA signaling, such as in coi1 mutants, lead to male sterility by failing to activate downstream genes like MYB21.^[103] Jasmonate signaling initiates when the bioactive form JA-isoleucine (JA-Ile) binds to the F-box protein CORONATINE INSENSITIVE1 (COI1) in the SCF^COI1 ubiquitin ligase complex, recruiting JAZ domain repressor proteins for 26S proteasome-mediated degradation and thereby derepressing transcription factors like MYC2 to activate defense and developmental genes.^[104] Recent studies highlight the volatile MeJA's role in root-emitted signaling, where it induces biofilm formation in soil microbiomes, enriching beneficial bacteria like Streptomyces and enhancing plant growth under stress conditions.^[105] Similarly, JA concentrations in root exudates modulate rhizosphere bacterial communities in maize, with higher JA levels promoting protective taxa and influencing community composition by up to 4.3% variance across growth stages.^[106] Jasmonates exhibit crosstalk with salicylic acid pathways in immunity, often antagonistically balancing local defenses against biotrophic versus necrotrophic pathogens.^[107]

Salicylic Acid

Salicylic acid (SA), chemically known as 2-hydroxybenzoic acid, is a simple phenolic compound derived from benzoic acid with a hydroxyl group at the ortho position.^[108] This structure enables SA to participate in hydrogen bonding and redox reactions critical for its biological activity in plants.^[109] In plants, SA is primarily biosynthesized through two main pathways: the isochorismate (IC) pathway in chloroplasts and the phenylalanine ammonia-lyase (PAL) pathway in the cytosol. The IC pathway, predominant in species like Arabidopsis thaliana, involves isochorismate synthase (ICS1) converting chorismate to isochorismate, followed by isochorismate pyruvate lyase (IPL) to form SA; this pathway accounts for over 90% of pathogen-induced SA accumulation.^[110] The PAL pathway, an alternative route active under certain stress conditions, starts with phenylalanine deamination to trans-cinnamic acid, which is then hydroxylated and modified to yield SA, though it contributes less to defense-related synthesis.^[111] These pathways allow flexible SA production in response to environmental cues, with localization influencing transport and signaling efficiency.^[112] SA plays a central role in plant defense, most notably by mediating systemic acquired resistance (SAR), a long-distance immune response that enhances resistance to subsequent pathogen attacks. Upon local infection, SA accumulation induces the expression of pathogenesis-related (PR) genes, such as PR1, through transcriptional reprogramming that establishes broad-spectrum, lasting protection in distal tissues.^[113] Additionally, SA regulates thermogenesis in certain thermogenic flowers, such as those of Arum lilies (Zantedeschia aethiopica), where elevated SA levels trigger cyanide-resistant alternative oxidase activity to generate heat for volatilizing attractants and protecting reproductive tissues.^[114] SA often antagonizes jasmonate signaling, promoting defenses against biotrophic pathogens while suppressing responses to necrotrophs.^[115] SA signaling is orchestrated by non-expressor of PR genes 1 (NPR1), the master regulator that translocates to the nucleus upon SA-induced redox changes, where it interacts with TGA-class transcription factors to bind SA-responsive promoter elements and activate defense gene expression.^[116] Recent advances highlight SA's involvement in epigenetic regulation, particularly through histone modifications; for instance, SA promotes H3K9 acetylation and inhibits histone deacetylase 6 (HDAC6) activity, facilitating chromatin remodeling for sustained PR gene activation during immunity.^[117] These mechanisms ensure precise control of immune outputs, integrating SA perception with downstream epigenetic marks to balance defense and growth.^[118]

