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Discovery of the rice gene promoting germ cells to enter meiosis.
Press Release PLoS Genetics 7(1): e1001265.
Nonomura Laboratory, Experimental Farm, Kurata Laboratory, Plant Genetics
A novel RNA-recognition-motif protein is required for premeiotic G1/S-phase transition in rice ( Oryza sativa L.)
Ken-Ichi Nonomura, Mitsugu Eiguchi, Mutsuko Nakano, Kazuya Takashima, Norio Komeda, Satoshi Fukuchi, Saori Miyazaki, Akio Miyao, Hirohiko Hirochika, Nori Kurata
PLoS Genetics 7(1): e1001265. DOI: 10.1371/journal.pgen.1001265

  Meiosis is a pivotal event to produce haploid spores and gametes in all sexually reproducing species, and a fundamentally different type of cell cycle from mitosis. Flowering plants have evolved an intricate network of regulatory mechanisms to ensure proper timing of the transition to flowering (Fig. 1). In addition, to achieve simultaneous fertilization within a limited season, the timing of meiotic entry is also strictly regulated. In yeast and metazoans, RNA-binding proteins are known to play important roles in the post-transcriptional regulation of genes implicated in the meiotic entry and meiosis. In contrast, in the plant kingdom, the mechanisms to control the meiotic entry have largely remained elusive.

  In this study, we discover a novel RNA-recognition-motif (RRM) protein in rice ( Oryza sativa L.), designated MEL2, and demonstrate that MEL2 is required for the faithful transition of germ cells from mitosis to meiotic cell cycle (Fig. 2). Rice MEL2 shows partial similarity with human DAZAP1, which is an RRM protein and relates to Azoospermia syndrome in human, while there are critical structural differences between germline-specific RRM proteins of mammals and plants. Our findings will lead the molecular-biological studies of plant meiotic entry to the next steps, and enable to compare the systems of meiotic entry between animals and plants.

Fig. 1. The synchronous formation of pollens is attributed to the synchronous progression of male meioses in the anther of angiosperm.
Fig. 2. Germ cells in the mel2 mutant are unable to transit to meiosis.
(A, B) The longitudinal section of rice anthers of the wild-type (A) and mel2 mutant (B). The MEL1 gene, which encodes an Argonaute protein and is expressed in premeiotic germ cells (Nonomura et al. 2007), is down-regulated in wild-type meiocytes (open arrow). In mel2 germ-cells at the same stage with panel A, the MEL1 is strongly expressed (closed arrow).
(C, D) The germ cell nuclei of the wild-type (C) and mel2 mutant (D). The rice meiotic protein PAIR2, which is required for synaptonemal complex formation (Nonomura et al. 2006), was stained immunofluorescently (magenta), and chromosomes were counter-stained with DAPI (blue). In the wild type, a filamentous structure of PAIR2 is clearly observed, inidcating that the formation of synaptonemal comprex is normally initiated. In mel2 germ-cells at the same stage with panel C, filamentous PAIR2 structure is barely observed.