Respiratory burst oxidase homolog 1 plays a critical role in spermidine-induced thermotolerance by enhancing CO2 assimilation and cellular redox homeostasis in tomato.

Heat stress increasingly threatens global agriculture by disrupting photosynthesis and cellular redox homeostasis in plants. Spermidine (Spd), a stress-responsive polyamine, enhances thermotolerance by bolstering antioxidant capacity and stabilizing membranes; however, the molecular mechanisms underlying these effects remain incompletely characterized. Here, we demonstrate that Respiratory burst oxidase homolog 1 (RBOH1) is essential for Spd-induced thermotolerance in tomato (Solanum lycopersicum L.). Deficiency of RBOH1 compromised the protective effects of Spd, resulting in exacerbated membrane damage and a pronounced decline in photosynthetic performance under heat stress. Although Spd upregulated RBOH1 expression, the absence of RBOH1 disrupted Calvin cycle activity and impaired antioxidant modulation. Consequently, Spd-induced improvements in CO2 assimilation and redox homeostasis were abolished in rboh1 mutants. Our findings identify RBOH1 as a central molecular link integrating Spd signaling with photosynthetic protection and redox regulation, revealing a critical polyamine-RBOH1 axis that not only advances the mechanistic understanding of thermotolerance but also provides actionable insights for breeding heat-resilient crops.