Phosphorylation of KRP3, a KIP-related protein by MPK3 modulates rice tiller and seed number by fine-tuning cell division

During the cell cycle process, multiple inhibitors function as checkpoint regulators ensuring an impeccable duplication of genetic material. Kip-Related Proteins (KRPs) are a group of plant-specific cell cycle inhibitors and are known to regulate plant architecture and yield by differentially regulating cell division. KRPs, though functionally conserved, are dynamic in their amino acid composition. Interestingly, some KRPs are specific to dicots, and some are for monocots. In this study, we have identified that the presence of KRP3 and KRP6 is specific to the Poaceae family of monocots. An in-depth study of KRP3 showed a strict regulation of its expression in actively dividing cells during the G1-S phase progression of cell division. Our study identified KRP3 as a phosphorylation target of MPK3. The phosphorylation enhances KRP3 protein stability, which leads to strong inhibition of cell proliferation. By generating knock-out lines of krp3, mpk3 and double knock-out krp3mpk3, our study demonstrated that the MPK3-KRP3 module regulates rice root and shoot development as well as tiller and seed numbers. It was observed that KRP3 regulate the rate of cell division in a dose-dependent manner. The study, in a nutshell, establishes the role of KRP3 as an important regulator of rice vigor and yield.