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Proanthocyanidins (PAs; or condensed tannins) are a major class of flavonoids that contribute to citrus fruit quality. However, the molecular mechanism responsible for PA biosynthesis and accumulation in citrus remains unclear. Here, we identify a PH4–Noemi regulatory complex that regulates proanthocyanidin biosynthesis in citrus. Overexpression of PH4 or Noemi in citrus calli activated the expression of PA biosynthetic genes and significantly increased the PA content. Interestingly, Noemi was also shown to be up-regulated in CsPH4 -overexpressing lines compared with wild-type calli. Simultaneously, CsPH4 partially complemented the PA-deficient phenotype of the Arabidopsis tt2 mutant and promoted PA accumulation in the wild-type. Further analysis revealed that CsPH4 interacted with Noemi, and together these proteins synergistically activated the expression of PA biosynthetic genes by directly binding to the MYB-recognizing elements (MRE) of the promoters of these genes. Moreover, CsPH4 could directly bind to the promoter of Noemi and up-regulate the expression of this gene. These findings explain how the CsPH4–Noemi regulatory complex contributes to the activation of PA biosynthetic genes via a positive feedback loop and provide new insights into the molecular mechanisms underlying PA biosynthesis, which can be effectively employed for metabolic engineering to improve citrus fruit quality. [ABSTRACT FROM AUTHOR]

Background: Citrus fruits are consumed freshly or as juice to directly provide various dietary flavonoids to humans. Diverse metabolites are present among Citrus genera, and many flavonoids biosynthetic genes were induced after abiotic stresses. To better understand the underlying mechanism, we designed experiments to overexpress a UDP-GLUCOSYL TRANSFERASE gene from sweet orange (Citrus sinensis) to evaluate its possible function in metabolism and response to stress. Results: Our results demonstrated that overexpression of Cs-UGT78D3 resulted in high accumulation of proanthocyanidins in the seed coat and a dark brown color to transgenic Arabidopsis seeds. In addition, the total contents of flavonoid and anthocyanin were significantly enhanced in the leaves of overexpressed lines. Gene expression analyses indicated that many flavonoid (flavonol) and anthocyanin genes were up-regulated by 4–15 folds in transgenic Arabidopsis. Moreover, after 14 days of high light stress, the transgenic Arabidopsis lines showed strong antioxidant activity and higher total contents of anthocyanins and flavonoids in leaves compared with the wild type. Conclusion: Our study concluded that the citrus Cs-UGT78D3 gene contributes to proanthocyanidins accumulation in seed coats and confers tolerance to high light stress by accumulating the total anthocyanin and flavonoid contents with better antioxidant potential (due to photoprotective activity of anthocyanin) in the transgenic Arabidopsis. [ABSTRACT FROM AUTHOR]