Time dependent heat stress responses in rice and the role of brassinosteroids in regulating thermotolerance

Understanding the dynamic responses of rice (Oryza sativa L.) to varying durations of heat stress is crucial for developing thermotolerant crops in the face of global climate trends. Here, we evaluated the responses of rice cultivar Zhonghua11 to short-term (0.5-1 h) and long-term (12-24 h) heat stress at 45 °C. Phenotypic observations showed that short-term heat stress reduced photosynthetic pigment content without major leaf morphological changes, while long-term stress caused severe leaf drying and reduced the survival rate to 13 %. Transcriptome analysis revealed persistent inhibition of chlorophyll metabolism and photosynthesis genes under heat stress. Short-term heat stress primarily induced genes involved in reactive oxygen species scavenging and heat shock proteins, while long-term stress shifted the transcriptional profile toward DNA damage repair and chloroplast reconstruction, indicating the dynamic molecular nature of rice's response. Furthermore, set of BR-linked genes were identified from transcriptome analysis and exogenous BR application was confirmed to improve plants survival rate. Functional analyses of BR-linked genes (OsBZR1, OsBU1, and OsGSR1) confirmed that BR signaling enhances thermotolerance. Based on these results, we conclude that rice employs distinct dynamic strategies to cope with heat stress of different durations. Importantly, this study clarifies rice's dynamic strategies under heat stress and highlights BR pathways as key regulators, providing a basis for breeding heat-resilient varieties.