Presence and function of small RNAs during plant reproduction

Plants possess an expansive suite of epigenetic control mechanisms to keep their large, repetitive, and unruly genomes in check. Small non-coding RNAs constitute one such mechanism by directing transcriptional and post-transcriptional gene silencing. Small RNAs come in several forms that are produced by distinct pathways, including microRNAs and small interfering RNAs, such as tasiRNAs, phasiRNAs, and RNA Polymerase IV (Pol IV)-dependent small RNAs. Compared to other classes of small RNAs, Pol IV-dependent small RNAs and phasiRNAs appear to have the greatest phenotypic impact during the reproductive phase of the life cycle, from meiosis through seed development. Here, we focus on the function of 21-24 nucleotide small RNAs during reproduction, with a primary focus on Pol IV-dependent small RNAs. We critically assess the evidence surrounding their mode of action, specifically whether they act in cis or in trans, cell autonomously or non-autonomously, and within or between generations.