The Heat Shock Transcription Factor OsHsfA2a Modulates Rice Seed Dormancy and Pre-Harvest Sprouting With OsSD6

Seed dormancy is a complex developmental and agronomic trait requiring a delicate balance: insufficient dormancy leads to pre-harvest sprouting (PHS), while excessive dormancy causes uneven germination. Understanding the intrinsic mechanisms resolving this trade-off is vital for crop improvement. In this study, our findings indicate that the heat shock transcription factor OsHsfA2a acts as a negative regulator of seed dormancy. The hsfa2a knockout mutants exhibited enhanced PHS resistance while maintaining yield potential and post-harvest germination rates, offering a potential strategy to uncouple PHS resistance from agronomic penalties. Mechanistically, our data suggest that OsHsfA2a self-associates and can activate the ABA catabolic gene OsABA8ox3, thereby promoting dormancy release. Furthermore, genetic analysis using the sd6 hsfa2a double mutant supports the hypothesis that the bHLH transcription factor OsSD6 is epistatic to OsHsfA2a, acting as an upstream initiator. Biochemically, OsSD6 transcriptionally activates OsHsfA2a. Once expressed, OsHsfA2a likely amplifies the signal via a self-activation loop. However, the accumulating OsSD6 protein physically interacts with OsHsfA2a, which we propose competitively attenuates its DNA-binding capacity. These results support a multi-tiered regulatory module operating across both transcriptional and post-translational levels. Collectively, our findings provide insights into an intrinsic physiological mechanism by which this module fine-tunes ABA catabolism to govern primary seed dormancy, providing precise genetic targets for mitigating PHS.