OsBT1 encodes an ADP-glucose transporter involved in starch synthesis and compound granule formation in rice endosperm

Starch is the main storage carbohydrate in higher plants. Although several enzymes and regulators for starch biosynthesis have been characterized, a complete regulatory network for starch synthesis in cereal seeds remains elusive. Here, we report the identification and characterization of the rice Brittle1 (OsBT1) gene, which is expressed specifically in the developing endosperm. The osbt1 mutant showed a white-core endosperm and a significantly lower grain weight than the wild-type. The formation and development of compound starch granules in osbt1 was obviously defective: the amyloplast was disintegrated at early developmental stages and the starch granules were disperse and not compound in the endosperm cells in the centre region of osbt1 seeds. The total starch content and amylose content was decreased and the physicochemical properties of starch were altered. Moreover, the degree of polymerization (DP) of amylopectin in osbt1 was remarkably different from that of wild-type. Map-based cloning of OsBT1 indicated that it encodes a putatively ADP-glucose transporter. OsBT1 coded protein localizes in the amyloplast envelope membrane. Furthermore, the expression of starch synthesis related genes was also altered in the osbt1 mutant. These findings indicate that OsBT1 plays an important role in starch synthesis and the formation of compound starch granules.