Strigolactones

Strigolactones (SLs) are a class of carotenoid-derived terpenoid hormones that regulate various aspects of plant growth and development, particularly shoot branching and interactions with symbiotic microorganisms. First identified as germination stimulants for parasitic plants in the 1960s, SLs were established as endogenous hormones in the mid-2000s through studies on mutants exhibiting excessive branching, such as the more axillary growth (max) series in Arabidopsis and dwarf mutants in rice.^[119] These hormones are exuded from roots and play dual roles as internal signals for architecture control and external cues for rhizosphere communication.^[119] The core structure of SLs consists of an ABC-ring lactone moiety connected via an enol-ether bridge to a D-ring lactone, with variations in the A/B rings leading to distinct types such as strigol (from cotton) and orobanchol (from plants like red broomrape). This tricyclic ABC scaffold is essential for bioactivity, with the D-ring acting as the key signaling component that interacts with receptors. Natural SLs exhibit structural diversity across species, influencing their specificity in functions like symbiosis.^[119] Biosynthesis of SLs begins in plastids with the cleavage of carotenoids by carotenoid cleavage dioxygenases 7 and 8 (CCD7 and CCD8), yielding carlactone as a central precursor. Carlactone is then oxidized by cytochrome P450 enzymes, such as MORE AXILLARY GROWTH 1 (MAX1) in Arabidopsis or orthologs in other species, to form active SLs like 5-deoxystrigol. This pathway is conserved in vascular plants, with upstream regulation involving feedback from environmental cues.^[120] SLs primarily inhibit axillary bud outgrowth to control shoot branching, acting through the α/β-hydrolase receptor DWARF14 (D14), which upon ligand binding undergoes conformational change and facilitates ubiquitination of target repressors. In the signaling cascade, D14 interacts with an F-box protein (e.g., D3 in rice) to target SMAX1-like/D53 repressors or DELLA proteins for proteasomal degradation, thereby derepressing genes like BRANCHED1 (BRC1) that promote bud dormancy. This mechanism integrates SLs with other hormones, including a brief crosstalk with auxin transport in axillary buds to fine-tune branching decisions.^[119] Beyond branching, SLs serve as signaling molecules for arbuscular mycorrhizal (AM) fungi, promoting hyphal branching and symbiosis establishment by mimicking host recognition cues exuded from roots. They also stimulate seed germination in root parasitic plants like Striga and Orobanche, though this ex planta role underscores their evolutionary origin in symbiosis before co-option as hormones.^[121] Recent studies highlight SLs' involvement in abiotic stress responses, particularly drought tolerance through crosstalk with abscisic acid (ABA). Under water deficit, SL biosynthesis increases, enhancing SMAX1 degradation to activate stress-protective pathways that overlap with ABA signaling, such as stomatal closure and antioxidant upregulation, thereby improving resilience without altering branching architecture. This integration positions SLs as key mediators in resource allocation during environmental challenges.^[122]

Other Recognized Hormones

Polyamines, such as spermidine, are aliphatic polycations that exhibit hormone-like functions in plants, particularly in modulating stress responses and senescence. They are synthesized primarily through the arginine decarboxylase (ADC) pathway, where ADC converts arginine to agmatine, leading to putrescine formation as a precursor for spermidine and spermine.^[123] This biosynthesis is upregulated under abiotic stresses like drought and salinity, enhancing osmotic adjustment, antioxidant enzyme activity, and membrane stabilization to improve tolerance.^[124] In senescence, declining polyamine levels correlate with chlorophyll degradation and oxidative damage, while exogenous spermidine application delays these processes by inhibiting reactive oxygen species accumulation and maintaining photosynthetic efficiency.^[125] Evidence supporting their hormone status includes regulated biosynthesis via stress-inducible genes like ADC2, intercellular transport through unidentified carriers that facilitate systemic signaling, and indirect receptor-like actions via hypusination of eukaryotic initiation factor 5A (eIF5A), which regulates translation during development and stress. However, specific membrane receptors remain elusive, distinguishing polyamines from classical hormones.^[126] Nitric oxide (NO) functions as a gaseous signaling molecule in plants, fulfilling key hormone criteria through endogenous production, diffusion-based transport, and receptor-mediated effects. Biosynthesized enzymatically by nitrate reductase or non-enzymatically from nitrite, NO diffuses freely across membranes due to its lipophilic nature, enabling rapid intercellular communication without dedicated transporters.^[127] In guard cells, NO promotes stomatal closure by activating guanylate cyclase to elevate cyclic GMP levels, which triggers calcium influx and ion channel modulation.^[127] It acts synergistically with abscisic acid (ABA) in stress responses, enhancing ABA-induced closure while also providing negative feedback through S-nitrosylation of OST1 kinase at cysteine 137, which inhibits its activity and prevents excessive stomatal response.^[128] This post-translational modification exemplifies NO's signaling versatility, extending to overlaps with ethylene in fruit ripening and defense. Receptors include H-NOX domain-containing guanylate cyclases like ATNOGC1, confirming NO's role as a multitasked hormone in development and abiotic stress adaptation.^[127] Karrikins, butenolide compounds originally identified in wildfire smoke, serve as exogenous germination stimulants but mimic endogenous signals in plants, meeting hormone criteria via perception and downstream effects. Although smoke-derived, they emulate an unidentified endogenous KAI2 ligand (KL) involved in seed dormancy release and early development, with biosynthesis of KL independent of strigolactone pathways like carlactone.^[129] Active at nanomolar concentrations, karrikins promote germination in fire-prone species by repressing dormancy genes through the KAI2 receptor, an α/β-hydrolase that initiates signaling via the SCF^MAX2 ubiquitin ligase complex, leading to degradation of SMXL transcriptional repressors.^[130] Transport occurs through tissue-specific metabolism or diffusion, as evidenced by hypocotyl elongation responses in Arabidopsis mutants.^[130] The conservation of KAI2 across seed plants underscores karrikins' role in an ancient signaling system for environmental cue integration during seedling establishment.^[129] Hydrogen sulfide (H₂S), recognized increasingly as a gasotransmitter hormone since the 2010s, exhibits antioxidant properties and fulfills hormone status through enzymatic biosynthesis, diffusion, and protein modification signaling. Produced endogenously by enzymes such as L-cysteine desulfhydrase and D-desulfhydrase, H₂S diffuses as a gas to coordinate responses without specific transporters.^[131] In oxidative stress, H₂S enhances antioxidant defenses by persulfidating (S-sulfhydration) key proteins, such as catalase and ascorbate peroxidase, to boost their activity and mitigate reactive oxygen species damage under drought, salinity, and heavy metal exposure.^[131] It regulates development, including seed germination and stomatal movement, often via crosstalk with ABA and NO pathways.^[132] Recent studies, including 2024 reviews, highlight H₂S-mediated persulfidation of transcription factors such as ABI4 for stress resilience, with no dedicated receptors identified but signaling integrated into thiol-based networks for broad physiological impacts.^[131]

