Rice Cytochrome P450 Protein CYP71P1 Is Required for Heat Stress Tolerance by Regulating Serotonin Biosynthesis and ROS Homeostasis

Heat stress is one of the major factors affecting crop growth and yield. However, the molecular mechanisms underlying rice heat stress tolerance remain largely unclear. In this study, we identified and characterized the rice <i>high temperature sensitive 2</i> (<i>hts2</i>) mutant, which is highly susceptible to heat stress. Map-based cloning revealed that the <i>HTS2</i> encodes a cytochrome P450 protein (CYP71P1) involved in serotonin biosynthesis. <i>HTS2</i> is ubiquitously expressed across plant tissues and shows strong upregulation in response to heat stress. The <i>HTS2</i> mutation significantly impairs basal serotonin synthesis in rice, and the heat-sensitive phenotype of the <i>hts2</i> mutant is completely rescued by exogenous serotonin supplementation. Compared to the wild type, the <i>hts2</i> mutant exhibits reduced antioxidant capacity, leading to excessive reactive oxygen species (ROS) accumulation and severe oxidative damage, ultimately reducing heat stress tolerance. Furthermore, disruption of <i>HTS2</i> significantly affects the rice heat shock response, with the heat-induced expression of <i>HsfA2s</i> and their downstream target genes, such as <i>HSP18.0</i> (<i>heat shock protein 18.0</i>) and <i>OsAPX2</i> (<i>ascorbate peroxidase 2</i>), markedly depressed in <i>hts2</i> mutant. Our results suggest a pivotal role of <i>HTS2</i> in modulating serotonin metabolism and maintaining ROS homeostasis during heat stress, offering new perspectives on the mechanisms underlying heat tolerance and potential strategies to enhance rice resilience to heat stress.