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Protonema

A protonema is the initial juvenile stage of the gametophyte generation in mosses (Bryophyta), consisting of a branched, filamentous network of haploid cells that develops directly from the germination of haploid spores released by the sporophyte.^[1] This structure typically forms a prostrate mat on the substrate, resembling a tangle of single-celled green threads, and serves as a transitional phase before the emergence of the upright gametophore—the leafy shoot that dominates the moss life cycle.^[2] Protonemata are characterized by tip-growing cells and exhibit two primary filament types: chloronema, which are photosynthetic with abundant chloroplasts and irregular branching, and caulonema, which have fewer chloroplasts, elongated cells, and more regular branching to facilitate rapid expansion and transport.^[3] In the moss life cycle, the protonema plays a crucial role in establishing the gametophyte by anchoring to the substrate via rhizoids and producing buds that develop into multiple gametophores, thereby enabling asexual propagation and genetic variation through spore-derived growth.^[1] Germination occurs rapidly under favorable moist conditions, often within 1–3 days in species like Funaria hygrometrica, with the chloronema emerging first to support initial photosynthesis before transitioning to caulonema for further development.^[3] Cellular differentiation within protonemata involves irreversible processes such as tonoplast disintegration, endoreduplication, and cytoskeletal reorganization, which are influenced by environmental cues like light and gravity, making this stage a key model for studying plant morphogenesis and development.^[2] Protonemata are haploid (1n) throughout, aligning with the dominant gametophytic phase of bryophytes, and contribute to the mosses' adaptation to diverse habitats, from arctic tundras to tropical forests, by facilitating efficient spore dispersal and establishment without vascular tissues.^[1] Species such as Physcomitrium patens (formerly Physcomitrella patens) are widely used in research due to their propensity for homologous recombination and ease of culturing protonemata, providing insights into evolutionary developmental biology (evo-devo) and comparative studies with vascular plants.^[2]

Overview

Definition

A protonema is a haploid, filamentous structure that arises from the germination of moss spores, serving as the juvenile phase of the gametophyte generation in the moss life cycle.^[2] This initial growth form enables the establishment of the gametophyte before the development of the more complex leafy structures.^[4] In the alternation of generations characteristic of bryophytes, the protonema represents the early haploid stage following spore release.^[5] The term "protonema" derives from the Greek words protos (first) and nēma (thread), aptly describing its pioneering, thread-like appearance as the primary growth from a spore.^[6] Morphologically, it comprises multicellular, branched filaments that grow by apical cell division and can extend up to several centimeters, forming a green, fuzzy mat on the substrate without developing true leaves or roots.^[7] Protonemata are exclusive to bryophytes and predominate in mosses (phylum Bryophyta), where they facilitate initial colonization; liverworts possess analogous but typically thalloid structures from spore germination.^[8]

Role in Moss Life Cycle

The protonema represents the initial stage of the haploid gametophyte phase in the moss life cycle, emerging from the germination of a haploid spore released by the diploid sporophyte.^[9] This placement underscores its role in the alternation of generations characteristic of bryophytes, where the gametophyte dominates the life history.^[10] In the developmental sequence, spore dispersal leads to protonema formation, followed by the production of buds on its filaments that develop into mature leafy gametophytes capable of producing gametes.^[9] These gametophytes then facilitate fertilization to form a zygote, which grows into the sporophyte that completes the cycle by generating new spores.^[10] This progression allows the protonema to serve as a transitional structure bridging spore establishment and the upright, photosynthetic gametophyte phase.^[11] The adaptive significance of the protonema lies in its capacity for rapid clonal propagation, enabling a single spore to produce multiple genetically identical gametophores that enhance colonization of suitable substrates before sexual reproduction occurs.^[10] By spreading vegetatively through branching filaments, it promotes efficient resource capture and population expansion in heterogeneous environments, providing a buffer against the uncertainties of spore dispersal and fertilization.^[9] As a temporary stage, the protonema typically persists for weeks to months, varying with species and environmental factors such as light, moisture, and nutrients, before transitioning to gametophore formation.^[12] This duration allows sufficient time for establishment while minimizing energy investment in the juvenile phase.^[13]

