miR397, the second miRNA found in this work to be specifically enhanced in abundance in infected nodules, shows an elevated expression in late nodule developmental stages (28 dpi). This is in contrast to miR171c, which retains stable expression levels as nodules undergo maturation, indicating a possible link between nodule functionality, or even the onset of
nodule senescence, and miR397 expression. The analysis of inefficient interactions of L. japonicus mutant sst1 (Krusell et al., 2005) and wild-type plants infected with the inefficient bacterial symbiont Bradyrhizobium sp. (Bek et al., 2010) confirmed that the accumulation of miR397 in both nodules and aerial plant parts correlates with the progression of a functional N2-fixing symbiosis. miR397 thus serves as a systemic marker for the presence of functional nodules in L. japonicus.