Physiological Roles in Plants

Growth and Development Processes

Plant hormones play a pivotal role in coordinating tropisms, which are directional growth responses that enable plants to adapt to environmental cues. In phototropism, the bending of shoots toward light, auxin gradients are established through the asymmetric redistribution of auxin via influx and efflux carriers, leading to differential cell elongation on the shaded side.^[133] Similarly, in geotropism (gravitropism), auxin accumulation in the lower parts of roots and shoots triggers downward root growth and upward shoot curvature, mediated by gravity-sensing statoliths that influence auxin transport.^[134] Ethylene contributes to thigmotropism, the touch-induced growth response seen in tendrils and roots, by promoting rapid cell expansion and coiling upon mechanical stimulation, often in coordination with calcium signaling.^[135] During organogenesis, the balance of auxin and cytokinin directs the formation of shoots and roots from meristematic tissues. A high auxin-to-cytokinin ratio promotes root initiation and development by enhancing cell division in the pericycle, while a low ratio favors shoot organogenesis through activation of cytokinin-responsive genes that stimulate axillary bud formation.^[136] Gibberellins further influence flowering as part of organogenesis by inducing the expression of the LEAFY gene, a key floral meristem identity regulator, particularly under short-day conditions where gibberellin biosynthesis is required for the transition from vegetative to reproductive growth.^[137] In vascular development, brassinosteroids and auxins interact to pattern xylem and phloem tissues, ensuring efficient nutrient transport. Auxin maxima, transported via polar efflux carriers, initiate procambial cell specification, while brassinosteroid signaling enhances this process by phosphorylating transcription factors that promote xylem differentiation and phloem unloading, as seen in mutants where disruptions lead to irregular vascular bundles.^[138] This crosstalk is essential for radial patterning in stems and roots, with brassinosteroids amplifying auxin responses in cambial initials. Strigolactones regulate phyllotaxy—the spatial arrangement of leaves—and leaf expansion by inhibiting axillary bud outgrowth, thereby maintaining apical dominance and optimizing light capture. Acting downstream of auxin, strigolactones reduce polar auxin transport in buds, preventing excessive branching and promoting a rosette-like phyllotactic pattern during vegetative growth.^[139] Recent advances in single-cell RNA sequencing and spatial transcriptomics have illuminated these hormone gradients at cellular resolution, revealing dynamic expression of hormone-responsive genes in developing meristems and vascular tissues, thus enhancing understanding of morphogenetic coordination.^[140]

