The CK1-Opaque2 module orchestrates endosperm filling and nutrient storage in maize seeds

Phosphorylation is one of the major posttranslational modifications to control plant growth and development. Opaque2 (O2) represents a central hub for endosperm filling, which largely determines seed yield and nutrient storage in maize. However, it still remains unclear how O2 phosphorylation orchestrates endosperm filling and nutrient quality. Here, we systematically identified the phosphorylation sites of O2 during endosperm filling. A total of 18 phosphorylation sites were found in O2 and five sites were identified to apparently modulate its subcellular localization and transactivation capacity. In addition, a conserved protein kinase CK1 was confirmed to interact with and phosphorylate O2 at the residue Threonine (T)202 to promote O2-mediated transactivation and protein stability. Overexpression of CK1 resulted in increased kernel size, 100-kernel weight and nutrient storage. Phosphorylation-mimic O2 seeds at T202 exhibited enhanced kernel dimension, test weight, vitreous endosperm area and nutrient accumulation, whereas the phosphorylation-deficient O2 seeds did not. Collectively, this study establishes a comprehensive phosphocode atlas of O2 during endosperm filling and highlights the importance of phosphorylation modification in O2 to precisely orchestrate maize yield and nutrient quality.