OsbZIP58, a basic leucine zipper transcription factor, regulates starch biosynthesis in rice endosperm
Jie-Chen Wang¹, Heng Xu², Qiao-Quan Liu³ and Xiu-Ling Cai¹*

¹National Key Laboratory of Plant Molecular Genetics, Institute of Plant Physiology and Ecology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai 200032, China;
²State Key Laboratory Breeding Base for Zhejiang Sustainable Pest and Disease Control, Institute of Virology and Biotechnology, Zhejiang Academy of Agricultural Science, Zhejiang 310021, China;
³Key Laboratories of Crop Genetics and Physiology of the Jiangsu Province and Plant Functional Genomics of the Ministry of Education, Yangzhou University, Yangzhou 225009, China.
*Correspondence: Xiu-Ling Cai, Tel: +86-21-54924078; Fax: +86-21-54924015, E-mail: xlcai@sibs.ac.cn

Abstract
Starch composition and content in endosperm are controlled by a series of complex and coordinated biochemical reactions, attributing dramatically to seed yield, cooking   quality, and taste in cereals. However, the mechanism regulating starch biosynthesis in cereal seeds is not well understood. Here we report that OsbZIP58, a bZIP transcription factor, serves as a master player in coordinating starch synthesis in rice endosperm. Consistent with this role, the expression of OsbZIP58 is detectable in endosperm during active starch synthesis in rice seeds. T-DNA insertion mutants and RNAi transgenic plants of OsbZIP58 displayed abnormal seed morphology with altered starch accumulation in white belly region, with decreased amount of total starch and amylose. Moreover, the osbzip58 null mutants had a higher proportion of short chains, but a lower proportion of intermediate chains of amylopectin. OsbZIP58 was shown to directly bind to and regulate the expression of starch synthesizing genes including OsAGPL3, Wx, OsSSIIa, OsSSIIIa, OsSSIVb, SBE1. This finding suggests that OsbZIP58 functions as a key genetic regulator in starch synthesis in rice seeds, providing an insight into seed quality control in this agricultural important crop.

Key words: OsbZIP58, coordinate, starch biosynthesis, endosperm, rice