Abstract
In plants, small non-coding RNAs (≈20–30 nt) play a major role in a gene regulation mechanism that controls development, maintains heterochromatin and defends against viruses. However, their possible role in cell division (mitosis and meiosis) still remains to be ascertained. ARGONAUTE (AGO) proteins are key players in the different small RNA (sRNA) pathways. Arabidopsis contains 10 AGO proteins belonging to three distinct phylogenetic clades based on amino acid sequence, namely: AGO1/AGO5/AGO10, AGO2/AGO3/AGO7, and AGO4/AGO6/AGO8/AGO9. To gain new insights into the role of AGO proteins, we have focused our attention on AGO2, AGO5, and AGO9 by means of the analysis of plants carrying mutations in the corresponding genes. AGO2 plays a role in the natural cis-antisense (nat-siRNA) pathway and is required for an efficient DNA repair. On the other hand, AGO5, involved in miRNA (microRNA)-directed target cleavage, and AGO9, involved in RNA-directed DNA methylation (RdDM), are highly enriched in germline. On these grounds, we have analyzed the effects of these proteins on the meiotic process and also on DNA repair. It was confirmed that AGO2 is involved in DNA repair. In ago2-1 the mean cell chiasma frequency in pollen mother cells (PMCs) was increased relative to the wild-type (WT). ago5-4 showed a delay in germination time and a slight decrease in fertility, however the meiotic process and chiasma levels were normal. Meiosis in PMCs of ago9-1 was characterized by a high frequency of chromosome interlocks from pachytene to metaphase I, but chiasma frequency and fertility were normal. Genotoxicity assays have confirmed that AGO9 is also involved in somatic DNA repair.
Highlights
ARGONAUTE (AGO) proteins are essential players in the different small RNA pathways. sRNAs are non-coding RNAs, about 20–30 nt in length, involved in regulating gene expression
To gain new insights into the role of AGO proteins, we have focused our attention on AGO2, AGO5, and AGO9 by means of the analysis of plants carrying mutations in the corresponding genes
There are three classes of sRNAs: microRNAs, which are genome-encoded and derived from imperfectly folded stem-loop structures of single stranded RNA precursors; small interfering RNAs, which derive from long double-stranded RNA precursors; and PIWI-interacting RNAs which are animal germline specific (Castel and Martienssen, 2013)
Summary
ARGONAUTE (AGO) proteins are essential players in the different small RNA (sRNA) pathways. sRNAs are non-coding RNAs, about 20–30 nt in length, involved in regulating gene expression. In Arabidopsis there are three classes of siRNAs: repeated associated siRNAs (ra-siRNAs), involved in RNA-directed DNA methylation pathway (RdDM) and acting at TGS level; natural antisense siRNAs (nat-siRNAs), derived from natural antisense transcription and involved in PTGS; and trans-acting siRNAs (ta-siRNAs) which act at PTGS level These sRNAs are firstly processed from longer RNA precursors by a member of the DICER RNase III like endonuclease family, that cleaves the long dsRNAs to generate small dsRNAs about 20–30 nt (Matzke and Birchler, 2005). A possible role of sRNAs during plant meiosis is only exemplified by rice MEIOSIS ARRESTED AT LEPTOTENE1 (MEL1, related to Arabidopsis AGO5), and maize AGO104 (related to Arabidopsis AGO9) The former regulates cell division of premeiotic germ cells and the faithful progression of meiosis, but not their initiation and establishment (Nonomura et al, 2007). We have included ago and ago mutants in the analysis because AGO3 (At1g31290) and AGO8 (At5g21030) are recent duplications of AGO2 and AGO9, respectively
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