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

Cytokinin (CK) plays a key role in responding to different abiotic stressors including salinity in delaying leaf senescence. cis-Zeatin (cZ) is a naturally occurring CK that has been less studied than its well-known isoform trans-Zeatin (tZ). Here, we investigate the function of cZ in regulating leaf senescence in Solanum lycopersicum (Tomato) using physiological and transcriptomic analyses. Mature leaves treated with cZ or tZ ± 150 mM NaCl were examined at early (2 h) and late (72 h) timepoints. Physiological analysis of Photosystem II efficiency (Fv/Fm) and chlorophyll content revealed that salt accelerates leaf senescence, but this effect can be delayed by both cZ and tZ 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. cZ 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 cZ and salt stress. Although cZ and tZ 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 cZ and tZ in salt stress-induced senescence. Overall, our findings demonstrate that cZ 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 cZ in plant resilience strategies.