A module involving the cyclin-dependent kinase OsCDKF3 regulates grain size in rice.

Grain size is a key agronomic trait determining grain yields in crops. However, the molecular mechanisms of grain size regulation are still not fully understood. Here, we identified a cyclin-dependent kinase OsCDKF3 that controls grain development in rice (Oryza sativa). Knocking out OsCDKF3 decreased grain size and thousand grain weight. Moreover, we found that OsMPK6 interacts with and phosphorylates OsCDKF3 at Tyr22. Phosphorylation of Tyr22 contributes to the stability and function of OsCDKF3 in grain size regulation. Consistent with this, the phosphomimic OsCDKF3Y22D but not the dephosphomimic OsCDKF3Y22A complemented the grain defects of the oscdkf3 mutant, and knocking down of OsMPK6 also reduced grain size. In addition, OsSCL7, a grain size negative regulator, inhibited the autophosphorylation activity and promoted the degradation of OsCDKF3, as well as suppressed the interaction between OsCDKF3 and OsMPK6. Conversely, OsMPK6 interacted with OsSCL7 to suppress its transcriptional activity. Taken together, our findings revealed the crucial role of the OsMPK6-OsSCL7-OsCDKF3 module in fine regulation of grain size, providing insights into the important function of CDKs in grain development.