LcNAC90 Transcription Factor Regulates Biosynthesis of Anthocyanin in Harvested Litchi in Response to ABA and GA3

The antagonism between ABA and GA in regulating accumulation of anthocyanin has been well documented. Lately we showed both ABA and GA3 alleviated litchi browning, but the underlying molecular mechanism remains unclear. Here we showed 6-d storage caused obvious browning in litchi, but ABA and GA3 increased anthocyanin and reduced color fading relative to the control. LC-MS analysis identified 21 anthocyanins from litchi, of which seven were upregulated and 14 downregulated for ABA treatment after storage relative to before, and four upregulated and 17 downregulated for GA3 treatment. Among the upregulated anthocyanins were three shared by ABA and GA3 treatments. RNA-seq revealed that ABA and GA3 upregulated genes for 7 anthocyanin biosynthetic enzymes, including LcPAL and LcCHS, whose expression patterns were consistent with that of LcNAC90. Moreover, ABA and GA3 activated transcription of LcNAC90 transiently expressed in tobacco, verifying the responsiveness of LcNAC90 to ABA and GA3. The regulatory network analysis suggested that LcPAL and LcCHS transcripts were highly correlated with LcNAC90. Sequence analysis revealed NAC binding sites in promoters of LcPAL and LcCHS. Biochemical evidences showed LcNAC90 specifically bound to the promoters and activated transcription of LcPAL and LcCHS. LcNAC90 overexpression in Arabidopsis increased leaf pigmentation and upregulated anthocyanin biosynthetic genes in Arabidopsis, e.g. AtPAL and AtCHS. Collectively, this work suggests that ABA and GA3 activate LcNAC90 to enhance anthocyanin biosynthesis in litchi during storage. The findings provide novel insight into mechanism underlying anthocyanin accumulation regulated by ABA and GA.