The chlorophyll and carotenoid metabolism in postharvest mandarin fruit peels is co-regulated by transcription factor CcbHLH35

Early-maturing mandarin fruit have the characteristic that the peel and flesh cannot reach the maturity standard synchronously. Ethylene degreening is a technique used in the industry to improve the coloration of these citrus fruit. In order to further elucidate the underlying mechanism of ethylene degreening, this study screened a CcbHLH35 transcription factor from ethephon-degreened mandarin fruit. The results showed that the expression level of CcbHLH35 was upregulated in ethephon-degreened mandarin fruit. CcbHLH35 is a nuclear localization protein and has transcriptional activation activity. Heterologous transient overexpression of CcbHLH35 in tobacco leaves resulted in a significant yellowing of tobacco leaves and a significant reduction in the content of chlorophyll and its metabolites. Overexpression of CcbHLH35 in mandarin fruit peels promoted the coloration of peel, up-regulated the expression of chlorophyll degradation-related genes, and thus accelerated the degradation of chlorophyll; at the same time, it promoted carotenoid biosynthesis and expression of the related gene. Electrophoretic mobility shift assay (EMSA) and dual-luciferase reporter (DLR) assays showed that CcbHLH35 activated the expression of four key genes in the chlorophyll degradation (CcChlase1, CcNYC1, CcPAO, CcRCCR) and three key genes in the carotenoid biosynthesis pathway (CcPSY1, CcBCH2, CcNCED5). Our results clarify the regulatory network by which CcbHLH35 accelerates mandarin fruit coloration. •CcbHLH35 is a transcriptional activator with nuclear localization signals.•Overexpression of CcbHLH35 promotes coloration of mandarin fruit.•CcbHLH35 binds directly to the G-box in the promoter of the target gene.•Overexpression of CcbHLH35 accelerates chlorophyll degradation of mandarin fruit.•Overexpression of CcbHLH35 accelerates carotenoid metabolism of mandarin fruit.