Abscisic Acid and Calcium Signals Convergently Regulate Sugar Accumulation by Orchestrating the SRK2A/CIPK6-ABI5-TST2 Module in Citrus.

Abscisic acid (ABA) and calcium respectively work as crucial plant hormones and second signalling molecules in the regulation of fruit development and quality formation, including the sugar content and flavour quality. However, the regulatory mechanisms of fruit sugar accumulation arising from the interaction between ABA and calcium have not yet been fully elucidated. Here, we show that the application of ABA or calcium enhances sugar accumulation in sweet orange (Citrus sinensis) fruit, accompanied by upregulation of the expression level of tonoplast sugar transporter 2 (CsTST2), which mediates the transport of sugars into the vacuole. Based on molecular and transgenic analyses, we identified CsABI5 as the core transcription factor that positively regulates CsTST2 expression for sugar accumulation. Both CsSRK2A, a key player in ABA signalling, and CsCIPK6, a kinase linked to calcium signalling, interact with and phosphorylate CsABI5 at distinct amino acid sites to enhance its transcriptional activity. Interestingly, CsABI5 also activates the expression of CsCIPK6 by binding to its promoter, thereby establishing a positive feedback loop between ABA and calcium signalling that cascades into higher expression of CsTST2 and sugar content. Collectively, these findings uncover a novel molecular network that underlies sugar accumulation in citrus, revealing the intricate coordination between ABA and calcium signalling in regulating tonoplast sugar transport in a non-climacteric fruit.