Ultraviolet-B (UV-B) radiation accelerates ripening and coloration of highbush blueberry (Vaccinium corymbosum) fruit through a stage-dependent transcriptional network: in green fruit the bZIP factor VcHY5 activates anthocyanin-promoting MYB genes and represses the repressor VcMYBC2, whereas in mature (ripening) fruit VcHY5 is not induced, yet UV-B still up-regulates both the activator VcMYBPA1 and the repressor VcMYBC2; the source study explicitly left the pathway up-regulating VcMYBC2 unidentified and offered only an unevidenced speculation (a COP1-mediated abscisic-acid response) for VcMYBPA1. The open problem is to identify the HY5-independent regulatory pathway(s) — the upstream signals and transcription factors — through which UV-B controls VcMYBC2 expression in mature blueberry fruit, thereby supplying the still-unidentified mature-fruit link of the proposed stage-dependent model of UV-B-regulated anthocyanin accumulation. No study has functionally addressed VcMYBC2 regulation since 2021; reviews as recent as 2026 still describe the stage-specific mechanism of blueberry UV-B response as an open distinction.

Background

Highbush blueberry (Vaccinium corymbosum) accumulates large amounts of anthocyanins — red-to-blue flavonoid pigments synthesized by a conserved phenylpropanoid pathway — during fruit ripening, and these pigments are major determinants of the fruit's visual and nutritional quality. The expression of the pathway's structural genes (for example VcUFGT, which encodes UDP-glucose:flavonoid 3-O-glucosyltransferase, the last step fixing anthocyanin structure) is controlled by R2R3-MYB transcription factors, DNA-binding proteins named for their two repeats of the MYB domain. In the blueberry system the key players are the activators VcMYBPA1 and VcMYBA1, which promote anthocyanin gene expression, and VcMYBC2, a repressor-type R2R3-MYB carrying the conserved C2 repressor motif characteristic of anthocyanin-inhibiting MYBs (related to strawberry FaMYB1 and grape VvMYBC2-L1). Light is the dominant environmental cue for this network. Ultraviolet-B radiation (UV-B, 280–315 nm) is perceived by the photoreceptor UVR8; photoactivated UVR8 binds the E3 ubiquitin ligase COP1 (CONSTITUTIVELY PHOTOMORPHOGENIC 1), which stabilizes the bZIP transcription factor HY5 (ELONGATED HYPOCOTYL 5), a central activator of flavonoid and anthocyanin genes. Because COP1 also degrades HY5 in darkness, the UVR8–COP1–HY5 axis is the canonical UV-B signaling route; however, UV-B signaling is known to include HY5-independent branches whose identities in fruit tissue are largely uncharacterized. Li, Yamane and Tao (2021) exposed greenhouse-grown southern highbush blueberry 'O'Neal' to long-term preharvest UV-B at environmentally relevant doses and showed that the treatment accelerated fruit growth, sugar accumulation, ripening and coloration. Using dual-luciferase reporter assays (a transient co-expression system quantifying whether a candidate transcription factor activates or represses a target promoter), they demonstrated that VcMYBA1 positively regulates the VcUFGT promoter while VcMYBC2 represses both VcMYBA1 and VcUFGT. Their gene-expression data produced a stage-dependent model: in green fruit, UV-B induces VcHY5, which up-regulates VcMYBPA1 (and VcMYBA1) and down-regulates VcMYBC2, jointly promoting anthocyanin accumulation; in mature fruit, UV-B does not significantly change VcHY5 transcript levels, yet VcMYBPA1 and VcMYBC2 are still up-regulated. The authors wrote that the up-regulation of VcMYBC2 in mature fruit occurs 'through unknown pathways', and only speculated — without evidence — that a COP1-mediated abscisic-acid (ABA) response pathway (via ABI, ABSCISIC ACID INSENSITIVE, transcription factors) might explain the rise of VcMYBPA1. Thus the mature-fruit branch of the UV-B response, and in particular the mechanism restraining anthocyanin accumulation via VcMYBC2, was left as an explicitly open question.

Problem Statement

Identify the HY5-independent regulatory pathway — the upstream signal(s) and transcription factor(s) — through which UV-B radiation up-regulates the anthocyanin repressor gene VcMYBC2 in mature (ripening) highbush blueberry (Vaccinium corymbosum) fruit, a developmental stage in which UV-B does not induce VcHY5 transcription, thereby supplying the still-unidentified mature-fruit link of the stage-dependent model in which green fruit uses HY5-dependent and mature fruit uses HY5-independent regulation of anthocyanin metabolism. The deliverable is confined to this VcMYBC2 link; the UV-B up-regulation of the activator VcMYBPA1 in mature fruit is a distinct link of the model and need not be resolved here. Identifying one sufficient HY5-independent pathway — a specified upstream signal and a specified transcription factor (or complex) acting on VcMYBC2 — resolves the task. Such an identification must be a specific, refutable pathway, not a bare unevidenced speculation or a restatement of a branch name: it must state the proposed signal–transcription-factor–VcMYBC2 chain and rest on mechanistic evidence (for example, stage-specific expression, promoter–factor interaction, or functional data), whether drawn from the literature or from a new analysis.

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