A cryptic inhibitor of cytokinin phosphorelay controls rice grain size

Plant hormone cytokinin signals through histidine-aspartic acid (H-D) phosphorelay to regulate plant growth and development. While it is well known that the phosphorelay involves histidine kinases, histidine phosphotransfer proteins (HPs), and response regulators (RRs), how this process is regulated by external components remains unknown. Here we demonstrate that protein phosphatase with kelch-like domains (PPKL1), known as a signaling component of steroid hormone brassinosteroid, is actually a cryptic inhibitor of cytokinin phosphorelay in rice (Oryza sativa). Mutation at a specific amino acid D364 of PPKL1 activates cytokinin response and thus enlarges grain size in a semi-dominant mutant named s48. Overexpression of PPKL1 containing D364, either with the deletion of the phosphatase domain or not, rescues the s48 mutant phenotype. PPKL1 interacts with OsAHP2, one of authentic HPs, and D364 resides in a region resembling the receiver domain of RRs. Accordingly, PPKL1 can utilize D364 to suppress OsAHP2-to-RR phosphorelay, whereas mutation of D364 abolishes the effect. This function of PPKL1 is independent of the phosphatase domain that is required for brassinosteroid signaling. Importantly, editing of the D364-residential region produces a diversity of semi-dominant mutations associated with variously increased grain sizes. Further screening of the edited plants enables the identification of two genotypes that confer significantly improved grain yield. Collectively, our study uncovers a noncanonical cytokinin signaling suppressor and provides a robust tool for seed rational design.