Structure and Development

Spore Germination

Moss spores are haploid, single-celled structures equipped with a thick, protective wall primarily composed of sporopollenin, which confers resistance to desiccation and environmental stresses; these spores typically range from 10 to 50 μm in diameter, with those of the model moss Physcomitrium patens (formerly Physcomitrella patens) measuring approximately 30 μm.^[14]^[15] Germination is triggered by rehydration in moist conditions, exposure to light—particularly red wavelengths that promote the process via phytochrome signaling, while far-red light inhibits it—and temperatures of 20–25°C, often on suitable substrates such as soil or bark that provide anchorage and nutrients.^[16]^[17] The process begins with the spore absorbing water, causing swelling and subsequent rupture of the exine (outer spore wall), followed by distension of the inner intine layer and protrusion of a germ tube—an initial, unbranched filamentous structure. This germ tube elongates through repeated divisions of an apical cell, establishing the foundational protonemal filament.^[18]^[17]^[19] Under optimal laboratory conditions (e.g., 20–25°C and appropriate light), spore germination typically initiates within 1–7 days, with the first chloronemal cells emerging around 8 hours post-rehydration and protonemal development progressing over 24–72 hours as metabolic shifts enable heterotrophic growth and eventual photoautotrophy.^[16]^[15] This initial filamentous stage transitions into the chloronema formation, marking the onset of specialized protonemal development.^[16]

Chloronema Formation

The chloronema represents the initial stage of protonemal development in mosses, consisting of green, filamentous structures that emerge as the primary photosynthetic tissue following spore germination. These filaments are composed of elongated, cylindrical cells that contain numerous large chloroplasts, typically arranged peripherally around a central vacuole, which impart a characteristic light green appearance.^[20] Chloronema cells are generally shorter than those in later stages, with lengths often ranging from 100 to 250 μm, and they feature cross walls oriented perpendicular to the filament axis.^[21] Chloronema formation begins directly from the germ tube produced during spore germination, where the initial cell undergoes repeated transverse cell divisions to elongate the filament. This process is accompanied by oblique side-branching, which occurs behind the cross walls and contributes to the highly branched, mat-like morphology of the protonema.^[2] The growth is primarily apical, with tip cells expanding via polarized tip growth at a slow rate of approximately 4–10 μm per hour, resulting in the addition of about one cell every 24 hours.^[22]^[21] This gradual extension fosters the formation of a dense, sprawling mat that maximizes surface area for light capture in the early post-germination phase. At the cellular level, chloronema cells possess homogeneous, cellulose-based walls approximately 500 nm thick, which provide structural support while allowing for the accumulation of high chlorophyll content within the abundant chloroplasts—often exceeding 20 per cell.^[2] These chloroplasts, which are ovoid and starch-filled, enable efficient photosynthesis and serve as the primary mechanism for initial energy capture and nutrient assimilation in the developing moss.^[2] The overall physiology of chloronema emphasizes slow, sustained growth under favorable light conditions, preceding a potential transition to caulonema in response to hormonal or environmental signals.^[20]

Transition to Caulonema

The transition from chloronema to caulonema in moss protonema represents a key differentiation event driven by hormonal and environmental cues. Auxin, specifically indole-3-acetic acid (IAA), accumulates at filament tips and promotes this shift by upregulating bHLH transcription factors PpRSL1 and PpRSL2 in species like Physcomitrium patens, leading to rapid cell elongation and altered morphology.^[23] Environmental triggers such as reduced light intensity (including darkness) and low nutrient availability further enhance differentiation, favoring caulonema formation over chloronema proliferation to optimize resource exploration in shaded or nutrient-poor microhabitats. Caulonema filaments exhibit distinct traits adapted for exploratory growth: they are largely colorless due to few and small chloroplasts, contrasting with the chlorophyll-rich chloronema. Cells are slimmer, typically 10-20 μm in diameter, enabling efficient tip-focused expansion. Elongation rates are approximately three times faster than in chloronema, reaching 20-50 μm per hour in P. patens and up to 100 μm per hour in species like Funaria hygrometrica under optimal conditions. Branching in caulonema occurs primarily apically, with lateral branches emerging from subapical cells (2-5 cells behind the tip) at oblique angles, resulting in sparse, elongated networks rather than the dense, perpendicular side-branching of chloronema. This pattern supports directed colonization. In mature protonema, caulonema dominates, though interconversion can occur under altered conditions; for instance, high light or nutrient shifts may induce chloronema-like traits from caulonema precursors, while auxin or low-nutrient stress reverts chloronema toward caulonema.^[23]

