Small RNA pathways responsible for non-cell-autonomous regulation of plant reproduction.

In angiosperms, germline precursors and germ cells are always attached to or engulfed within somatic companion cells until just before fertilization. This is because sperm and egg cells develop as part of the multicellular gametophyte. Thus, the non-cell-autonomous regulation by somatic companions plays important roles in efficient reproduction, in addition to the cell-autonomous regulation. Epigenetic silencing of transposable elements is one of the central events by which the germline transmits the error-free genome to the next generation. This review focuses on small RNA-mediated epigenetic regulation of meiosis, spore formation and pollen development. Besides microRNA (miRNA) and small interfering RNA (siRNA), animals express PIWI-interacting RNA (piRNA), a germline-specific class of small RNAs. Plants lack piRNA-like RNAs and, instead, express unique classes of small RNAs: trans-acting siRNA (tasiRNA) and phased secondary siRNA (phasiRNA). Especially in grass species, 21- and 24-nucleotide phasiRNAs are abundant in anthers during premeiosis and meiosis. This review also describes recent progress in reproductive phasiRNA research.