Using a combination of high-throughput sequencing, northern blotting, and primary microRNA transcript expression analysis, we have identified members of five MIRNA families in L. japonicus with strong expression in symbiotic nodules: miR167, miR172,miR390, miR397, and the miR171 isoform lja-miR171c.To dissect whether these miRNAs are potentially involved in processes related to either symbiotic bacterial infection or nodule organogenesis, we made use of the availability of L. japonicus snf mutants spontaneously producing uninfected nodule organs but generating wild-type-like symbiotic nodules in the presence of M. loti. A comparison of symbiotic and uninfected snf mutant nodules showed that, although strongly
expressed in infected nodules, levels of miR171c and miR397 were low in nodule organs devoid of the symbiont M. loti. As spontaneous and infected nodules are indistinguishable in terms of developmental and morphological characteristics (Tirichine et al.,2006b), these results strongly suggest that enhanced expression of both miR171c and miR397 in nodules is specifically linked to the presence of the bacterial symbiont.