Variegation
Variegation is a striking visual trait observed in many plants, characterized by the presence of distinct patches or zones of non-green coloration—typically white, yellow, or pale—interspersed with green areas on leaves, stems, or sometimes flowers and fruits.[1] This pattern arises primarily from irregularities in chloroplast development or pigment distribution, leading to sectors where chlorophyll is absent or reduced, while other pigments like carotenoids may persist.[1] In biological terms, variegated plants exhibit a mosaic-like appearance in their vegetative parts due to these localized differences in pigmentation.[1] The phenomenon can be classified into several types based on its origins. Genetic variegation, the most stable and heritable form, results from mutations in nuclear, chloroplast, or mitochondrial genes that disrupt chloroplast biogenesis or function. Mutations in nuclear genes follow Mendelian inheritance, whereas mutations in chloroplast or mitochondrial genes are transmitted cytoplasmically in a non-Mendelian fashion, often involving plastid sorting-out.[1] Examples include the Arabidopsis thaliana mutant immutans, caused by a mutation in a gene encoding plastid terminal oxidase involved in carotenoid biosynthesis leading to carotenoid imbalances and photooxidative damage in white sectors, and the maize iojap mutant, linked to ribosomal deficiencies in plastids.[1] Viral variegation, in contrast, stems from infections by plant viruses that induce chlorotic patches by interfering with chlorophyll synthesis, as seen in tulips during historical outbreaks or in ornamentals like abutilon.[2][3] This type is non-heritable but can persist through vegetative propagation if the virus is maintained.[2] Environmental or non-heritable variegation occurs due to external stresses such as low light, temperature extremes (e.g., cool weather below 10°C), or chemical exposures, temporarily altering pigment expression without genetic changes.[4] These forms highlight variegation's dual role as both a natural adaptation and a response to perturbation. In horticulture, variegation is highly prized for its aesthetic appeal, enhancing the ornamental value of species like caladiums, hostas, and certain conifers, where selective breeding stabilizes desirable patterns through chimeras or transposon activity.[3] However, it often imposes physiological costs, such as reduced photosynthetic efficiency in non-green tissues, leading to slower growth and greater susceptibility to environmental stress compared to uniform green counterparts.[2] Research into variegation mechanisms, including gene sorting and photo-protection—as advanced by studies on PTOX function as of 2023—continues to inform plant breeding and our understanding of plastid biology.[1][5]Overview and Characteristics
Definition
Variegation in botany is the appearance of differently colored zones, patches, spots, or streaks in the leaves, stems, flowers, or fruits of plants, typically involving contrasts between green and non-green areas such as white, yellow, or red.[6] This phenomenon manifests as stable patterns of pigmentation variation across plant tissues, often resulting from underlying biological processes that affect chlorophyll distribution or other pigments.[7] Unlike uniform coloration, which presents a consistent hue throughout a plant part, or simple spotting from temporary environmental damage, variegation emphasizes irregular or structured contrasts that are integral to the plant's morphology and may be heritable.[6] The scope of variegation is confined to plants, particularly vascular species, where it serves as a distinctive trait; analogous color variations in animals or fungi are not included under this botanical definition unless used for comparative clarity.[7] Historical records of variegation trace back to ancient observations, with Roman naturalist Pliny the Elder documenting it in the 1st century AD in his Natural History, where he described three kinds of ivy (Hedera helix), including one with a variegated leaf known as Thracian ivy.[8] Such early accounts highlight variegation's recognition as a natural curiosity long before modern scientific classification.[9]Patterns and Forms
