Transcriptomic insights into ABA- and ethephon-mediated regulation of flavonoid biosynthesis during pericarp coloration in Litchi chinensis cv. Deshi

Coloration of the pericarp in Litchi chinensis Sonn. is a major factor in determining the economic value of litchi fruits, and this is primarily mediated by anthocyanin accumulation. The application of plant hormones is a common practice for inducing anthocyanin accumulation in fruits. The mechanism by which hormone signaling is involved in flavonoid biosynthesis in litchi fruits is not very clear. The present study aimed to investigate the physiological, biochemical, and transcriptional responses in litchi fruits cv. Deshi to abscisic acid (ABA), ethephon (ETH), and their combined application in the field. Application of both ABA and ETH resulted in significant increases in pericarp redness and anthocyanin accumulation in litchi fruits compared to the control, and the promotive effects of ABA were more significant. The high-quality transcriptomes generated in this study using RNA sequencing technology provided comprehensive information for analyzing differentially expressed genes (DEGs), functional annotations, and pathway enrichments in response to ABA and ETH application in litchi fruits. Transcriptomic analysis revealed that ABA-induced coloration is primarily driven by the early-stage phenylpropanoid pathway (e.g., LcHCT, LcCYP98A), whereas Ethephon significantly upregulates LcCYP75B1 and LcANR. Notably, individual applications of ABA or Ethephon were more effective than the combined treatment, suggesting potential hormonal antagonism at the transcriptional level. These results provide a molecular basis for improving 'Deshi' litchi marketability through targeted hormonal regulation. The major structural genes involved in anthocyanin biosynthesis, such as PAL, CHS, F3H, DFR, and UFGT, showed significant upregulation in response to ABA application. At the same time, the genes involved in ABA signaling, such as those in the PYR/PYL-PP2C-SnRK2 pathway, also showed transcriptional activation in response to ABA application. The transcription factors involved in anthocyanin biosynthesis, such as MYB, bHLH, and ERF, also showed differential regulation in response to ABA application, suggesting a hormone-dependent transcriptional regulation mechanism in anthocyanin biosynthesis in litchi fruits. Correlation analysis also showed a positive correlation between anthocyanin accumulation and structural gene expression in the anthocyanin biosynthetic pathway, suggesting transcriptional regulation as the major mechanism in anthocyanin biosynthesis in litchi fruits. The effects of ABA application in activating the flavonoid pathway in litchi fruits were more significant than those of ethylene, suggesting the major role of ABA in regulating anthocyanin biosynthesis in litchi fruits. Based on the results, a model is proposed for ABA signaling in anthocyanin biosynthesis in litchi fruits.