The variegated pattern in P0429 is unlike the familiar mottling seen in ornamental plants, where chlorophyll defects create white or yellow sectors. Instead, the red zones owe their color to betacyanins, a class of nitrogen-containing pigments characteristic of the plant order Caryophyllales, to which quinoa belongs. Betalains, the broader pigment family that includes red-violet betacyanins and yellow betaxanthins, functionally replace anthocyanins in most members of this lineage and are synthesized from the amino acid L-tyrosine. Beyond their role as natural colorants, these pigments are potent antioxidants that help plants cope with salinity, drought, and intense light by scavenging reactive oxygen species and providing photoprotection, making them doubly interesting for a crop prized for its stress tolerance.

When the researchers examined cross-sections of variegated leaves, they found something unexpected: the pigmentation was confined almost entirely to the epidermal bladder cells, or EBCs, that blanket most of the plant’s aerial surfaces. These large, two-celled structures, consisting of a stalk and a swollen bladder, are often considered the simplest form of salt glands and are known to help quinoa manage salinity, deter herbivores, and regulate osmotic pressure. When the team mechanically brushed the EBC layer off the leaf surface, the red sectors became visually indistinguishable from the green ones, proving that the entire mosaic resides in this specialized epidermal layer. The boundary between red and green EBC territories was sharp, developmentally stable, and occasionally aligned with the leaf’s vascular tissue.

Quantitative measurements drove the point home. Spectrophotometric analysis at 538 nanometers, the absorption maximum of betacyanins, revealed that red EBCs contained 6.23 micrograms of betacyanins per gram of tissue, roughly fifty times the 0.12 micrograms per gram found in colorless EBCs from green sectors. Extracts from red EBCs displayed a deep purple-red hue that rapidly turned yellow upon the addition of sodium hydroxide, a classic chemical signature of betacyanins. By contrast, the underlying leaf lamina retained only low pigment levels, and the difference between red and green EBCs was an order of magnitude larger than any difference measured in the lamina beneath. No significant variation in yellow betaxanthins was detected between the two cell types, confirming that betacyanin accumulation in the bladder cells is the dominant driver of the variegation.

To find the molecular switch behind this dramatic difference, the team performed RNA sequencing on red and colorless EBCs isolated from the very same variegated leaves. Because the cells were harvested from adjacent sectors of a single organ, the genetic background was held constant, and the differential expression analysis identified only seven genes, all upregulated in the red EBCs. Quantitative PCR validation narrowed the field further. Among the candidates, one gene stood out: Cqu0091301, a member of the CYP76ADα lineage of cytochrome P450 enzymes, showed both the highest absolute mRNA level and the largest fold change between red and colorless EBCs. The CYP76ADα family catalyzes the first committed step of betalain biosynthesis, the hydroxylation of tyrosine to L-DOPA, and is widely regarded as a rate-limiting component of the pathway.

But the story took a twist when the researchers inspected the gene model of Cqu0091301 in the available quinoa reference genomes. In those assemblies, the gene’s second exon appeared truncated, deleting a conserved P450 domain and rendering the predicted enzyme non-functional. Clues from the sequencing data suggested otherwise: no reads aligned to the short second exon, while a homologous gene, Cqu0091280, showed a peculiar enrichment of mismatched reads over part of its second exon. De novo transcriptome assembly recovered a transcript matching Cqu0091301 but with an extended three-prime region, hinting that the reference genomes were missing something. Whole-genome resequencing of P0429, followed by targeted PCR, revealed the answer: a 3,957-base-pair genomic insertion, absent from the reference assemblies, that restores the full-length second exon and the complete, conserved P450 domain of Cqu0091301.

This structural variation proved to be the linchpin of the entire phenotype. When the team re-analyzed their RNA sequencing data using the corrected gene model, the expression difference between red and colorless EBCs became even more pronounced, while the apparent differential expression of the homolog Cqu0091280 diminished, indicating that reads previously misassigned to the homolog actually belonged to the restored gene. Other betalain pathway genes, including tyrosine decarboxylase, DODA, and several glucosyltransferases, showed extremely low expression in EBCs and no differential upregulation in red sectors. The only genes consistently and markedly elevated in red EBCs were the three B-subgenome CYP76ADα copies, Cqu0091280, Cqu0091301, and Cqu0091310, cementing the conclusion that upregulation of this P450 lineage, and Cqu0091301 in particular, is the key regulatory step driving betacyanin synthesis in the pigmented bladder cells.

Genomic context added yet another layer of intrigue. The quinoa genome harbors seventeen CYP76AD family members distributed across three subfamilies, and all five CYP76ADα genes are physically linked to DODA genes, forming adjacent CYP76ADα–DODA pairs on two homoeologous chromosomes. Such metabolic gene clusters are a recurring theme in plant secondary metabolism and are thought to facilitate coordinated regulation of enzyme production, conferring evolutionary advantages and enhanced environmental adaptability. Expression profiling across tissues showed that CYP76ADα genes and their linked DODA partners share highly similar relative expression patterns, consistent with transcriptional co-regulation, even though the P450 genes are generally expressed at an order of magnitude higher basal levels than their DODA neighbors.