Variegation exhibits a range of distinctive patterns that contribute to its visual diversity in plants. Striping consists of linear zones of contrasting colors, often appearing as elongated bands parallel to the veins or midrib, creating a zebra-like effect along leaves or stems. Marbling presents as intermingled patches of color that blend irregularly, producing a mottled or veined appearance reminiscent of marble stone. Marginal variegation is restricted to the edges of leaves, forming borders of lighter or differently colored tissue around a central green area. Sectoral variegation manifests as pie-shaped sectors, where large, wedge-like portions of a leaf or other organ display uniform coloration distinct from the surrounding tissue.[10][7][11] These patterns can occur across various plant parts, with foliar variegation being the most prevalent, affecting leaf blades in numerous species. Stem variegation appears as longitudinal stripes or sectors on branches and trunks, while floral variegation introduces multicolored zones on petals or sepals. Fruit variegation, though less common, results in patchy or striped coloration on the pericarp or skin.[12][3] Variegation patterns vary in stability, with stable forms maintaining consistent coloration throughout the plant's life, whereas transient patterns may shift or diminish over time. A notable tendency in unstable variegation is reversion, where sectors of variegated tissue revert to uniform green, often due to selective growth advantages of chlorophyll-rich cells that outcompete non-green areas.[13][14] The expression of variegation can also be influenced by growth stages, as juvenile leaves frequently display subtler or less defined patterns compared to mature leaves, where colors may intensify or sectors become more pronounced during development.[15]Biological Causes
Chimerism
Chimerism in plants manifests as the coexistence of two or more genetically distinct cell populations within a single organism, typically arising from the shoot apical meristem and resulting in variegated tissues through differential pigmentation across cell lineages.[16] This genetic mosaicism contrasts with uniform genotypes and often produces stable patterns of color variation, such as green and white sectors on leaves, due to mutations affecting chloroplast development or pigment synthesis in specific tissues.[16] Plant chimeras are classified into three main types based on the spatial arrangement of the distinct genotypes: periclinal, sectorial, and mericlinal. Periclinal chimeras feature genetically distinct layers running parallel to the surface, offering the most stable variegation; these align with the tunica-corpus model of the shoot apical meristem, where the outer L1 layer forms the epidermis, the subsurface L2 layer contributes to palisade mesophyll, and the inner L3 layer supports vascular and pith tissues.[17] Sectorial chimeras display radial sectors of differing genotypes extending from the meristem center, creating stripe-like patterns that may vary in stability.[16] Mericlinal chimeras involve partial layering, where only portions of a layer differ genetically, leading to unstable variegation prone to reversion as the chimera propagates.[16] These chimeras form primarily through somatic mutations in meristematic cells, which disrupt pigment pathways and cause unequal distribution of functional chloroplasts or pigments across the L1, L2, and L3 layers—for instance, a mutation in L2 may yield white mesophyll while L1 and L3 remain green.[16] Grafting between genetically distinct plants can also generate chimeras at the union site, perpetuating mixed tissues if meristem initials incorporate both genotypes.[16] In the tunica-corpus structure, L1 divisions are mostly anticlinal to maintain layering, but occasional periclinal divisions can introduce variability, as seen in tissue cross-sections where variegated leaves show distinct green (chlorophyllous) and white (achlorophyllous) zones corresponding to layer contributions. A representative example is Ficus rubiginosa 'Variegata', a periclinal chimera denoted as GWG (green L1, white L2, green L3), where the chlorophyll-deficient L2 layer produces achlorophyllous mesophyll, resulting in gold-edged leaves with mosaic green patches from L1-derived vascular sheaths.[18] Similarly, variegated Pelargonium (geranium) cultivars arise from periclinal chimeras with pigmentation mutations in meristem layers, yielding stable white-margined leaves.[16] Tissue layering in such chimeras can be illustrated as follows:| Layer | Function | Genotype Example (GWG Chimera) | Pigment Outcome |
|---|---|---|---|
| L1 (Outer) | Epidermis, some vascular sheaths | Green | Chlorophyllous (green edges) |
| L2 (Subsurface) | Palisade and spongy mesophyll | White | Achlorophyllous (white areas) |
| L3 (Inner) | Vascular core, pith | Green | Chlorophyllous (green core) |