Five Beta-Expansin Genes Sharing Common Expression Patterns are Redundantly Involved in Pollen Tube Growth in Rice (Oryza sativa)

The transfer of sperm cells to the egg cell during pollen germination and pollen tube (PT) growth is an essential process for successful reproduction in higher plants. In this process, proper cell wall assembly and remodeling is important for the normal growth of PTs. The functions of members of the Beta(β)-Expansin (EXPB) family, which encode proteins that loosen cell walls, remain poorly understood. Here, we performed a meta-expression analysis of all OsEXPBs sourced from anatomical samples comprising 22 tissues and/or organs. We identified five pollen-preferred OsEXPBs (i.e., OsEXPB1a, OsEXPB1b, OsEXPB9, OsEXPB10, and OsEXPB13). We also identified gene duplication events that specifically involved pollen-preferred OsEXPBs. Subcellular localization of the OsEXPB proteins was found to match well with their roles as cell wall loosening factors, and these were also visible when in transit through the secretory pathway. Further functional characterization of OsEXPBs using a gene editing system for all five targets after removing probable redundancy revealed that a quintuple expb1a;1b;9;10;13 mutant was sterile due to defects in PT elongation. Taken together, the results of our study suggest that the role of five pollen-preferred OsEXPBs that share common expression patterns is important for normal PT growth in rice.