A heat stress responsive NAC transcription factor heterodimer plays key roles in rice grain filling

High temperature often leads to the failure of grain filling in rice (Oryza sativa) to cause yield loss, while the mechanism is not well elucidated yet. Here, we report that two seed-specific NAM/ATAF/CUC domain transcription factors, ONAC127 and ONAC129, are responsive to heat stress and involved in the grain filling process of rice. ONAC127 and ONAC129 are dominantly expressed in the pericarp and can form a heterodimer during rice grain filling. CRISPR/Cas9 induced mutants and overexpression lines were then generated to investigate the functions of these two transcription factors. Interestingly, both knock-out and overexpression plants showed incomplete grain filling and shrunken grains, which became more severe under heat stress. Transcriptome analysis revealed that ONAC127 and ONAC129 mainly regulate stimulus response and nutrient transport. ChIP-seq analysis identified that the direct targets of ONAC127 and ONAC129 in developing rice seeds include monosaccharide transporter OsMST6, sugar transporter OsSWEET4, calmodulin-like protein OsMSR2 and AP2/ERF factor OsEATB. These results suggest that ONAC127 and ONAC129 may regulate grain filling through affecting sugar transportation and abiotic stress responses. Overall, this study demonstrates a transcriptional regulatory network involving ONAC127 and ONAC129 and coordinating multiple pathways to modulate seed development and heat stress response at rice reproductive stage.HighlightA NAC transcription factor heterodimer plays vital roles in heat stress response and sugar transportation at rice grain filling stage.