Phased, secondary siRNAs in plant reproduction

In plants, 21 or 22-nt miRNAs or siRNAs typically negatively regulate target genes through mRNA cleavage or translational inhibition. Heterochromatic or Pol IV are 24-nt and function to maintain heterochromatin and silence transposons. Phased “secondary” siRNAs (phasiRNAs) are generated from mRNAs targeted by a typically 22-nt “trigger” miRNA and are produced as either 21- or 24-mers via distinct pathways. Our prior work in maize, wheat, rice and other flowering plants demonstrates the temporal and spatial distribution of up to three sets of “reproductive phasiRNAs”, which are extraordinarily enriched in the male germline of many of these species. These include 21-nt and 24-nt siRNAs abundant in pre-meiotic and meiotic anthers, support tapetal and meiocyte development. Both classes are produced from long, non-coding RNAs, generated by hundreds to thousands of loci, depending on the species.  The functions for these small RNAs in plants remain poorly characterized. I will describe our recent work investigating the biogenesis, evolution, and functions of plant phasiRNAs and their roles in modulating male fertility, primarily centered on our work in grasses.