Identification of Fertility Genes Required for Microgametogenesis in Rice

The process of microgametogenesis occurs within the developing pollen. It depends on two rounds of meiosis of microspore, and sporophitic functions provided by the surrounding anther tissues. Employing our rice T-DNA insertional mutant library, we identified three mutants exhibit a phenotype of completely sterile compared with their wild types. Two genes, PAIR3 and OsRPA1a, play essential roles in DNA metabolism during meiosis process; and another gene OsAPI5, is required for tapetal cell degenenation and pollen development. PAIR3 encodes a protein that contains putative coiled-coil motifs, and plays a crucial role in homologous chromosome pairing and synapsis in meiosis. OsRPA1a plays an essential role in DNA repair but may not participate in, or at least is dispensable for, DNA replication and homologous recombination in rice. Recently, we examined the role of OsAPI5, a homolog of animal antiapoptosis proteins, in the degeneration of the tapetum during the formation of male gametophytes in rice. Mutation in OsAPI5 results in delayed degeneration of the tapetum due to inhibition of the tapetal programmed cell death process leading to defects in formation of male gametophytes. OsAPI5 is a nuclear protein that interacts with two DEAD-box ATP-dependent RNA helicases, AIP1 and AIP2 (for API5 INTERACTING PROTEIN 1 AND 2). OsAIP1 and AIP2 can form dimers and interact directly with the promoter region of CP1, a rice cysteine protease gene. Our results uncover a previously unknown pathway for regulating programmed cell death during tapetum degeneration in rice, one that may be conserved among eukaryotic organisms.