Physiological and Molecular Responses to High-Temperature Stress at Anthesis in Brazilian Flooded Rice

High-temperature events are projected to increase in frequency under future climate scenarios, threatening rice yields globally. This study investigated the physiological and molecular responses of two Brazilian flooded rice varieties, IRGA 428 and BR-IRGA 409, during the anthesis stage under high-temperature stress, aiming to uncover mechanisms of heat tolerance. Plants were exposed to a daytime temperature of 38°C for seven hours across three, five, or seven days. Prolonged heat stress led to a significant reduction in filled grain in both cultivars, although BR-IRGA 409 demonstrated greater heat tolerance, particularly under three days of stress, as it maintained higher spikelet fertility compared to IRGA 428. Comparative transcriptome analysis revealed that BR-IRGA 409 had more differentially expressed genes (DEGs) in response to heat stress, including a significant upregulation of canonical heat-responsive genes such as heat shock factor (HSFs), heat shock proteins (HSPs), and peptidyl-prolyl isomerase FK506-binding proteins (FKBPs). Furthermore, BR-IRGA 409 displayed enhanced modulation of the mitochondrial electron transport pathway, which is crucial for ATP synthesis and cellular energy production. Interestingly, while photosynthetic performance varied between cultivars, only a few genes associated with photosynthesis were significantly altered in response to heat stress. Instead, BR-IRGA 409 displayed a higher basal expression of photosynthesis-related genes, suggesting that this pre-adaptation might mitigate heat stress impacts on photosynthesis. The ability to preserve functional photosynthetic activity is critical for sustaining the energy-intensive process required to cope with heat stress. This study highlights the difference between the varieties in their response to heat stress and identifies candidate molecular and physiological mechanisms that contribute tomaintaining cellular energy homeostasis and heat tolerance in Brazilian rice, providing valuable insights for crop improvement strategies.