cis-Zeatin is an Active Cytokinin that Mediates Gene Expression and Delays Leaf Senescence in Tomato Under Salt Stress

Abstract Cytokinin (CK) plays a key role in responding to different abiotic stressors including salinity in delaying leaf senescence. cis -Zeatin ( c Z) is a naturally occurring CK that has been less studied than its well-known isoform trans- Zeatin ( t Z). Here, we investigate the function of c Z in regulating leaf senescence in Solanum lycopersicum (Tomato) using physiological and transcriptomic analyses. Mature leaves treated with c Z or t Z ± 150 mM NaCl were examined at early (2 h) and late (72 h) timepoints. Physiological analysis of Photosystem II efficiency ( F v /F m ) and chlorophyll content revealed that salt accelerates leaf senescence, but this effect can be delayed by both c Z and t Z in a dark-induced leaf senescence bioassay. Transcriptomic analysis indicated increasing number of differential expressed genes (DEGs) from early to late senescence under salt stress. c Z treatment uniquely regulates genes ± NaCl, including multiple signaling pathways and regulatory networks involved in salt stress. Weighted Gene Co-expression Network Analysis revealed modules with unique cZ elevated eigengene values, and correlations indicate a coordinated regulatory response to c Z and salt stress. Although c Z and t Z gene regulations largely overlapped in direction and response, some antagonistic patterns were observed, highlighting divergent CK functions in fine-tuning the salt stress response. Further organelle-level analysis of DEGs indicated distinct involvement of c Z and t Z in salt stress-induced senescence. Overall, our findings demonstrate that c Z is an active CK form that regulates gene expression and delays tomato leaf senescence under salinity. This reveals novel aspects of CK-mediated stress adaptation underscoring the importance of c Z in plant resilience strategies.