Rice gene, OsCKX2-2, regulates inflorescence and grain size by increasing endogenous cytokinin content

Panicle size is one of the yield determining traits in rice (Oryza sativa L.). In the current study, a rice mutant named cytokinin oxidase/dehydrogenase (Osckx2-2) was developed from the indica cultivar 9311 by ethyl methanesulfonate (EMS) mutagenesis. A comparison of agronomic traits showed that the panicle of the Osckx2-2 mutant was longer with dense branching pattern and higher seed setting rate compared to the wild-type (WT). In addition, the grains in the Osckx2-2 mutant were markedly longer, wider, and heavier compared to the WT. Histological analysis revealed that a higher number of the spikelet cells with bigger size resulted in larger grains in the Osckx2-2 mutant. By map-based cloning, SNP was detected in the 3′-UTR of the OsCKX2-2 gene which encodes cytokinin dehydrogenase and the cleaved amplified polymorphic sequence (CAPS) marker was developed based on the detected SNP. The RT-qPCR analysis displayed a decreased OsCKX2-2 expression in the Osckx2-2 mutant panicles. Meanwhile, the enzymatic activity assay showed the reduced cytokinin dehydrogenase enzyme activity, whereas the enzyme-linked immunosorbent assay (ELISA) revealed remarkably higher endogenous cytokinin content in the Osckx2-2 mutant in contrast to WT. The analysis of grain quality parameters showed that the degree of chalkiness and gel consistency decreased, while the amylose content and alkaline spreading value increased in the Osckx2-2 mutant. The OsCKX2-2 gene and Osckx2-2 mutant would be beneficial in marker-assisted backcrossing to improve yield in rice.