The papain-like cysteine proteases (PLCP) in tomato: Identification, expression analysis, and functional...

Papain-like cysteine proteases (PLCPs) are key enzymes involved in protein hydrolysis and play critical roles in plant growth, development, and responses to stresses. Although PLCPs have been systematically identified in various plant species, their functions in tomato remain largely unexplored, particularly their roles in salt stress adaptation. In this study, we identified 32 PLCP genes in the tomato genome and classified them into nine subfamilies. We found that the promoter regions of SlPLCP genes are enriched with stress-responsive elements. Combining transcriptome data and qRT-PCR analysis showed that SlRD19B was the most significantly upregulated gene under salt stress. Further functional studies demonstrated that silencing SlRD19B enhanced tomato sensitivity to salt stress, as evidenced by severe leaf wilting, increased membrane damage, reduced osmotic adjustment capacity, inhibited root growth, and an elevated Na⁺/K⁺ ratio. Additionally, SlRD19B-silenced plants exhibited excessive reactive oxygen species (ROS) accumulation and disrupted antioxidant enzyme activities under salt stress, characterized by increased superoxide dismutase (SOD) and catalase (CAT) activities but decreased peroxidase (POD) activity. This study systematically reveals the crucial role of the SlPLCP gene family in tomato salt stress response and provides new insights into the functional mechanisms of PLCP genes in plants.