Editing of rice (Oryza sativa L.) OsMKK3 gene using CRISPR/Cas9 decreases grain length by modulating the expression of photosystem components

Grain size is one of the most important agronomic traits for grain yield determination in rice. To better understand the proteins that are regulated by the grain size regulatory gene OsMKK3, this gene was knocked out using the CRISPR/Cas9 system, and tandem mass tag (TMT) labeling combined with liquid chromatograph-tandem mass spectrometry analysis was performed to study the regulation of proteins in the panicle. Quantitative proteomic screening revealed a total of 106 differentially expressed proteins (DEPs) via comparison of the OsMKK3 mutant line to the wild-type YexiangB, including 15 and 91 up-regulated and down-regulated DEPs, respectively. Pathway analysis revealed that DEPs were enriched in metabolic pathways, biosynthesis of secondary metabolites, phenylpropanoid biosynthesis, and photosynthesis. Strong interactions were detected among seven down-regulated proteins related to photosystem components in the protein-protein interaction network, and photosynthetic rate was decreased in mutant plants. The results of the liquid chromatography-parallel reaction monitoring/mass spectromery analysis and western blot analysis were consistent with the results of the proteomic analysis, and the results of the quantitative reverse transcription polymerase chain reaction analysis revealed that the expression levels of most candidate genes were consistent with protein levels. Overall, OsMKK3 controls grain size by regulating the protein content in cells. Our findings provide new candidate genes that will aid the study of grain size regulatory mechanisms associated with the mitogen-activated protein kinase (MAPK) signaling pathway.