Transactivation of <i>AGPL2</i> by Opaque2 is responsible for starch accumulation and kernel hardness in sweet corn

Kernel hardness is an important indicator of quality in sweet corn (Zea mays subsp. saccharata), but the mechanism underlying its regulation remains poorly understood. In this study, we determined that the starch and protein composition of the endosperm was positively correlated with kernel hardness. AGPL2, encoding a subunit of AGPase, was differentially expressed in the endosperm of different cultivars and was responsible for residual AGPase activity and starch biosynthesis. We found that Opaque2 (O2), a key regulatory transcriptional factor for endosperm development, directly binds to the promoter of AGPL2 and activates its expression, resulting in the accumulation of starch. Taken together, our results reveal a novel mechanism whereby the O2-AGPL2 module participates in regulation of kernel hardness in sweet corn.