Transcriptome profiling of Litchi chinensis pericarp in response to exogenous cytokinins and abscisic acid

Anthocyanin biosynthesis in Litchi chinensis is promoted by exogenous abscisic acid (ABA) treatment and inhibited by exogenous N-(2-chloro-pyridin-4-yl)-N'-phenylurea (CPPU) application. However, the mechanisms by which ABA or CPPU regulates anthocyanin biosynthesis are still unclear. To understand the global molecular events of these physiological changes, transcriptome profiling was analyzed in L. chinensis cv. Feizixiao pericarps after 0, 10, and 20 days of exogenous ABA (25 mg/L ABA) and CPPU (4 mg/L) treatment using RNA-seq. Compared with the control, a total of 579 and 827 genes were differently expressed [vertical bar log(2) fold change vertical bar >= 1 and P value <= 0.005] in ABA- and CPPU-treated pericarp, respectively. Exogenous ABA up-regulated the expressions of genes involved in flavonoid and anthocyanin biosynthesis, including PAL, C4H, CHS, CHI, DFR, LDOX, and GTs. In contrast, exogenous CPPU induced genes related to carbon metabolism, amino acids biosynthesis, and photosynthesis, and down-regulated genes related to anthocyanin biosynthesis. Comparison of transcriptomes in responses to individual treatments with ABA or CPPU revealed that there were cooperative and antagonistic interplay between ABA and cytokinins in litchi fruit ripening. ABA treatment had no significant effect on the genes related to chlorophyll catabolism. On the other hand, CPPU treatment significantly increased the expression of chlorophyll synthesis genes and inhibited the expression of chlorophyll degradation gene (SGR). In addition, ABA and CPPU treatment also affected gene expression in other plant hormone signaling pathways, such as auxin, GA, and ethylene, forming a complex network to regulate anthocyanin biosynthesis. This study provides a valuable overview of global molecular events for studying the mechanisms by which ABA and cytokinins influence anthocyanin biosynthesis in litchi and other fruit trees enriched with anthocyanins.