Global identification of key genes for pollen germination in rice through high-throughput screening and gene editing.

Abstract

Successful reproduction depends on the stable germination and growth of the pollen tubes (PT). However, the molecular mechanisms involved in rice PT growth and development remain largely unknown. In a previous study, microarray transcriptome analysis identified 627 genes preferentially expressed in the tricellular and germinating pollen of rice (i.e., Oryza sativa ssp. japonica). To elucidate key genes involved in the gene transfer process facilitated by male gametophytes, we systematically screened T-DNA lines containing disrupted sequences that corresponded to these 627 genes and analyzed the genotypes of heterozygote progeny from 107 T-DNA-indexed lines covering 105 genes. We found that 42 lines exhibited a distorted segregation ratio among the wild-type (WT), heterozygote (HT), and homozygote (HM) genotypes, which deviated from the expected Mendelian ratio of 1:2:1 (WT:HT:HM). Further characterization using CRISPR/Cas9 mutants revealed that knockout mutants of certain genes that exhibited segregation distortion in the T-DNA insertion region were completely sterile. Moreover, even when T-DNA insertion lines followed Mendelian segregation patterns, sterility could be induced by simultaneously mutating functionally redundant genes, thereby overcoming genetic compensation. Interestingly, although some T-DNA insertion lines exhibited segregation ratios approximating 1:1:0, the corresponding CRISPR/Cas9 mutants produced homozygous seeds and showed partial sterility. Partial sterility suggests that despite mutant pollen grains being less competitive than WT pollen, they retain their fertilization potential under relaxed competition from WT pollen. Beyond mutant-based analysis, transcriptomic profiling of sterile mutant lines provided additional insight into the regulatory relationship between key germination regulators and the 105 target genes studied here. Overall, this study demonstrates the effectiveness of a multi-pronged strategy to accelerate the identification of defective phenotypes using mutant studies and provides valuable genetic resources for inducing novel male sterility in rice.