Perhaps the most striking finding concerns the division of labor between quinoa’s two subgenomes. As an allotetraploid, quinoa carries A and B subgenomes derived from distinct ancestral species, and the study revealed that their CYP76ADα copies have evolved non-overlapping tissue preferences. The three B-subgenome genes dominated expression in EBCs, while the A-subgenome copies Cqu0282960 and Cqu0282940 were undetectable in bladder cells; instead, Cqu0282960 was preferentially expressed in seedlings and Cqu0282940 in stems and flowers, both upregulated in red tissues of those organs. Across most pigmented organs, the B-subgenome pairs made the greater overall contribution to betalain biosynthesis, a clear demonstration of subgenome functional divergence. Because the reference variety Real carries a structurally incomplete Cqu0091301, the functional burden in its tissues likely falls on the second-highest expressed B-subgenome pair, underscoring how a single insertion can reshape the architecture of pigment regulation in a polyploid crop.

The implications extend well beyond explaining a curious color pattern. The study provides the first report of genomic structural variation driving betalain biosynthesis and links it directly to cell-type-specific pigmentation in a crop of growing global importance. It also highlights a practical lesson for the genomics era: reference genomes, however high quality, can silently truncate genes and mislead expression analyses, and resequencing diverse accessions can recover functional alleles hidden in the pan-genome. For breeders, the restored Cqu0091301 allele offers a potential target for tuning betalain accumulation, with consequences for stress tolerance and natural colorant production. For evolutionary biologists, the work illuminates how duplicated genes in polyploid genomes partition their duties across tissues, and how a single insertion of roughly four kilobases can tip the balance between green and red, one bladder cell at a time.

Subject of Research: Genomic structural variation controlling cell-type-specific betalain pigment biosynthesis in allotetraploid quinoa

Article Title: Genomic structural variation underlies cell type-specific betacyanin variegation in Chenopodium quinoa

Article References: Zhang, Z., Wang, Y., Hu, X., Yu, T., Feng, Y., Zhang, J., Zhang, T., Feng, G., & Zhang, H. (2026). Genomic structural variation underlies cell type-specific betacyanin variegation in Chenopodium quinoa. Stress Biology, 6(1), Article 15. https://doi.org/10.1007/s44154-025-00284-z

Image Credits: AI Generated

DOI: 10.1007/s44154-025-00284-z

Keywords: quinoa, betacyanin, betalain, epidermal bladder cells, CYP76ADα, structural variation, variegation, subgenome, gene cluster, polyploidy, plant pigments, Stress Biology

The research team, led by Ying Xu, Yan Dong, Dandan Chai, Xiaozhi Huang, Longxiao Gao, Jun Zhang and Minsheng Yang of Hebei Agricultural University in Baoding, China, together with a colleague from Tangshan Hongsen Flowers & Trees Planting Co., subjected ‘Xuanyang 2′ saplings to four light regimes: full sunlight as a control, and shade treatments that reduced available light by 30, 50 and 80 percent. Because the leaves of this cultivar carry distinct yellow and green sectors on the same plant, the scientists could compare the two tissue types side by side under identical conditions, an arrangement that turns the chimeric leaf into a natural laboratory for studying how pigment systems respond to light stress.

The physiological measurements told a clear story. Chlorophyll content was significantly higher in the green regions than in the yellow regions under every treatment, and in the green tissue it climbed steadily as shade deepened. The yellow sectors behaved differently: their chlorophyll levels rose at first, peaked under the moderate 50 percent shading treatment, and then declined under the most severe 80 percent shade. In other words, a moderate dimming coaxed extra pigment out of the yellow tissue, while extreme shade pushed it past a tipping point. Net photosynthetic rate, stomatal conductance, initial fluorescence and maximum fluorescence were all significantly lower in the yellow regions than in the green ones, confirming that the yellow sectors are fundamentally weaker photosynthetic machines even before any stress is applied.

Shading also reshuffled the plant’s defensive chemistry, and the two sectors responded in opposite ways. Antioxidant enzymes such as superoxide dismutase, peroxidase and catalase, along with osmolytes like soluble sugars and soluble proteins, were differentially affected in green and yellow tissue depending on the shade intensity. The most telling marker was malondialdehyde, a breakdown product that signals damage to cell membranes from reactive oxygen species. Under the harshest 80 percent shade, malondialdehyde rose sharply in the yellow sectors compared with full sunlight, indicating that the yellow tissue was suffering genuine oxidative stress precisely at the light level where it also began to re-green. The plant, it seems, was rebuilding its photosynthetic apparatus under duress rather than thriving.

