Small RNA Pathways and Their Function in the Male Gametophyte

Abstract

In animals, small RNAs are essential for reproductive development. Without the activity of members of the Piwi class of the Argonaute protein family, and their associated small RNAs, the formation and maintenance of the germline, and gametogenesis, cannot take place correctly. Reproductive development in plants is more complex than in animals; instead of the haploid products of meiosis leading directly to gametes, further mitotic divisions contribute to the formation of a separate haploid generation called the gametophyte. As might be expected, the formation of the gametophyte and gametogenesis also relies on small RNA systems. In plants, almost all of the information so far gathered on the gametophyte has come from studies of the male gametophyte (pollen). Here, recent studies have revealed all the families of small RNAs known from the somatic cells in the diploid sporophyte – microRNAs, trans-acting siRNAs, natural antisense siRNAs and siRNAs to be involved in RNA-dependent DNA methylation. Given the apparent simplicity of the development of the male gametophyte, microRNAs and tasiRNAs are unexpectedly diverse. As in animals, plants use small RNA systems to control transposable element activity in the germline. There is also recent evidence that a specific regulatory module of a natural antisense gene pair, that spawns nat-siRNAs in the sperm cells, has a key role in fertilisation.