Functions and Physiology

Photosynthesis and Nutrient Uptake

Photosynthesis in protonema occurs primarily within the chloronema cells, which are densely packed with numerous chloroplasts that serve as the sites for photosynthetic activity.^[24] These chloroplasts house thylakoid membranes where light-dependent reactions take place, capturing photons to split water molecules and generate ATP and NADPH through electron transport chains.^[24] In the chloroplast stroma, the Calvin cycle utilizes these energy carriers to fix atmospheric CO₂ into organic sugars, such as glucose, supporting the energy demands of filament growth.^[25] Unlike vascular plants, protonema gametophytes lack stomata, relying instead on passive diffusion of CO₂ across the moist cell surfaces, which enhances efficiency under elevated CO₂ levels but limits rates in dry conditions.^[25] The filamentous structure of protonema, particularly the branched chloronema, provides a high surface area-to-volume ratio that optimizes light capture in shaded or understory habitats typical of moss colonization.^[26] This morphology favors diffuse light for maximal photosynthetic output, as direct sunlight induces photoinhibition by overwhelming the photosynthetic apparatus with excess energy, leading to damage in photosystem II.^[26] Chloroplasts in chloronema cells respond dynamically to light quality, migrating to optimize positioning under low-intensity red or blue light while avoiding high blue light to prevent oxidative stress.^[25] Nutrient uptake in protonema occurs directly through the permeable cell walls, as the absence of vascular tissue necessitates reliance on surface absorption and diffusion from the surrounding medium.^[27] Essential ions such as K⁺ and NO₃⁻ are transported across the plasma membrane via secondary active mechanisms powered by H⁺-ATPases, which establish an electrochemical gradient by pumping protons out of the cell. Specific channels and transporters, including potassium channels and nitrate-permeable channels in the tonoplast, facilitate the influx of these ions once the proton motive force is generated.^[28] This process depends heavily on moist environments, where water films enable ion diffusion to the filaments without specialized roots or xylem.^[29] Hormonal regulation enhances nutrient acquisition by influencing morphology; cytokinins promote side-branch formation in protonema, increasing the overall surface area available for absorption and thereby improving resource uptake efficiency.^[30] Reduced cytokinin levels lead to fewer branches, underscoring their role in optimizing filament networks for nutrient foraging in nutrient-poor substrates.^[30]

Bud Formation and Gametophore Initiation

Bud formation in protonema primarily occurs on caulonema filaments, where side branch initials develop into multicellular outgrowths at subapical regions or branch points.^[31] These sites are preferred due to the elongated, oblique cell divisions characteristic of caulonema, which facilitate the transition from filamentous growth to three-dimensional structures.^[31] Exogenous application of auxins, such as indole-3-acetic acid (IAA), promotes bud initiation specifically in caulonema cells by inducing localized cell division and differentiation, often at low concentrations that establish auxin gradients to regulate polarity. Cytokinins, acting synergistically, further enhance this process by stimulating nuclear migration and asymmetric cell division in responsive subapical cells, typically the third to sixth from the tip.^[32] The initial bud forms as a small multicellular outgrowth originating from the asymmetric division of a target caulonema cell under hormonal influence.^[33] Auxin gradients, mediated by polar transport proteins like PIN orthologs, accumulate in these initials to maintain stem cell identity and drive initial elongation, resembling protonemal filaments before differentiation. Cytokinin signaling, particularly through extracellular forms like N6-(Δ2-isopentenyl)adenine, triggers calcium influx and gene expression changes that commit the outgrowth to gametophore development, preventing reversion to filamentous growth. Strigolactones also coordinate with auxin and cytokinin to regulate branching and bud formation.^[32]^[30] This hormonal interplay ensures precise timing, with buds emerging 7-14 days post-germination under optimal conditions. As the bud elongates, its apical cell undergoes spiral divisions to establish the gametophore's shoot apical meristem, while basal cells differentiate into rhizoids for anchorage and nutrient absorption.^[31] Subsequent development involves the outgrowth of protonemata-like structures that transition into stems and leaves, with auxin promoting vascular-like tissue patterning and cytokinin supporting meristem proliferation. A single protonema can produce dozens of such buds, enabling extensive clonal propagation and rapid colony expansion in favorable environments.^[32] This multiplicity is regulated by hormonal thresholds, where balanced auxin-cytokinin ratios prevent over-branching while maximizing reproductive potential.^[30]