Transmission electron microscopy added a physical dimension to the biochemical data. In full sunlight, the chloroplasts inside the yellow sectors displayed conspicuously irregular shapes and blurred membrane boundaries, evidence that these organelles are particularly sensitive to high light intensity and struggle to maintain their architecture when illumination is strong. Under severe shading, the picture changed again: thylakoid membranes, the stacked internal sheets where the light reactions of photosynthesis take place, showed increased local stacking, while dark osmiophilic granules accumulated within the plastids. These ultrastructural shifts mirror the chlorophyll measurements, suggesting that the yellow sectors actively remodel their chloroplasts in an attempt to capture more of the scarce light, with mixed success.

To connect physiology with genetics, the team turned to transcriptomics and network analysis. A graphical Gaussian model, which infers regulatory relationships from correlations among gene expression patterns, identified four top-tier transcription factor families responding to low-light stress: C2H2, MBF1, C2C2-CO-like and HMG. Complementary weighted gene co-expression network analysis, integrated with the physiological measurements, revealed four gene modules significantly correlated with net photosynthetic rate, soluble sugar content, leaf lightness and the yellow-blue color parameter. Within those modules, the transcription factors bZIP, C2H2 and MYB emerged as hub regulators, sitting at the intersection of chloroplast development and pigment synthesis pathways. The implication is that shade does not merely starve the leaf of energy; it rewires a coordinated genetic program that governs how chloroplasts are built and how pigments are made and broken down.

According to the authors’ conclusions, shading promoted chlorophyll synthesis while simultaneously inhibiting chlorophyll degradation, modulated pigment metabolism and altered the expression of light-harvesting protein genes, all under the differential control of MYB, C2C2 and bZIP transcription factors. This dual action, building pigment up and slowing its teardown, explains why the yellow sectors re-greened under shade and why the green sectors deepened their color as light diminished. For nursery growers and landscape designers, the practical message is that the ornamental value of ‘Xuanyang 2’ is highly light-dependent: plant it in deep shade and the golden patches that justify its place in the design may gradually disappear into uniform green.

The most provocative result, however, came from the tissue culture experiments. When the researchers attempted to purify the cultivar through in vitro propagation, the chimeric tissue senesced and died in culture, while stable green plantlets regenerated from the explants. Those regenerated plantlets remained green even when returned to full sunlight, the very condition under which the parent plant displays its yellow variegation. This outcome suggests that the chimeric trait may not be genetically stable, and that the yellow sectors could arise from a non-genetic mechanism rather than a fixed mutation that tissue culture could simply lock in.

The authors point to generational segregation of the variegated traits as further support for a possible non-genetic or epigenetic basis. In classic chimeras, the yellow and green layers carry distinct genotypes, and tissue culture typically allows one genotype to overrun the other, producing either all-yellow or all-green offspring depending on which layer dominates. Here, the fact that only green plantlets survived and stayed green under full light hints that the yellow phenotype depends on an unstable balance between tissue layers or on regulatory states that collapse when the plant is isolated in culture. If confirmed, this would reframe ‘Xuanyang 2’ not as a stable ornamental cultivar but as a living equilibrium whose signature colors exist only as long as the underlying tissues remain in tension.

For the broader plant science community, the study offers a technically rich template for dissecting variegation: paired sector sampling, gradient shading, chlorophyll fluorescence, antioxidant profiling, electron microscopy and co-expression network modeling, all integrated around a single cultivar. It also raises questions that extend beyond ornamental horticulture. Understanding how bZIP, C2H2 and MYB factors coordinate chloroplast biogenesis and pigment turnover under fluctuating light could inform breeding of crops and forestry species that maintain photosynthetic efficiency in shaded environments, from understory plantings to agroforestry systems. Meanwhile, for anyone tempted to plant a golden-leaved poplar along a shaded walkway, the science delivers a caution: the color you buy is a negotiation between the plant’s genome and the light you give it, and in ‘Xuanyang 2’, shade wins the argument.

Subject of Research: Shade-induced leaf color change and tissue culture purification in the variegated poplar cultivar 'Xuanyang 2'

Article Title: Response of colored poplar ‘Xuanyang 2’ to shading and purification by tissue culture

Article References: Xu, Y., Dong, Y., Chai, D., Huang, X., Gao, L., Zhang, J., & Yang, M. (2026). Response of colored poplar ‘Xuanyang 2’ to shading and purification by tissue culture. BMC Plant Biology. https://doi.org/10.1186/s12870-026-10014-w

Image Credits: AI Generated

DOI: 10.1186/s12870-026-10014-w

Keywords: colored poplar, Xuanyang 2, variegation, shading, chlorophyll, chloroplast ultrastructure, photosynthesis, transcription factors, WGCNA, oxidative stress, tissue culture, chimera