The E3 ubiquitin ligase OsSAP3 mediates small heat shock protein OsHSP16 degradation to modulate thermotolerance in rice

Abstract

Stress-associated proteins (SAPs), a class of zinc-finger proteins, play crucial roles in plant responses to abiotic stresses, including high temperature; however, their underlying mechanism is largely unknown. Here, we report that a heat-induced E3 ubiquitin ligase OsSAP3 negatively regulates thermotolerance in rice (Oryza sativa) by specifically interacting with the small heat shock protein OsHSP16. Knockout of OsSAP3 improves seedling survival and maintains grain-setting rates under heat stress. OsSAP3 ubiquitinates the lysine residues K128 and K145 in OsHSP16 to regulate its stability. Unlike OsSAP3, OsHSP16 functions as a positive regulator of thermotolerance in rice, and this enhanced heat resistance correlates with increased antioxidant enzyme activity, reduced reactive oxygen species (ROS) accumulation, decreased relative electrolyte leakage, and upregulation of heat stress defense genes. Genetic interaction analysis supports that OsSAP3 and OsHSP16 operate in the same pathway to modulate heat stress responses, albeit with opposing roles. Furthermore, OsHSP16 binds to and promotes the degradation of putatively misfolded or damaged OsAPX2 (Ascorbate Peroxidase 2) proteins, which may contribute to its role in enhancing thermotolerance in rice. Collectively, our findings reveal a key molecular framework controlling rice heat tolerance and provide a potential gene-editing-based strategy to enhance crop heat resilience.