OsMAPK20-1 and OsMAPK20-4 phosphorylate OsCK2β3 to regulate its stability in response to phosphate starvation in rice

Phosphorus is crucial for plant growth. Rice (Oryza sativa) OsCK2, composed of the catalytic subunit OsCK2α3 and the regulatory subunit OsCK2β3, plays a role in regulating phosphate (Pi) homeostasis. The function of OsCK2 activity depends on phosphorylation of OsCK2β3. We show that the plant-specific N-terminal domain in OsCK2β3, rather than the conserved β subunit domain, is responsible for its stability. Both OsMAPK20-1 and OsMAPK20-4 interact with OsCK2β3 and phosphorylate it at Ser-45 in the plant-specific N-terminal domain. Mutation of either OsMAPK20-1 or OsMAPK20-4 leads to more Pi accumulation in rice, while overexpression of the constitutively active MAPKK1 variant, OsMEK1DD, decreases the rice Pi concentration. Moreover, phospho-mimicry in OsCK2β3 increases its stability and enhances its interaction with OsCK2α3 to form the OsCK2α3/β3 holoenzyme. Overexpression of phosphorylatable-mimicking forms of OsCK2β3 reduces Pi levels even under Pi-deficient conditions. Finally, we verify that overexpression of OsMEK1DD inhibits the trafficking of the Pi transporter targeting to the plasma membrane, similar to the effect of OsCK2β3. Collectively, our results suggest that OsMAPK20-1 and OsMAPK20-4 phosphorylate OsCK2β3 at Ser-45 in the plant-specific N-terminal domain to maintain Pi homeostasis in response to the Pi supply in rice.