FLOURY AND SHRUNKEN ENDOSPERM6 Encodes a Glycosyltransferase and is Essential for the Development of Rice Endosperm

Composition and structure of endosperm starch are major determinant factors for rice quality. Glycosylinositol phosphorylceramides (GIPC) is a kind of sphingolipid and it accounts for ~ 25% of total plasma membrane lipids in plants. The relationship between synthesis of GIPC and development of endosperm starch, however, remains unclear. We here identified a mutant with a floury and opaque endosperm, named floury and shrunken endosperm 6 (fse6). The mutant seeds displayed a shrunken grain appearance. Physicochemical analysis showed that both total starch and amylose contents were decreased, while lipid and protein contents were increased in the mature mutant seeds, compared to their counterparts in the wild type. Further observation of semi-thin sections indicated the development of mutant amyloplasts was defective. The mutant seeds germinated normally but failed to survive in a later stage of seedling growth. Map-based cloning and genetic complementation revealed that FSE6 encodes a glycosyltransferase and is homologous to Arabidopsis GLUCOSAMINE INOSITOLPHOSPHORYLCERAMIDE TRANSFERASE1 (GINT1), an enzyme vital for GIPC synthesis. We further found that cellulose content and starch biosynthesis in the mutant were altered. This study connects a gap between a rice GINT1 and starch synthesis, which will be helpful for rice quality improvement.