Functional Characterization of OsRac6 Involved in Pollen Germination in Rice

For a successful reproduction in angiosperms, pollen germination and pollen tube growth must proceed normally. The processes are regulated by transcriptional regulation and cellular activities, in which signaling proteins are the key. Ras homolog (Rho)-type GTPases (Rops), which are known as RACs, are a group of Rho small GTP-binding proteins which evolved in plants. However, the molecular mechanisms of the rice Rac family (OsRac) genes in pollen germination are not yet understood. In this investigation, the OsRac6 gene, which is only expressed only during the pollen development stage, was identified. A functional characterization of OsRac6 using gene-edited mutants exhibits male sterility due to the disruption of pollen germination. Then, to determine downstream candidate genes modulated by the OsRac6 genes during pollen development, we performed RNA-seq analysis by comparing osrac6-1 and wild-type (WT) anthers. Subsequently, we identified 652 genes with pollen-preferential expression which were upregulated compared with the WT, and 608 genes which were downregulated. Furthermore, gene ontology (GO) enrichment and MapMan analyses show that downregulated genes are closely associated with transcription, protein ubiquitination, proteolysis, cell wall modification, cell organization, cell cycle, and calcium regulation. The results of this study will help in elucidating the molecular mechanisms of pollen germination in rice mediated by OsRac6.