Abscisic Acid Lowers Postharvest Grape Acidity via PP2C-ALMT9-Dependent Malate Transport.

This study uncovers a novel concentration-dependent signaling network through which abscisic acid (ABA) modulates acid metabolism in grape berries. Treatment with 800 μM ABA significantly decreased malate and citrate synthesis and transport while enhancing glucose and fructose accumulation, optimizing the sugar-acid ratio. Multi-omics analysis revealed differential changes in 96 organic acids and identified a key malate transporter, VvALMT9. Through yeast two-hybrid screening, the VvPP2C-VvALMT9-VvSnRK2.3-VvCDPK interaction network was characterized in a dual regulatory mode. Under low-ABA conditions, VvSnRK2.3 and VvPP2C are synergistically upregulated to impair VvALMT9 function via direct binding and transcriptional repression, respectively, while reduced VvCDPK expression leads to insufficient VvSnRK2.3 phosphorylation, establishing a "low ABA-high VvALMT9" state with high acid accumulation; when low temperature is coupled with low ABA, the inactivation of the temperature-sensing domain of VvPP2C further relieves this repression, delaying acid degradation. Conversely, a high-ABA environment suppresses the basal expression of VvPP2C/VvSnRK2.3 and activates the VvCDPK-VvSnRK2.3 cascade to efficiently inhibit ALMT9 phosphorylation, blocking malate accumulation and reducing acidity. Transgenic validation demonstrated that VvALMT9 overexpression promotes acid accumulation under ambient temperature, whereas high temperature induces VvPP2C to inhibit VvALMT9, an effect reversed by ABA. Overall, this study elucidates the mechanism of ABA-temperature synergistic regulation of VvALMT9-mediated acid metabolism homeostasis, providing a theoretical basis for targeted fruit quality improvement.