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Editor-in-Chief of The Plant Cell

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 and rice demonstrated the temporal and spatial distribution of two sets of “reproductive phasiRNAs”, which are extraordinarily enriched in the male germline of the grasses. These two sets are the 21-nt (pre-meiotic) and 24-nt (meiotic) siRNAs. Both classes are produced from long, non-coding RNAs, generated by hundreds to thousands of loci, depending on the species.  These phased siRNAs show striking similarity to mammalian piRNAs in terms of their abundance, distribution, distinctive staging, and timing of accumulation, but they have independent evolutionary origins. The functions for these small RNAs in plants remain poorly characterized. In monocots, the 24-nt phasiRNA pathway, triggered by miR2275 and abundant during meiosis, requires a recently-diverged Dicer known as DCL5, an interesting evolutionary elaboration of this pathway. I will describe our recent work investigating the functions of plant phasiRNAs, their diversification across crop plants including in eudicots, and their roles in modulating traits of agronomic importance in plants, including male fertility. I will also discuss how we are implementing new imaging techniques for visualization of these RNAs and their precursors at a single-molecule level.