Insights into the function of the phasiRNA-triggering miR1514 in response to stress in legumes.

We recently described the activity of miR1514a in response to water deficit in Phaseolus vulgaris. Pvu-miR1514a targets a NAC transcription factor mRNA for cleavage and subsequently triggers NAC-derived phasiRNA formation. Here we show that accumulation and activity of miR1514a are also conserved in the model legume Medicago truncatula. Consistently, we identified Mtr-miR1514a and detected its increased accumulation in response to stress conditions, targeting a NAC TF mRNA for cleavage and triggering phasiRNA production. In P. vulgaris, miR1514a inhibition in transgenic hairy roots was reported to increase NAC 700 mRNA levels and to affect expression patterns of several genes, including that of a Sec 14 homolog. We report here that in adult plant roots exposed to dehydration conditions, where miR1514a levels increased and NAC 700 mRNA decreased, there was a reduction of Sec 14 homolog mRNA levels, suggesting a direct transcriptional effect. The functions of miR1514a, NAC 700 and derived phasiRNAs have just begun to be elucidated in common bean; future understanding of their activities in this and other legumes species will advance our knowledge of microRNA functions in plants.