Multiple roles for small RNAs during plant reproduction

Abstract

Germline development and early embryogenesis in eukaryotes are characterized by large-scale genome reprogramming events. In companion cells of the Arabidopsis male gametophyte, epigenome reorganization leads to loss of heterochromatin and production of a distinct small RNA (sRNA) population. A specific class of sRNA derived from transposons appears to be mobile and can accumulate in germ cells. In the germline of maize, rice, and Arabidopsis, specific ARGONAUTE-sRNA silencing complexes appear to play key roles in reproductive development, including meiosis and regulation of germ cell fate. These results reveal new roles for sRNAs during plant reproduction and suggest that mobility of sRNAs could be critical for some of these functions.

Figure 1. Overview of small RNA (sRNA) functions during different stages of reproduction in Arabidopsis

In the ovules, sRNAs-AGO9 complexes suppress gametogenesis in the somatic cells surrounding the megaspore mother cell. Reactivated transposable elements (TEs) are presumably posttranscriptionally regulated during meiosis. In pollen, reactivation of certain TEs in the VN leads to the production of TE-derived sRNAs that likely move to the sperm cells. Before and after fertilization, maternal expression of TEs leads results in sRNAs that can silence paternal TEs in the endosperm and perhaps also TEs in the embryo. The antipodal cells and synergids of the seven-celled embryo sac are shown in dark- and light grey, respectively. TE = Transposable elements. CC = Central cell. EC = Egg cell. VN = Vegetative nuclei. SC = Sperm cell. PG = Paternal genome. MG = Maternal genome.

Figure 2. Source to sink movement of mobile silencing signals in Arabidopsis

A mobile signal can move from shoot to root (white arrows) or from root to shoot (red arrows). It remains to be determined whether the silencing signal can travel from root to the male or the female germline (inset).