ZmMKK4, a mitogen-activated protein kinase kinase, influences maize kernel size

Maize kernel size is regulated by multiple genetic pathways, including the evolutionarily conserved mitogen-activated protein kinase (MAPK) cascade, which plays crucial roles in plant growth and development. However, the function of MAPKK kinases in maize kernel development remains poorly understood. Here, we identified mitogen-activated protein kinase kinase 4 (ZmMKK4) as a positive regulator of kernel development in maize. ZmMKK4 is localized primarily in the nucleus and cytoplasm and shows high expression levels across multiple tissues, including kernels. Knockout mutants of zmmkk4 resulted in significantly reduced kernel size and weight, accompanied by severe impairments in kernel development. Furthermore, we observed a decrease in starch content accompanied by an increase in protein content in the mutant kernels. In summary, our study elucidates the role of ZmMKK4 in promoting kernel size and modulating starch and protein accumulation in maize, providing important insights into the molecular mechanisms underlying kernel development. These findings offer a theoretical foundation and a candidate gene for further research into maize kernel development and molecular breeding strategies. • ZmMKK4 functions as a positive regulator of maize kernel size. • The zmmkk4 gene knockout mutant has a decreased starch content and an increased protein content. • A candidate gene was provided that can be used for molecular marker-assisted selection or gene editing.