Ecological and Evolutionary Aspects

Establishment in Natural Habitats

Moss spores, the primary dispersal units for protonema establishment, are primarily transported by wind over long distances, with additional short-range dispersal facilitated by water or animals such as mammals and birds.^[34] Upon settlement, the sticky exine layer of spores, often mucilaginous, aids initial attachment to substrates like soil, decaying wood, or rocks, while developing protonemata produce rhizoid-like extensions that further anchor the structure through thigmotropic growth and a protective mucilage sheath.^[2] These mechanisms enable protonema to secure positions in unstable microhabitats, preventing dislodgement during environmental disturbances. Protonema preferentially establishes in moist, shaded environments such as forest floors, coniferous or mixed woodlands, and rocky outcrops, where high humidity and organic litter support germination and growth.^[34]^[35] These habitats provide decaying wood or peat substrates that retain moisture, with protonema exhibiting resilience to periodic desiccation through cellular adaptations that allow survival in fluctuating water availability, as seen in species thriving on less decomposed wood or xeric soils.^[34]^[36] The rapid vegetative growth of protonema confers competitive advantages by forming dense filamentous mats that colonize substrates quickly, often outpacing algae through priority effects and resource monopolization in humid niches.^[36] Additionally, symbiotic associations with fungi, such as occasional arbuscular mycorrhizal interactions, enhance nutrient acquisition like phosphorus in nutrient-poor natural settings, bolstering establishment success.^[37] Optimal environmental conditions include a pH range of 4-7, with many species favoring acidic substrates around 5-6 for protonemal elongation, and temperatures between 15-25°C that promote filament extension without stress.^[38]^[35] However, protonema shows high sensitivity to pollutants like sulfur dioxide and heavy metals, which inhibit growth and limit urban or industrially impacted habitats.^[39]

Evolutionary Origins and Comparisons

The protonema stage in mosses represents a key innovation in the early evolution of land plants, with mosses (Bryophyta) diverging from other land plants around 450–470 million years ago during the Ordovician-Silurian periods.^[40] Microfossils suggest bryophyte-like affinities by the mid-Ordovician (~470 mya). Takakia, an early-diverging moss lineage that separated from other mosses approximately 390 million years ago, exemplifies primitive features but has adapted to extreme environments; recent studies (as of 2023) indicate it faces heightened extinction risks from climate change in its high-altitude habitats.^[41] This filamentous structure is homologous to the branched filaments of charophycean green algae, the closest aquatic relatives of land plants, but underwent significant adaptations for terrestrial environments, including enhanced desiccation tolerance and the development of septate rhizoids for anchorage. These modifications allowed spores to germinate and establish on land surfaces, marking a critical transition from aquatic algal ancestors to embryophytes.^[40] Within bryophytes, the protonema is universal and often persistent in mosses, where it can produce multiple gametophores from a single spore, facilitating clonal propagation and extensive colonization. In contrast, it is reduced or transient in liverworts, which develop thalloid prothalli directly from spores, and in hornworts, where the protonema quickly gives rise to a single thallus without significant branching or persistence. This variation highlights the moss-specific elaboration of the protonema as an evolutionary specialization within the bryophyte clade.^[9]^[40] Comparatively, the protonema differs markedly from the embryonic development in vascular plants, which occurs protected within seeds or fruits and leads directly to a diploid sporophyte-dominant life cycle without a free-living filamentous gametophyte phase. It shares functional similarities with the prothalli of ferns, both serving as independent haploid gametophytes that produce gametes, but the protonema is characteristically thread-like and branched, whereas fern prothalli are typically heart-shaped and more compact. These distinctions underscore the primitive gametophyte dominance in bryophytes versus the advanced sporophyte dominance in vascular plants.^[42]^[40] The evolutionary significance of the protonema lies in its role as a dispersal and establishment mechanism, enabling early land plants to colonize barren substrates without vascular tissues or true roots, thereby pioneering terrestrial ecosystems during the Paleozoic era. As a precursor to more elaborate gametophyte architectures, it facilitated the diversification of bryophytes and laid the groundwork for subsequent land plant radiations.^[43]

