Melatonin promotes anthocyanin biosynthesis in postharvest litchi fruit via regulation of the transcription factor LcWRKY17

Pericarp discoloration and browning in postharvest litchi fruit extremely reduce marketability of products. Exogenous melatonin (MEL), a robust bioactive molecule, has been previously demonstrated to delay quality deterioration in litchi fruit. The present study further uncovered the molecular mechanism of MEL in regulating postharvest quality in litchi fruit. The results showed that the application of 400 μM MEL to ‘Baitangying’ litchi fruit appreciably delayed redness loss and browning while inhibiting the changes in DPPH radical scavenging activity and relative conductivity. RTqPCR analysis revealed that MEL upregulated key anthocyanin biosynthesis genes, including LcPAL1, LcPAL2, LcPAL3, Lc4CL, LcCHS, LcF3H and LcANS, which paralleled with increased anthocyanin content in the MEL-treated fruit. Furthermore, a MEL-inducible WRKY transcription factor was identified, which had a conserved WRKY-binding W-box motifs in the promoter regions of LcPAL3, Lc4CL and LcANS. Functional validation via promoter-binding assays revealed that LcWRKY17 is a nuclear protein with transcriptional activation and it directly activated the transcription of LcPAL3, Lc4CL and LcANS. In summary, the results suggest that LcWRKY17 mediates MEL-induced anthocyanin biosynthesis in litchi, at least partially through direct transcriptional activation of LcPAL3, Lc4CL and LcANS, thereby advancing our understanding of the mechanism by which MEL delays quality deterioration in harvested litchi fruit.