Identification of heat-tolerant germplasm and elucidation of heat tolerance mechanisms in polyploid rice.

Heat stress has become one of the major abiotic stresses limiting rice growth and development. Polyploid rice represents an important approach in rice breeding and may contribute to heat tolerance improvement; however, the processes by which polyploid rice responds to heat stress remain mainly ambiguous. In this study, we evaluated the heat tolerance of 40 neo-tetraploid rice lines at the seedling stage. Among them, six lines displayed high heat tolerance (HD441-HD446), and 16 lines showed low heat tolerance (HD465-HD480). Cytological observations of the two lines, HD441 and HD477, which exhibited marked differences in survival rate, indicated that HD441 maintained a more stable root structure. Physiological investigations indicated that HD441 exhibited more stable antioxidant enzyme activities and accumulated less reactive oxygen species (ROS) under heat stress. Transcriptomic analysis revealed that the expression levels of heat shock protein genes (e.g., OsHSP18.2 and OsHSP60-3B) under both physiological and heat stress conditions differed significantly between the two lines. Temporal differential expression analysis further demonstrated that the expression levels of photosynthesis-related genes (e.g., OsRBCS2 and OsRBCS5) were significantly upregulated in HD441 under heat stress, and that the chloroplast structure of HD441 remained more stable than that of HD477. Integrated metabolo-transcriptomic analysis revealed that flavonoid biosynthesis was a key pathway in the heat stress response of HD441. These findings collectively offer significant insights into the regulatory processes governing high-temperature responses in polyploid rice.