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

Abstract

In cereal crops, the endosperm is responsible for synthesizing large amounts of proteins, including storage proteins and functional factors essential for the accumulation of storage substances. 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 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, which we named floury endosperm27 (flo27). Molecular cloning revealed that FLO27 encodes RISBZ1/bZIP58, an endosperm-specific transcription factor that is co-expressed with seed storage protein (SSP) genes and starch biosynthesis-related genes in rice. We found that the flo27 mutant exhibits severe ER stress, accompanied by the upregulation of UPR-related genes. Notably, RISBZ1 interacts with bZIP50 and bZIP60 to antagonistically downregulate the expression of downstream UPR genes. These interactions simultaneously suppress the expression of SSP genes and starch biosynthesis-related genes, ultimately leading to reduced dry matter accumulation. In conclusion, our findings demonstrate that RISBZ1 acts as a "brake signal" to mitigate ER stress, thereby broadening our understanding of the delicate trade-off between grain-filling and adaptation to adverse environmental conditions in rice.