Stress and Defense Responses

Plant hormones play pivotal roles in orchestrating responses to abiotic and biotic stresses, enabling adaptations that enhance survival under adverse conditions. Abiotic stresses, such as drought and cold, trigger specific hormonal pathways that regulate physiological adjustments like stomatal closure and gene expression for protective proteins.^[141] Biotic stresses from pathogens and herbivores activate defense signaling cascades involving antagonistic interactions between hormones to tailor responses to the threat type.^[118] Recent multi-omics studies have revealed intricate hormone networks that integrate these responses, contributing to climate resilience by modulating gene regulatory and metabolic pathways.^[142] In response to drought, abscisic acid (ABA) acts as a central mediator by inducing the expression of late embryogenesis abundant (LEA) proteins, which stabilize cellular structures and prevent protein denaturation under water deficit.^[143] ABA signaling upregulates LEA genes through transcription factors like ABI5, enhancing osmotic adjustment and membrane integrity, as demonstrated in Arabidopsis and crop species.^[141] This pathway is activated rapidly upon dehydration, with ABA levels increasing to balance water loss and maintain turgor.^[144] Biosynthesis of ABA is upregulated under drought, amplifying these protective mechanisms.^[145] Brassinosteroids (BRs) contribute to cold stress tolerance by activating the CBF (C-repeat binding factor) pathway, which induces cold-responsive genes such as COR (cold-regulated) genes for membrane stabilization and osmoprotectant accumulation.^[146] BRs enhance CBF expression via the BRI1 receptor and transcription factors like BES1, improving freezing tolerance in species like Arabidopsis and tomato.^[147] This regulation involves crosstalk with redox signaling to fine-tune cold acclimation, reducing oxidative damage during low temperatures.^[148] For biotic stresses, jasmonates (JA) and ethylene synergistically defend against herbivory by inducing vegetative storage protein (VSP) genes, which mobilize resources for defense compound synthesis and deter feeding.^[149] In tomato and Arabidopsis, JA-ethylene signaling converges on ERF (ethylene response factor) transcription factors to activate VSP expression within hours of insect attack, inhibiting herbivore growth.^[150] This pathway is triggered by oral secretions from herbivores, leading to systemic defense propagation.^[102] Salicylic acid (SA) is essential for systemic acquired resistance (SAR) against biotrophic pathogens, mediated by the NPR1 (non-expressor of pathogenesis-related genes 1) regulator, which translocates to the nucleus upon SA accumulation to activate PR (pathogenesis-related) genes.^[118] NPR1 forms complexes with TGA transcription factors, enhancing defenses like cell wall fortification and antimicrobial protein production in distal tissues.^[113] This establishes long-term immunity, as seen in tobacco and Arabidopsis models.^[151] Crosstalk between JA and SA often results in trade-offs, where JA-SA antagonism prioritizes defenses against necrotrophic pathogens or herbivores via JA dominance, while SA prevails against biotrophs to avoid mutual suppression.^[152] In Arabidopsis, this balance is regulated by WRKY and MYC2 transcription factors, with JA inhibiting SA signaling through protein degradation, optimizing resource allocation based on pathogen lifestyle.^[153] Such trade-offs ensure effective, non-redundant responses, though they can limit broad-spectrum resistance.^[154] Polyamines, including putrescine and spermidine, interact with hormones to mitigate oxidative stress by scavenging reactive oxygen species (ROS) and stabilizing membranes during abiotic challenges like salinity and drought.^[155] They enhance ABA and JA signaling to upregulate antioxidant enzymes, reducing lipid peroxidation in crops such as wheat.^[124] Polyamine catabolism generates H₂O₂, which acts as a signal to amplify hormone-mediated defenses without overwhelming cellular redox homeostasis.^[156] Hormones modulate ROS signaling to maintain balance, preventing oxidative damage while enabling stress perception and adaptation. ABA and SA promote ROS bursts for hypersensitive responses against pathogens, while JA and BRs activate ROS scavengers like superoxide dismutase to counteract excess accumulation during abiotic stress.^[157] This integration involves MAPK cascades that link hormone receptors to ROS-producing enzymes like RBOH, ensuring precise signaling in Arabidopsis and rice.^[158] Disruptions in this balance can lead to programmed cell death or enhanced tolerance.^[159] Multi-omics analyses since 2022 have elucidated hormone networks underlying climate resilience, revealing how ABA-JA-SA hubs regulate transcriptomic and metabolomic shifts for combined drought-heat tolerance in tomato and maize.^[142] Integrated genomics and proteomics identify key nodes like NPR1-JA antagonists that enhance polyamine-ROS interactions, informing breeding for resilient varieties.^[160] These networks highlight emergent properties, such as feedback loops that amplify defenses under multifactorial climate stresses.^[161]

