SmCDPK5 Phosphorylation Protein Modification-Mediated Mechanism of Reactive Oxygen Species Accumulation in Eggplant at Low Temperatures

In this study, we performed a comprehensive phosphoproteomic analysis of a cold-sensitive eggplant ( Solanum melongena ) cultivar (R5) and a cold-tolerant wild relative, Solanum aculeatissimum (Sk), before and after exposure to 4 °C for 12 h. A total of 6962 proteins, 8235 phosphorylation sites, and 71 phosphorylated motifs were identified. Functional and domain enrichment analyses revealed that differentially expressed proteins under cold stress were primarily involved in sugar, protein, and lipid metabolism, as well as peroxidase and superoxide dismutase activities. Differential phosphorylation sites were mainly enriched in sugar metabolism and redox pathways. Among the candidates, the calcium-dependent protein kinase SmCDPK5 was selected for functional validation. Silencing SmCDPK5 in eggplant reduced reactive oxygen species (ROS) accumulation under cold stress, whereas transient overexpression in S. aculeatissimum induced ROS bursts. Further analysis showed that SmCDPK5 interacts with and likely phosphorylates SmRBOHH1, a key regulator of ROS production.