The <scp>ZmCOP1s</scp> – <scp>ZmCOL3</scp> Module Enhances Late Flowering, Grain Yield and Grain Quality in Maize

Flowering time is a key determinant of yield and regional adaptation in crops and is largely controlled by light signalling. In this study, we identified two maize orthologs of Arabidopsis CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1), designated ZmCOP1a and ZmCOP1b, which activate light signalling and reduce plant height. Loss-of-function mutants of ZmCOP1a and ZmCOP1b flowered early, whereas overexpression of either gene significantly delayed flowering under both long- and short-day conditions. Mechanistically, both ZmCOP1a and ZmCOP1b interacted with and stabilised the C2C2-CO-like transcription factor ZmCOL3. Together, they upregulated key repressors of flowering (ZmPRR37, ZmPRR95, ZmCCA1 and ZmCCT9), leading to suppression of florigen genes (ZCN7, ZCN8 and ZCN12) and floral meristem identity genes (ZMM3, ZMM4 and ZMM15). Field trials demonstrated that overexpression of ZmCOP1a or ZmCOP1b reduced starch content but increased protein levels, kernel numbers and kernel length, width and weight. In contrast, overexpression of ZmCOL3 decreased both starch and protein content. Collectively, our results demonstrate that the ZmCOP1s-ZmCOL3 module is a key regulator of flowering time and source-sink reallocation, providing valuable genetic targets for breeding high-yield, high-quality maize with optimised flowering time and enhanced carbon and nitrogen allocation.