Reproduction, Dormancy, and Senescence

Plant hormones play crucial roles in regulating reproductive processes, including the transition to flowering and the development of fruits and seeds. Gibberellins (GAs) are key promoters of the floral transition in many plants, acting by integrating environmental and endogenous signals to induce flowering through the activation of floral identity genes such as LEAFY and FLOWERING LOCUS T. For instance, in long-day plants like Arabidopsis thaliana, bioactive GAs like GA4 stimulate the expression of SOC1 (SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1), a MADS-box transcription factor that coordinates the switch from vegetative to reproductive growth. Cytokinins, often in synergy with GAs, maintain and specify floral meristems by promoting cell division in the shoot apical meristem and inhibiting the differentiation of floral organs, ensuring proper flower architecture; this is evident in cytokinin-overproducing mutants that exhibit enhanced floral meristem size. In fruit and seed development, ethylene is central to ripening in climacteric fruits, such as tomatoes and apples, where it triggers a burst of autocatalytic production that coordinates softening, color change, and flavor development via the activation of ripening-specific genes like those encoding polygalacturonase and ACC oxidase. This process involves ethylene receptors that, upon binding, deactivate downstream repressors like EIN3, allowing the expression of senescence-associated genes (SAGs) adapted for fruit maturation. Conversely, abscisic acid (ABA) enforces seed dormancy to prevent premature germination under unfavorable conditions, primarily through the transcription factor ABI3 (ABSCISIC ACID INSENSITIVE 3), which ABI3 represses genes involved in seed maturation and promotes the accumulation of storage proteins and lipids while inhibiting embryo growth. ABI3 achieves this by binding to RY motifs in target promoters, integrating ABA signaling with developmental cues to maintain dormancy until stratification or after-ripening breaks it. Dormancy regulation also involves epigenetic mechanisms modulated by ABA, particularly histone modifications that reinforce transcriptional repression. Recent studies highlight ABA's role in inducing H3K27me3 (trimethylation of histone H3 at lysine 27) marks via Polycomb Repressive Complex 2 (PRC2) recruitment to dormancy-related loci, such as DOG1 (DELAY OF GERMINATION 1), thereby epigenetically silencing germination-promoting genes during seed maturation. This chromatin-based control, observed in species like Arabidopsis and cereals, ensures heritable dormancy states that respond to environmental cues like temperature, with disruptions in histone demethylases leading to reduced dormancy depth. Senescence, the programmed aging of plant organs, is finely tuned by antagonistic hormone actions. Cytokinins delay senescence by maintaining chloroplast integrity and photosynthetic gene expression, for example, by upregulating CYCD3 cyclins to sustain cell cycle activity in leaves and inhibit SAG expression; exogenous application of benzyladenine can extend leaf longevity in crops like barley by weeks. In contrast, ethylene and jasmonic acid (JA) promote senescence through the induction of SAGs, such as SAG12 and SAG101, which encode cysteine proteases and lipid hydrolases that dismantle cellular structures; ethylene acts via EIN2-mediated signaling to activate NAC transcription factors, while JA receptors like COI1 trigger similar cascades, accelerating chlorophyll degradation and nutrient remobilization. This balance ensures timely resource reallocation to reproductive sinks, with JA also briefly referenced in pollen defense contexts where it protects gametophytes from pathogens during reproduction. Strigolactones (SLs) influence reproductive interactions indirectly by regulating parasitic plant associations, particularly through inducing "suicidal germination" in root parasites like Striga and Orobanche. SLs, exuded from host roots as signaling molecules, mimic host presence to trigger germination of parasite seeds, but in the absence of a suitable host, the parasites perish; this mechanism, exploited in "suicide germination" strategies, involves SL perception via receptors like DWARF14, leading to downstream hormonal crosstalk that activates germination genes in parasites. Such interactions highlight SLs' dual role in plant reproduction by facilitating or disrupting seed bank dynamics in agroecosystems.

