ZmSSRP1, transactivated by OPAQUE11, positively regulates starch biosynthesis in maize endosperm

Starch synthesis is crucial for crop yield and quality. This study reveals an O11-ZmSSRP1 module of kernel starch biosynthesis in maize (Zea mays). We identify STARCH SYNTHESIS REGULATING PROTEIN 1 (ZmSSRP1) positively regulates amylose and amylopectin-dependent starch synthesis in maize endosperm. ZmSSRP1 encodes a highly conserved plastid-localized protein that is highly expressed in developing endosperm. Loss-of-function zmssrp1 mutants created by CRISPR/Cas9 exhibit much smaller starch granules and significantly reduced amylose and amylopectin synthesis, while overexpression of ZmSSRP1 causes larger starch granules and an increased amylose and amylopectin accumulation. RNA-seq analysis revealed that the expression levels of several genes involved in the synthesis of amylose and amylopectin were significantly down-regulated in the zmssrp1 mutant compared to the wild-type. Furthermore, we found that OPAQUE11 (O11), a core transcription factor essential for maize endosperm development and nutrient metabolism, transactivates the expression of ZmSSRP1 by binding to its promoter, and functions upstream of ZmSSRP1 in the regulatory pathway governing starch synthesis in the maize endosperm. The present work demonstrates that the O11-ZmSSRP1 module positively regulates starch synthesis in maize kernels, potentially paving the way for future genetic improvements of maize quality.