CONICET | Buscador de Institutos y Recursos Humanos

Legume plants form two types of root lateral organs, nodules and lateral roots (LRs), which serves for nitrogen fixation and water/nutrient uptake, respectively. Root nodules originate as the result of the symbiotic interaction with nitrogen-fixing bacteria, known as rhizobia. In the last years, microRNAs (miRNAs) have emerged as post-transcriptional regulators of gene expression that play essential roles during the development of new organs. miR390 targets the non-coding transcript TAS3, promoting TAS3 cleavage by Argonaute 7 (AGO7) and triggering the production of trans-acting small interference RNAs (tasiRNAs). In turn, these TAS3 derived-tasiRNAs (known as tasiARFs) control the stability of transcripts encoding the Auxin Response Factors ARF2, ARF3 and ARF4. Expression analysis in M. truncatula indicated that levels of miR390, as well as tasiARFs, decreased at early stages of the symbiotic interaction. Overexpression of the miR390 precursor (Ox390) promoted elongation of emerged LRs, but decreased the number of nodules and infection events. On the other hand, expression of a target mimicry of the miR390 (MIM390), which significantly reduced tasiARFs production, increased nodule number and altered the morphology and distribution of the nodules. A similar phenotype was observed in M. truncatula ago7 mutants, which produced null levels of tasiARFs. These results suggest that the miR390/TAS3 pathway is involved in the regulation of LR growth, nodule development and rhizobial infection in M. truncatula. Promoter studies, as well as the expression of miR390 and tasiARFs GFP sensors, would allow further characterization of this pathway at cellular-or tissue-type level during the establishment of this agronomical important symbiosis.