Overexpression of ZmbZIP22 gene alters endosperm starch content and composition in maize and rice.

Starch content and composition are major determinants of yield and quality in maize. In recent years, the major genes for starch metabolism have been cloned in this species. However, the role of transcription factors in regulating the starch metabolism pathway remains unclear. The ZmbZIP22 gene encodes a bZIP transcription factor. In our study, plants overexpressing ZmbZIP22 showed reductions in the size of starch granules, the size and weight of seeds, reduced amylose content, and alterations in the chemical structure of starch granules. Also, overexpression of ZmbZIP22 resulted in increases in the contents of soluble sugars and reducing sugars in transgenic rice and maize. ZmbZIP22 promotes the transcription of starch metabolism genes by binding to their promoters. Screening by yeast one-hybrid assays indicated a possible interaction between ZmbZIP22 and the promoters of eight key starch enzyme genes. Collectively, our results indicated that ZmbZIP22 functions as a negative regulator of starch synthesis, and suggest that this occurs through the regulation of key sugar and starch metabolism genes in maize.