A Germ Cell–Specific Gene of theARGONAUTEFamily Is Essential for the Progression of Premeiotic Mitosis and Meiosis during Sporogenesis in Rice

The rice (Oryza sativa) genome contains 18 copies of genes of the ARGONAUTE (AGO) family. Although AGO members play important roles in RNA-mediated silencing during plant development, a family member that is specifically involved in sexual reproduction has not been identified in plants. We identified the rice AGO gene MEIOSIS ARRESTED AT LEPTOTENE1 (MEL1) from the analysis of seed-sterile mutants. In the mel1 mutant, chromosome condensation was arrested at early meiotic stages and irregularly sized, multinucleated, and vacuolated pollen mother cells (PMCs) frequently appeared in developing anthers. In addition, histone H3 lysine-9 dimethylation of pericentromeres was rarely reduced and modification of the nucleolar-organizing region was altered in mel1 mutant PMCs. The mutation also affected female germ cell development. These results indicate that the germ cell–specific rice MEL1 gene regulates the cell division of premeiotic germ cells, the proper modification of meiotic chromosomes, and the faithful progression of meiosis, probably via small RNA–mediated gene silencing, but not the initiation and establishment of germ cells themselves.