Applications and Uses

In Horticulture and Agriculture

Plant hormones play a pivotal role in horticulture and agriculture by enabling targeted interventions to improve propagation, crop yield, quality, and post-harvest storage. Synthetic analogs and inhibitors of these hormones are applied to manipulate growth processes, enhance uniformity, and control weeds, leading to more efficient farming practices and reduced losses. In vegetative propagation, auxins such as indole-3-butyric acid (IBA) are commonly used in rooting powders to promote adventitious root formation in cuttings. Dipping cuttings in IBA solutions or powders increases rooting percentage, accelerates root initiation, and improves uniformity, particularly for difficult-to-root species like woody ornamentals.^[162] For optimal results, the auxin-to-cytokinin ratio in applied mixtures is adjusted to favor root development; higher auxin levels relative to cytokinins stimulate rooting in species like roses and tomatoes.^[163]^[4] Fruit management benefits significantly from hormone applications to extend shelf life and enhance marketable traits. Ethylene inhibitors like 1-methylcyclopropene (1-MCP) are applied post-harvest to apples, blocking ethylene receptors and delaying ripening, which maintains firmness, reduces volatile organic compound emissions, and preserves quality during cold storage for up to several months.^[164]^[165] Gibberellins, particularly gibberellic acid (GA3), are sprayed on grapevines to induce seedlessness in seeded varieties and increase berry size; concentrations of 30–50 ppm at early fruit set promote parthenocarpic development without seeds, boosting yield and consumer appeal in table grapes.^[166]^[167] Herbicides mimicking auxins, such as 2,4-dichlorophenoxyacetic acid (2,4-D), are widely used for selective weed control in broadleaf crops like cereals. As a synthetic auxin, 2,4-D disrupts normal growth in susceptible broadleaf weeds by overstimulating cell division and elongation, leading to twisting, abnormal proliferation, and death, while grasses remain largely unaffected due to metabolic differences.^[168]^[169] Hormone treatments also aid in breaking or inducing dormancy for synchronized crop establishment. Gibberellins like GA3 are applied to potato tubers to shorten dormancy periods and promote uniform sprouting; optimal concentrations (e.g., 100–500 ppm) with extended exposure enhance sprout vigor and emergence, facilitating earlier harvests.^[170]^[171] Conversely, abscisic acid (ABA) applications or analogs are used to enforce dormancy in seeds, preventing premature germination and ensuring uniform field emergence; by elevating ABA levels during storage, seed vigor is maintained, reducing variability in crops like lettuce and grains.^[172]^[173] Regulatory frameworks in the European Union continue to evolve regarding synthetic plant growth regulators (PGRs), with ongoing approvals, extensions, and restrictions on certain herbicides and PGRs to minimize environmental impact, alongside promotion of bio-based alternatives for sustainable agriculture. In May 2025, the Standing Committee on Plants, Animals, Food and Feed proposed segregating products containing phytohormones as PGRs under Regulation (EC) No 1107/2009.^[174]^[175]^[176]

