Transcriptomic analyses provide new insights into jujube fruit quality affected by water deficit stress

Water availability is a crucial factor affecting fruit quality and productivity. A period of mild drought stress can increase fruit sweetness and improve fruit quality. However, the mechanism underlying the transcriptional regulation in Chinese jujube under water deficit conditions remains relatively uncharacterized. In this study, we analyzed the protein and sugar contents, the average weight, and transcript expression (RNA sequencing) of jujube fruit at different ripening stages (i.e., white mature and half-red fruit) under mild and severe water deficit conditions. The sucrose and protein contents increased significantly under water deficit conditions. The increase in water deficit stress from mild to severe did not significantly alter the sugar accumulation, but it decreased the average fruit weight. The sugar-and amino acid-related pathways, flavonoid biosynthesis, xyloglucan meta-bolism, and phenylpropanoid biosynthesis were affected by water deficit stress. Gene co-expression analyses identified many ABA-and ethylene-related genes with expression levels modulated by water deficit conditions, especially in the half-red stage. Additionally, we identified the key genes affecting fruit sugars, pigments, cell wall polysaccharides, and phenylpropanoids, which influence fruit quality. Notably, genes related to sugar transporters rather than sugar metabolism contributed to the sucrose accumulation. Flavonoid synthesis-related gene expression contributed to the earlier coloration of the stressed fruit than of the control fruit. Severe water deficit stress up-regulated the expression of pectinesterase and endotransglucosylase/hydrolase genes, which accelerated fruit ripening and softening. Regarding the phenylpropanoid biosynthesis pathway, the exposure to mild water deficit stress promoted the synthesis of secondary metabolites useful to humans (e.g., coniferin and syringin), whereas the severe water deficit treatment increased lignin accumulation. Considered together, these findings revealed the transcriptional regulatory mechanism that improved jujube fruit quality under mild drought conditions. Furthermore, the results of this study may be useful for optimizing the irrigation during jujube cultivation.