The study began with a question that has long occupied plant biochemists: which enzymes and which genetic controllers determine the flux of carbon into the flavonol branch of the vast flavonoid pathway? Tea plants are famously rich in flavonoids, but the family is crowded. Catechins, anthocyanins, flavones and flavonols all branch off shared precursors, and the balance among them defines not only taste but also nutritional value. To find the enzyme most responsible for quercetin production in tea leaves, the researchers turned to bioinformatic mining of the tea plant transcriptome, scanning gene expression data for candidates that looked like flavonol synthases, the enzymes that convert dihydroflavonol intermediates into flavonols such as quercetin and kaempferol.

That search surfaced a candidate the team designated CsFLSd. The name reflects its membership of the flavonol synthase family, a group of dioxygenases first characterized decades ago in parsley and petunia and since identified across the plant kingdom. What distinguishes a good candidate from a confirmed player, however, is functional evidence, and the researchers gathered it from two directions. First, they transiently suppressed CsFLSd expression in tea plant leaves using an antisense oligonucleotide approach, a technique that temporarily blocks a specific messenger RNA without permanently altering the plant’s genome. When CsFLSd activity was dampened, quercetin content in the treated leaves fell significantly, a result that directly ties the gene’s expression to the metabolite’s accumulation.

The second line of evidence came from heterologous expression. The team transferred the CsFLSd gene into Arabidopsis thaliana, the thale cress that serves as the workhorse of plant molecular genetics, and generated stable overexpression lines. In these engineered plants, quercetin levels rose, confirming that the tea enzyme is not merely correlated with quercetin production but is sufficient to promote it when introduced into another species. This combination of loss-of-function evidence in the native plant and gain-of-function evidence in a heterologous system is the standard gold standard for assigning gene function, and CsFLSd passed both tests cleanly.

With the enzyme identified, the researchers moved upstream to ask what controls it. Transcription factors, the proteins that bind to specific DNA sequences in gene promoters and either encourage or block the recruitment of the transcriptional machinery, are the master regulators of metabolic pathways. To find the one acting on CsFLSd, the team performed a yeast one-hybrid screen, a technique in which the promoter sequence of the target gene is baited in yeast cells and a cDNA library from tea plants is introduced as prey. Any transcription factor capable of binding the promoter activates a reporter gene, allowing the interacting protein to be fished out of a library containing thousands of candidates. The screen flagged CsWRKY22, a member of the WRKY family, one of the largest families of transcription factors in plants and one with a well-documented history of involvement in secondary metabolism.

WRKY proteins are named for a conserved amino acid motif, WRKYGQK, that recognizes W-box DNA elements in target promoters. Family members across many species have been implicated in controlling flavonoid biosynthesis: in Arabidopsis, WRKY23 regulates flavonol production during root development; in grape, VvWRKY70 inhibits flavonol biosynthesis; and in tea itself, several WRKY factors have been shown to govern the accumulation of catechins and other signature metabolites. The identification of CsWRKY22 as a CsFLSd promoter partner therefore fits a growing pattern in which WRKY factors serve as tunable valves on the flavonoid pipeline, with each species deploying its own cast of characters.

Crucially, the team did not stop at the yeast screen. Follow-up validation experiments confirmed that CsWRKY22 binds specifically to the CsFLSd promoter, and a dual-luciferase reporter assay, in which the promoter drives luciferase expression and the transcription factor is supplied in trans, demonstrated that the binding is activating rather than repressive. The researchers then manipulated CsWRKY22 expression directly in tea leaves. Transient overexpression of the transcription factor elevated quercetin content, while transient suppression reduced it, mirroring the effects seen when CsFLSd itself was targeted. This symmetry establishes CsWRKY22 as a positive regulator positioned upstream of CsFLSd, completing what the authors describe as the CsWRKY22-proCsFLSd module.

The significance of the finding extends beyond basic plant biology. Quercetin is one of the most abundant and biologically valuable flavonoids in the human diet, with a substantial literature linking its consumption to chemoprotective and antioxidant effects. In tea, flavonol derivatives contribute to the beverage’s nutritional profile, but they also shape its sensory character. Astringency, the drying, puckering sensation prized in some teas and avoided in others, arises largely from polyphenolic compounds, and understanding the genetic dials that set flavonol levels opens the door to breeding or engineering tea cultivars with tailored flavor and health profiles. A regulatory module like CsWRKY22-CsFLSd is precisely the kind of target that breeders and metabolic engineers look for, because manipulating a single upstream switch can shift an entire branch of metabolism without disrupting the rest of the plant’s chemistry.

The work also adds a piece to the broader puzzle of how plants allocate shared metabolic precursors among competing branches. Dihydroflavonols sit at a metabolic crossroads: they can be converted to flavonols by flavonol synthase, or channeled toward anthocyanins and proanthocyanidins by other enzymes such as dihydroflavonol-4-reductase. The relative expression of these competing enzymes, and of the transcription factors that control them, determines the pigment and flavor chemistry of leaves, flowers and fruits. Previous studies in tea have identified other regulators, including NAC and bZIP and MYB family factors, that influence flavonol and catechin production, and the new study enriches this regulatory map by adding a WRKY component that acts specifically on the flavonol synthase step. Such layered control, with multiple transcription factor families converging on overlapping targets, gives plants the flexibility to adjust their chemistry in response to developmental cues and environmental stresses.

For the researchers, the immediate achievement is the characterization of a defined genetic module in a crop whose genome and metabolome have only recently become tractable to this kind of analysis. The transient expression systems used in the study, which allow genes to be overexpressed or silenced in tea leaves within days, have made functional validation in this woody perennial far more practical than it once was. Combined with yeast one-hybrid screening and dual-luciferase assays, these tools enabled the team to move from candidate identification to regulatory confirmation in a single study. The CsWRKY22-proCsFLSd module now stands as a verified node in tea’s flavonol network, a finding that deepens fundamental understanding of flavonoid metabolic regulation while offering a concrete molecular handle for improving the nutritional quality and taste of one of humanity’s oldest and most beloved drinks. As tea science continues to mature into a genomics-driven discipline, modules like this one are likely to multiply, bringing the ancient art of tea cultivation ever closer to the precision of modern molecular design.

Subject of Research: Transcriptional regulation of quercetin biosynthesis by the CsWRKY22-CsFLSd module in tea plants

Article Title: CsWRKY22 positively regulates quercetin biosynthesis by activating the CsFLSd promoter in tea plants (Camellia sinensis)

Article References: Yan, X., Rao, M., Tao, Y., Ran, W., Wang, Y., Lv, W., Ren, H., Chen, Y., Lu, M., Jing, T., & Li, C. (2026). CsWRKY22 positively regulates quercetin biosynthesis by activating the CsFLSd promoter in tea plants (Camellia sinensis). Plant Cell Reports, 45(10), Article 321. https://doi.org/10.1007/s00299-026-04011-4

Image Credits: AI Generated

DOI: 10.1007/s00299-026-04011-4

Keywords: Camellia sinensis, quercetin, flavonols, CsWRKY22, CsFLSd, flavonol synthase, transcription factor, yeast one-hybrid, tea plant, plant molecular biology, biosynthesis, astringency