Arbuscular mycorrhizal (AM) symbiosis associates most plants with fungi of the phylum Glomeromycota. The fungus penetrates into roots and forms within cortical cell branched structures called arbuscules for nutrient exchange. We discovered that miR171b has a mismatched cleavage site and is unable to downregulate the miR171 family target gene, LOM1 (LOST MERISTEMS 1). This mismatched cleavage site is conserved among plants that establish AM symbiosis, but not in non-mycotrophic plants. Unlike other members of the miR171 family, miR171b stimulates AM symbiosis and is expressed specifically in root cells that contain arbuscules. MiR171b protects LOM1 from negative regulation by other miR171 family members. These findings uncover a unique mechanism of positive post-transcriptional regulation of gene expression by miRNAs and demonstrate its relevance for the establishment of AM symbiosis.

Dyed roots reveal inner complexity Plant roots do so much more than just hold a plant up. As a site for air storage during flooding, mycorrhizal symbiosis, or carbohydrate storage, the more complex root can tap more complicated functions. Taking advantage of a dye that stains less the deeper it penetrates the tissue, Ortiz-Ramírez et al . applied fluorescence-activated cell sorting to the complex cell layers of the maize root. RNA sequencing applied to the single-cell pools defined a developmental map and showed that the mobile transcription factor SHORT-ROOT travels through multiple cell layers and directs this grass root’s anatomical complexity. —PJH