Grupo de Desarrollo Reproductivo y Apomixis, Laboratorio Nacional de Genómica para la Biodiversidad y Departamento de Ingeniería Genética de Plantas, Cinvestav Irapuato CP36500 Guanajuato, México.
Nature. 2010 Mar 25;464(7288):628-32. doi: 10.1038/nature08828. Epub 2010 Mar 7.
In the ovules of most sexual flowering plants female gametogenesis is initiated from a single surviving gametic cell, the functional megaspore, formed after meiosis of the somatically derived megaspore mother cell (MMC). Because some mutants and certain sexual species exhibit more than one MMC, and many others are able to form gametes without meiosis (by apomixis), it has been suggested that somatic cells in the ovule are competent to respond to a local signal likely to have an important function in determination. Here we show that the Arabidopsis protein ARGONAUTE 9 (AGO9) controls female gamete formation by restricting the specification of gametophyte precursors in a dosage-dependent, non-cell-autonomous manner. Mutations in AGO9 lead to the differentiation of multiple gametic cells that are able to initiate gametogenesis. The AGO9 protein is not expressed in the gamete lineage; instead, it is expressed in cytoplasmic foci of somatic companion cells. Mutations in SUPPRESSOR OF GENE SILENCING 3 and RNA-DEPENDENT RNA POLYMERASE 6 exhibit an identical defect to ago9 mutants, indicating that the movement of small RNA (sRNAs) silencing out of somatic companion cells is necessary for controlling the specification of gametic cells. AGO9 preferentially interacts with 24-nucleotide sRNAs derived from transposable elements (TEs), and its activity is necessary to silence TEs in female gametes and their accessory cells. Our results show that AGO9-dependent sRNA silencing is crucial to specify cell fate in the Arabidopsis ovule, and that epigenetic reprogramming in companion cells is necessary for sRNA-dependent silencing in plant gametes.
在大多数有性开花植物的胚珠中，雌性配子体发生是从一个经过减数分裂的体细胞来源的大孢子母细胞（MMC）产生的一个存活的配子细胞，即功能大孢子开始的。因为一些突变体和某些有性物种表现出不止一个 MMC，而且许多其他物种能够不经过减数分裂（无融合生殖）形成配子，所以有人认为胚珠中的体细胞有能力响应局部信号，这个信号可能在决定中具有重要作用。在这里，我们表明，拟南芥蛋白 ARGONAUTE 9（AGO9）通过剂量依赖的、非细胞自主的方式限制配子体前体的特化来控制雌性配子体的形成。AGO9 突变导致多个能够起始配子发生的配子细胞的分化。AGO9 蛋白不在配子系中表达；相反，它在体细胞伴胞的细胞质焦点中表达。SUPPRESSOR OF GENE SILENCING 3 和 RNA-DEPENDENT RNA POLYMERASE 6 的突变与 ago9 突变体表现出相同的缺陷，这表明小 RNA（sRNAs）沉默的运动从体细胞伴胞中移出对于控制配子细胞的特化是必要的。AGO9 优先与来自转座元件（TEs）的 24 个核苷酸的 sRNAs 相互作用，并且其活性对于在雌性配子及其附属细胞中沉默 TEs 是必需的。我们的结果表明，AGO9 依赖的 sRNA 沉默对于在拟南芥胚珠中指定细胞命运至关重要，并且伴胞中的表观遗传重编程对于植物配子中 sRNA 依赖性沉默是必要的。