Research Applications

Use as a Model System

Physcomitrium patens, formerly known as Physcomitrella patens, serves as a premier model organism for studying protonema due to its facile culture and genetic tractability, particularly in the filamentous protonemal stage that dominates early development.^[44] The protonema's tip-growing filaments, including chloronema and caulonema, facilitate investigations into polarized cell growth and differentiation, making it an accessible system for experimental manipulation.^[45] Cultivation of P. patens protonema occurs aseptically on simple mineral media, such as Knop's solution supplemented with agar, allowing rapid growth under controlled conditions like 25°C and a 16-hour light/8-hour dark cycle.^[46] Protoplasts isolated from protonemal tissue via enzymatic digestion regenerate efficiently into whole plants, often using PEG-mediated transformation for genetic introductions, with regeneration rates exceeding 50% within weeks.^[47] This ease of propagation supports high-throughput studies. A key advantage is the haploid-dominant gametophyte phase, which simplifies genetic analysis by avoiding diploid masking of recessive mutations and enabling direct phenotypic observation of knockouts.^[44] P. patens exhibits exceptionally high rates of homologous recombination—up to 100% efficiency for targeted integrations—facilitating precise gene knockouts and insertions without random events common in seed plants.^[48] Protonema in P. patens is widely used to elucidate tip growth mechanisms, including actin cytoskeleton dynamics and microtubule organization that drive polarized expansion.^[49] It also models hormone signaling pathways, such as auxin-mediated branching and abscisic acid responses conserved across land plants.^[50] Additionally, the system provides insights into non-seed plant development, bridging algal ancestors and vascular plants through evolutionary developmental studies of filament-to-organ transitions.^[44]

Key Studies and Discoveries

The term "protonema" was coined by Wilhelm Hofmeister in his seminal 1851 work on the germination and development of cryptogams, where he described the initial filamentous stage emerging from moss spores as a thread-like structure analogous to algal filaments.^[51] Hofmeister's observations laid the foundation for understanding moss life cycles, highlighting the protonema's role in spore germination and early growth.^[52] In the late 19th century, further microscopic examinations by botanists distinguished the two primary filament types—chloronema (chlorophyll-rich, branched filaments) and caulonema (slender, less branched filaments)—based on their morphological and physiological differences during protonemal development.^[9] Hormonal regulation of protonema development emerged as a key research focus from the 1970s onward, with studies demonstrating that auxins promote the transition from chloronema to caulonema filaments in species like Funaria hygrometrica.^[53] For instance, experiments in the 1970s showed that low concentrations of indole-3-acetic acid (IAA) induced caulonema formation, addressing earlier gaps in understanding environmental cues for filament differentiation.^[53] Concurrently, research on cytokinins revealed their role in bud formation on caulonema cells, with applications of benzyladenine triggering multicellular bud initials in the 1970s and 1980s, as detailed in work by Martin Bopp and colleagues.^[54] Building on this, 2000s studies integrated auxin and cytokinin interactions, showing that auxin positively regulates transcription factors like PpRSL1 and PpRSL2 to drive the chloronema-to-caulonema switch in Physcomitrium patens, providing molecular insights into hormonal crosstalk.^[55] Genetic advances accelerated in the 1990s with the development of transformation techniques in P. patens, enabling targeted gene disruptions to probe protonema functions.^[56] A landmark discovery was the 2008 draft genome sequence of P. patens, which revealed over 32,000 protein-coding genes and facilitated comparative analyses with vascular plants, uncovering conserved pathways for land plant adaptation.^[57] This genomic resource enabled functional studies, such as the 2011 identification of strigolactones as regulators of protonema branching in P. patens, where exogenous application reduced side branch formation and acted as a quorum-sensing signal to optimize resource allocation in dense cultures.^[58] Post-2020 research has emphasized protonema's resilience to climate stressors, particularly drought, through signaling pathways involving abscisic acid (ABA). Studies on desiccation-tolerant mosses like Syntrichia caninervis demonstrated that protonema activates ABA-dependent gene expression for osmoprotectant synthesis and membrane stabilization, enhancing survival during prolonged dry periods.^[59] In P. patens, transcriptomic analyses post-2020 revealed calcium-mediated signaling cascades in protonema that reprogram development under drought, integrating stress responses with filament growth to maintain viability.^[60] These findings highlight protonema as a model for elucidating early land plant adaptations to arid environments amid climate change.^[25] More recent work from 2023 to 2025 has advanced understanding of protonemal stem cell regulation and development. A 2023 study showed that ABA signaling converts stem cell fate in apical protonemal cells of P. patens by balancing growth inhibition and survival tradeoffs.^[61] In 2024, single-nucleus RNA sequencing identified key regulatory mechanisms governing pluripotent stem cells and their origins in P. patens protonema.^[62] A 2025 investigation demonstrated that ROP GTPases are essential for critical developmental processes in protonemal growth and differentiation.^[63] Additionally, research in 2025 confirmed P. patens' heavy reliance on homologous recombination for efficient DNA double-strand break repair in protonemal cells, underscoring its genetic robustness.^[64]

