CcWRKY31 Regulates Postharvest Mandarin Fruit Coloration in a Temperature-Dependent Manner

Storing mandarin fruit at 25 °C accelerates postharvest coloration of mandarin fruit, while 32 °C inhibits it. From the mandarin fruit stored at these two temperatures, we identified the temperature-responsive transcription factor CcWRKY31 as a key regulator of pigment metabolism. CcWRKY31 localizes to the nucleus and exhibits transcriptional activation activity in tobacco cells but not in yeast. Its transient overexpression in tobacco leaves caused visible yellowing and significantly reduced chlorophyll and its metabolites, with faster yellowing at 25 °C than at 32 °C. In mandarin fruit peel, transient overexpression of CcWRKY31 induced distinct color change around the injection site and strongly upregulated CcWRKY31 expression at both temperatures, though the induction was markedly lower at 32 °C. This overexpression promoted substantial accumulation of carotenoids and their metabolites while accelerating chlorophyll degradation. Mechanistically, CcWRKY31 directly binds to the W-box in the promoters of carotenoid biosynthesis genes (CcCRTZ, CcLCYE, CcCCS, CcZEP, CcNCED5) and chlorophyll degradation genes (CcNYC1, CcRCCR) to positively activate their expression. These results suggest that at the optimal temperature of 25 °C, CcWRKY31 accelerates postharvest mandarin peel coloration by coordinately promoting chlorophyll breakdown and carotenoid accumulation.