SlIAA9 mutation enhances tomato seed resilience to heat stress

Heat stress during seed germination represents a critical constraint to crop establishment, yet the hormonal and genetic mechanisms governing seed resilience to heat stress remains poorly understood. In tomato (Solanum lycopersicum), the role of auxin signaling repressor SlIAA9 in regulating seed germination and responses to heat stress has not been defined. Here, we investigated how loss of function mutation in SlIAA9 affects seed resilience under high temperature and post stress recovery. Utilizing two SlIAA9 mutant lines (iaa9-5 and iaa9-3) and Wild-Type Micro-Tom tomatoes, we assessed germination behaviors, seed quality parameters, reactive oxygen species (ROS) contents, and transcriptional responses during heat stress and recovery. Both mutants exhibited enhanced resilience to heat stress, with iaa9-5 maintaining high germination rate, normal seed and seedling qualities, and rapid post-stress recovery. This phenotype was associated with reduced accumulation of H₂O₂ and O₂^- and elevated expression of antioxidant and heat-responsive genes. Heat stress triggered stronger induction of HSFA9 and HSP70 in the mutants, while dormancy associated abscisic acid (ABA) biosynthesis genes were suppressed and ethylene biosynthesis genes were upregulated during stress recovery. Together, these findings identify SlIAA9 as a negative regulator of seed resilience to heat stress and loss of SlIAA9 function enhances antioxidant capacity and heat-responsive transcriptional programs during germination and recovery. Highlighting SlIAA9 as a potential genetic target for improving seed resilience to heat stress.