OsbZIP60-mediated unfolded protein response regulates grain chalkiness in rice

Grain chalkiness, an undesirable trait caused by complex factors, has great negative impacts on the quality and economic value of rice. However, little is known about the regulatory mechanism of grain chalkiness, particularly the effect of endoplasmic reticulum (ER) stress. Here, a genome-wide association study (GWAS) reveals that the transcription factor OsbZIP60 is a vital regulator of rice grain chalkiness. Genetic analysis demonstrates that knockout of OsbZIP60 results in extremely high grain chalkiness and aberrant structure of storage substances. Notably, the expression of unfolded protein response (UPR) genes, such as OsbZIP50, OsBiP1, OsBiP2 and OsBiP3, is up-regulated in the endosperm cells of osbzip60, and overexpression of all these UPR genes causes various degrees of chalkiness. Furthermore, OsbZIP60 is found to activate the expression of key genes related to grain chalkiness, such as GPA3, FSE1, FLO7, Chalk5, OsNF-YB1, and OsPK2, whose expression is significantly suppressed in osbzip60 and overexpression lines of OsbZIP50, OsBiP1, OsBiP2, and OsBiP3. Our study provides novel insights into the function of OsbZIP60 and the role of the UPR pathway in the formation of grain chalkiness in rice.