In Biotechnology and Research

Plant hormones serve as essential tools in biotechnology and research, enabling precise manipulation of plant growth, signaling pathways, and stress responses through genetic engineering and advanced imaging techniques. Mutants and transgenic lines have been instrumental in dissecting hormone signaling pathways. For instance, auxin-resistant mutants such as axr1 in Arabidopsis thaliana exhibit reduced sensitivity to auxin, leading to altered root growth, rosette expansion, and inflorescence development, which has facilitated the identification of the AXR1 gene as a key regulator in auxin-mediated processes by encoding a protein related to ubiquitin-activating enzymes.^[177] These mutants, including double mutants like axr1-12 sar1-1, have revealed interactions between auxin and other pathways, such as those involving jasmonic acid responses, through suppressor screens that partially restore signaling defects.^[178] Similarly, CRISPR/Cas9-mediated editing of abscisic acid (ABA) receptors, such as the PYL/RCAR family in crops like rice, has generated drought-tolerant models by knocking out or modifying these receptors, enhancing water-use efficiency and survival under abiotic stress without compromising yield.^[179] These edited lines provide platforms for studying ABA's role in stomatal closure and gene expression during drought, accelerating the development of resilient varieties.^[180] In tissue culture applications, plant hormones are optimized for regeneration protocols, where Murashige and Skoog (MS) medium supplemented with cytokinins like 6-benzylaminopurine (BAP) and auxins such as naphthaleneacetic acid (NAA) promotes callus induction and shoot organogenesis. Combinations of 5.0 mg/L BAP and 0.5 mg/L NAA in MS medium have achieved up to 100% regeneration frequency in species like Himalayan rice, yielding multiple shoots per explant for efficient propagation.^[181] This balance of cytokinin-to-auxin ratios drives cell division and differentiation, as seen in turmeric and tea plant cultures, where higher BAP concentrations favor shoot proliferation while NAA supports rooting, enabling large-scale production of genetically uniform plants.^[182]^[183] Biosensors have revolutionized the real-time visualization of hormone dynamics, particularly for auxin. The DII-VENUS reporter, a fluorescent fusion of the auxin-inducible degron domain from AUX/IAA proteins with Venus yellow fluorescent protein, degrades rapidly in response to auxin, allowing imaging of concentration gradients at cellular resolution in Arabidopsis roots and shoots.^[184] This tool has quantified auxin maxima during embryogenesis and vascular patterning, revealing how polar transport shapes tissue development.^[185] Advanced variants, including mutated DII for stable baselines, enable semi-quantitative measurements of auxin flux in response to environmental cues.^[186] High-throughput screening methods leverage hormone-specific assays to identify mutants efficiently. For ethylene, traps that capture the gas in sealed chambers combined with phenotypic readouts like the triple response in etiolated seedlings have isolated insensitive loci such as ein2 and ctr1 in Arabidopsis, elucidating the receptor-mediated signaling cascade.^[187] These screens, often using low-dose ethylene exposures, detect weak mutants with subtle defects, accelerating gene discovery for hormone biosynthesis and perception.^[188] Recent advancements incorporate machine learning to model complex hormone interaction networks, addressing gaps in traditional approaches. A 2025 machine learning meta-analysis of abiotic stress transcriptomes identified ethylene as a central hub integrating signals from auxin, ABA, and jasmonates, predicting regulatory modules with high accuracy across species.^[189] AI-driven tools, including predictive models for hormone crosstalk, optimize experimental designs in hormone research, such as simulating network perturbations for drought or development studies, and have potential extensions to field trials for trait validation.^[190]

In Human Health and Industry

Salicylic acid, a plant hormone derived from sources like willow bark (Salix spp.), serves as the foundational compound for acetylsalicylic acid, commonly known as aspirin.^[191] In 1897, Felix Hoffmann at Bayer synthesized aspirin by acetylating salicylic acid, marking a pivotal advancement in pain relief and anti-inflammatory medications.^[192] Aspirin's therapeutic effects stem from its irreversible inhibition of cyclooxygenase (COX) enzymes, particularly COX-1 and COX-2, which reduces prostaglandin synthesis and thereby alleviates inflammation, fever, and pain.^[193] Jasmonic acid and its derivative methyl jasmonate (MeJA) have garnered attention in oncology for their selective cytotoxicity toward cancer cells, sparing normal cells. MeJA induces apoptosis in tumor cells, including those from breast, lung, and prostate cancers, by disrupting mitochondrial function and elevating reactive oxygen species levels.^[194]^[195] Derivatives of jasmonic acid also promote wound healing by enhancing extracellular matrix remodeling and proteoglycan expression in skin tissues, leading to their incorporation in topical creams for anti-aging and repair applications.^[196] Recent advancements include jasmonate-loaded nanoparticles for targeted drug delivery, improving bioavailability and efficacy in preclinical cancer models since 2022, though human clinical trials remain limited.^[197] Brassinosteroids, structurally akin to animal steroids, exhibit anti-inflammatory properties by suppressing pro-inflammatory cytokines such as TNF-α and IL-6, positioning them as potential therapeutics for conditions like arthritis and skin inflammation.^[198] Cytokinins, such as kinetin, delay cellular senescence in human skin by inhibiting age-related changes like wrinkle formation and roughness, making them key ingredients in anti-aging cosmetics that promote collagen maintenance and epidermal vitality.^[199]^[200] In industrial applications, plant-derived ethylene supports green chemistry initiatives by serving as a renewable feedstock for plastics and chemicals, produced via bio-based fermentation of biomass sources like sugarcane, reducing reliance on petrochemicals and cutting greenhouse gas emissions by up to 40%.^[201] Strigolactone mimics, such as GR24, demonstrate promise in human health beyond agriculture, with anti-cancer effects through apoptosis induction in breast and hepatocellular carcinoma cells, and anti-inflammatory activation of Nrf2 pathways, highlighting their transition to biomedical drug candidates.^[202]

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