Abstract
RNA interference (RNAi) is a mechanism that regulates genes by either transcriptional (TGS) or posttranscriptional gene silencing (PTGS), required for genome maintenance and proper development of an organism. Small non-coding RNAs are the key players in RNAi and have been intensively studied in eukaryotes. In plants, several classes of small RNAs with specific sizes and dedicated functions have evolved. The major classes of small RNAs include microRNAs (miRNAs) and small interfering RNAs (siRNAs), which differ in their biogenesis. miRNAs are synthesized from a short hairpin structure while siRNAs are derived from long double-stranded RNAs (dsRNA). Both miRNA and siRNAs control the expression of cognate target RNAs by binding to reverse complementary sequences mediating cleavage or translational inhibition of the target RNA. They also act on the DNA and cause epigenetic changes such as DNA methylation and histone modifications. In the last years, the analysis of plant RNAi pathways was extended to the bryophyte Physcomitrella patens, a non-flowering, non-vascular ancient land plant that diverged from the lineage of seed plants approximately 450 million years ago. Based on a number of characteristic features and its phylogenetic key position in land plant evolution P. patens emerged as a plant model species to address basic as well as applied topics in plant biology. Here we summarize the current knowledge on the role of RNAi in P. patens that shows functional overlap with RNAi pathways from seed plants, and also unique features specific to this species.
Highlights
Small non-coding RNAs have been increasingly investigated as important regulators of gene expression
P. patens ∆PpDCL1a, ∆PpRDR6 and ∆PpDCL4 mutants lack ta-small interfering RNAs (siRNAs) production based on specific functions of the respective proteins in ta-siRNA biogenesis: the absence of ta-siRNAs in the PpDCL1a mutant is due to the lack of the specific miRNAs that are required to initiate the ta-siRNA pathway by the cleavage of the respective TAS precursors, PpRDR6 the essential enzyme catalysing the conversion of cleaved precursors into double-stranded RNAs (dsRNA) while
Another conserved miRNA-mediated feedback was reported for AGO1 mRNAs in A. thaliana and P. patens, AGO1 is a key protein in RNA-induced silencing complex (RISC) and is required for miRNA directed cleavage of target transcripts [89,150]
Summary
Small non-coding RNAs have been increasingly investigated as important regulators of gene expression. They were first discovered in the nematode Caenorhabditis elegans [6] and are responsible for the phenomenon known as RNAi, co-suppression, gene silencing, or quelling [7,8,9,10] After these reports were published, it was shown that PTGS in plants is correlated with the activity of small RNAs [11]. Besides the analysis of P. patens small RNA pathways, molecular tools were developed exploiting the mode of action of small RNAs for the down-regulation of genes in reverse genetics applications. These approaches include the use of conventional inverted RNAi constructs [24,25] as well as the expression of highly specific artificial miRNAs [26]. These findings underline that P. patens serves as a valuable model system to study the evolution, diversity, and complexity of plant RNAi pathways
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