Heat Shock Transcription Factor <scp>OsHsfc1a</scp> Enhances Rice Seedling Thermotolerance by Regulating <i>OsMFT1</i> and Preserving Chloroplast Structure Under Heat Stress

High-temperature stress severely threatens rice (Oryza sativa L.) growth and productivity, particularly during the seedling stage, making heat tolerance an essential breeding target. In this study, we identified the heat shock transcription factor OsHsfc1a as a positive regulator of thermotolerance in rice seedlings. OsHsfc1a expression was strongly induced by heat stress, and overexpression lines exhibited enhanced heat tolerance, whereas knockout mutants were hypersensitive. Integrative transcriptome and DAP-seq analyses revealed that OsHsfc1a directly binds to and represses the transcription of OsMFT1, a gene previously implicated in stress responses. Molecular and genetic assays confirmed that OsHsfc1a-mediated suppression of OsMFT1 helps maintain the chloroplast structural integrity, reduces reactive oxygen species accumulation, and alleviates programmed cell death under heat stress. In contrast, OsMFT1 overexpression impaired chloroplast organization and decreased seedling survival, whereas its knockout enhanced heat tolerance. Furthermore, comparative transcriptome analysis revealed that the OsHsfc1a-OsMFT1 regulatory module maintains the chloroplast structural integrity during heat stress by modulating the expression of multiple chloroplast structure-related genes. Analysis of natural haplotypes revealed that OsHsfc1a allelic variation is associated with rice subspecies differentiation, providing useful genetic resources for breeding heat-resilient varieties. In summary, we demonstrate that OsHsfc1a enhances heat tolerance by repressing OsMFT1 to preserve chloroplast stability, providing both mechanistic insights and practical genetic resources for rice improvement under global warming.