A Significant Fraction of 21-Nucleotide Small RNA Originates from Phased Degradation of Resistance Genes in Several Perennial Species

Small RNAs (sRNAs), including microRNA (miRNA) and short-interfering RNA (siRNA), are important in the regulation of diverse biological processes. Comparative studies of sRNAs from plants have mainly focused on miRNA, even though they constitute a mere fraction of the total sRNA diversity. In this study, we report results from an in-depth analysis of the sRNA population from the conifer spruce (Picea abies) and compared the results with those of a range of plant species. The vast majority of sRNA sequences in spruce can be assigned to 21-nucleotide-long siRNA sequences, of which a large fraction originate from the degradation of transcribed sequences related to nucleotide-binding site-leucine-rich repeat-type resistance genes. Over 90% of all genes predicted to contain either a Toll/interleukin-1 receptor or nucleotide-binding site domain showed evidence of siRNA degradation. The data further suggest that this phased degradation of resistance-related genes is initiated from miRNA-guided cleavage, often by an abundant 22-nucleotide miRNA. Comparative analysis over a range of plant species revealed a huge variation in the abundance of this phenomenon. The process seemed to be virtually absent in several species, including Arabidopsis (Arabidopsis thaliana), rice (Oryza sativa), and nonvascular plants, while particularly high frequencies were observed in spruce, grape (Vitis vinifera), and poplar (Populus trichocarpa). This divergent pattern might reflect a mechanism to limit runaway transcription of these genes in species with rapidly expanding nucleotide-binding site-leucine-rich repeat gene families. Alternatively, it might reflect variation in a counter-counter defense mechanism between plant species.