The bZIP transcription factor RISBZ1 balances grain filling and ER stress response in rice grains

In cereal crops, the endosperm synthesizes large quantities of proteins, including storage proteins and functional factors essential for the accumulation of storage reserves. The unfolded protein response (UPR) monitors the folding of nascent polypeptides in the endoplasmic reticulum (ER) to alleviate cellular stress. However, the molecular mechanisms linking the UPR to endosperm development in plants remain poorly understood. In this study, we isolated and characterized a rice (Oryza sativa L.) mutant with defective endosperm development, designated as floury endosperm27 (flo27). Molecular cloning revealed that FLO27 encodes RISBZ1/bZIP58, an endosperm-specific transcription factor co-expressed with seed storage protein (SSP) genes and starch biosynthesis-related genes in rice. The flo27 mutant exhibited severe ER stress, along with upregulation of UPR-related genes. Notably, RISBZ1 interacts with bZIP50 and bZIP60 to antagonistically suppress downstream UPR genes. These interactions simultaneously downregulate SSP and starch biosynthesis-related genes, ultimately leading to reduced dry matter accumulation. In conclusion, these findings demonstrate that RISBZ1 functions as a "brake signal" to mitigate ER stress, thus enhancing our understanding of the trade-off between grain filling and adaptation to adverse environmental conditions in rice.