Since their discovery and characterization, small non-coding RNAs have emerged as key post-transcriptional regulators of plant gene expression. The presence and activity of small RNA pathways have principally been described in the sporophyte whereas events in the gametophyte remain largely unexplored – which is unexpected in view of the pivotal role of small RNAs in animal germline development. Small non-coding RNAs can be roughly divided into two main categories dependent on their origin; microRNAs (miRNAs) are produced from a single-stranded RNA that base-pairs to form a hairpin structure, whereas short interfering RNAs (siRNAs) result from the processing of longer double-stranded RNAs. The miRNA pathway has been well-characterized and plays a variety of developmental roles during the plant life-cycle through the regulation of a wide range of specific transcripts. By contrast, a range of different siRNAs pathways coexist in the plant cell, some dedicated to the regulation of specific transcripts, while others are employed as signals to guide DNA methylation (RNA-directed DNA methylation or RdDM) at specific locations of the genome, generally enriched in repeated sequences and transposable elements (TEs).
Although first discovered in sporophytic tissues, information on small non-coding RNAs is starting to emerge from the study of male and female gametophyte development. Data on the presence, synthesis, and activity of non-coding RNAs in angiosperm gametophytes are reviewed in a new paper from the University of Leicester, with a focus on events accompanying sporogenesis and gametogenesis (pollen development) in the model plant Arabidopsis thaliana.
Small non-coding RNAs are key post-transcriptional and transcriptional regulators of plant gene expression in angiosperm sporophytes. In recent years, gametophytic small RNAs have also been investigated, predominantly in Arabidopsis male gametophytes, revealing features in common with the sporophyte as well as some surprising differences. Transcriptomic and deep-sequencing studies confirm that multiple small RNA pathways operate in male gametophytes, with over 100 miRNAs detected throughout development. Trans-acting siRNA pathways that are associated with novel phased transcripts in pollen, and the nat-siRNA pathway have important roles in pollen maturation and gamete function. Moreover, a role for siRNA-triggered silencing of transposable elements in male and female germ cells has been established, a feature in common with the role of piRNAs in animal germlines. Current evidence supports an integral role for small RNAs in angiosperm gametophyte development and it can be anticipated that novel small RNAs with significant roles in germline development and genome integrity await discovery.