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May 23, 2023 · Physcomitrium patens (Hedw.) Mitt. strain Gransden 2004 (Rensing et al., 2008) was generally cultivated on Knop's medium (Reski & Abel, 1985; ...
[46]
Isolation and Regeneration of Protoplasts of the Moss ...
This protocol describes how to isolate individual protoplasts from young gametophyte tissue of the moss Physcomitrella patens and how to regenerate them into ...
[47]
Targeted Gene Knockouts by Protoplast Transformation in the Moss ...
Dec 17, 2021 · The moss Physcomitrella patens is a known model organism for a high frequency of homologous recombination and thus harbors a remarkable rate of ...
[48]
Physcomitrella patens: a model for tip cell growth and differentiation
This transition is regulated by environmental factors such as light and nutrient availability as well as by the phytohormone auxin [3, 15, 16, 17].
[49]
Phytohormone biosynthesis and signaling pathways of mosses
Jul 10, 2021 · Biosynthesis and signaling pathways are best described in P. patens for the three classical hormones auxins, cytokinins and abscisic acid.Missing: studies | Show results with:studies
[50]
On the germination, development, and fructification of the higher ...
This is the English translation of Hofmeister's 1851 work on ferns, mosses, and algae (cryptogams) and revealed how the process of fertilization was due to the ...
[51]
Molecular Control of Sporophyte-Gametophyte Ontogeny and ...
The concept of alternation of generations was first proposed by the German botanist Hofmeister (1851). Hofmeister (1851) termed these fundamental phase ...<|separator|>
[52]
Auxin Regulation of Caulonema Formation in Moss Protonema
Auxin Regulation of Caulonema Formation in Moss Protonema. Nat New Biol. 1973 Oct 17;245(146):223-4. doi: 10.1038/newbio245223a0. Authors. M M Johri, S Desai.Missing: cytokinin transition studies 1970s- 2000s Frescas
[53]
Studies on Cytokinin-Controlled Bud Formation in Moss Protonemata
Abstract. Application of cytokinins to moss protonemata of the proper physiological age causes bud formation on specific cells (caulonema).Missing: auxin transition 1970s- 2000s Frescas
[54]
Auxin promotes the transition from chloronema to caulonema in ...
These data indicate that auxin positively regulates PpRSL1 and PpRSL2 whose expression is sufficient to promote caulonema differentiation in moss protonema.Missing: IAA | Show results with:IAA
[55]
Evolutionary crossroads in developmental biology: Physcomitrella ...
Nov 1, 2010 · The availability of a complete genome sequence, together with the ability to perform gene targeting efficiently in P. patens has spurred a ...Events In Land Plant... · Key Recent Findings And... · Evolution Of Gene Expression...Missing: advantages | Show results with:advantages
[56]
The Physcomitrella Genome Reveals Evolutionary Insights into the ...
We report the draft genome sequence of the model moss Physcomitrella patens and compare its features with those of flowering plants.
[57]
Strigolactones regulate protonema branching and act as a quorum ...
Apr 15, 2011 · Strigolactones are a novel class of plant hormones controlling shoot branching in seed plants. They also signal host root proximity during ...Missing: triggers | Show results with:triggers<|separator|>
[58]
Molecular and physiological responses to desiccation indicate the ...
Abscisic acid is a key conserved physiological component in the molecular mechanisms underlying desiccation responses in the moss Sphagnum.
[59]
Stress-associated developmental reprogramming in moss ...
Feb 18, 2022 · Parallel signaling pathways may be one mechanistic explanation; oscillatory calcium signals may be another mechanism for specificity and ...