Subunit 3 of the SUPERKILLER complex mediates microRNA172-directed cleavage of <i>Nodule Number Control 1</i> in <i>Medicago truncatula</i>

Legumes and rhizobia establish a nitrogen-fixing symbiosis that involves the formation of a lateral root organ, the nodule, and the infection process that allows intracellular accommodation of rhizobia within nodule cells. This process involves substantial gene expression changes regulated at the transcriptional and post-transcriptional levels. We have previously shown that a transcript encoding subunit 3 of the SUPERKILLER Complex (SKI), which guides mRNAs to the exosome for 3´-to-5´ degradation, is required for nodule formation and bacterial persistence within the nodule, as well as the induction of early nodulation genes, including early nodulin40 (MtENOD40), during the Medicago truncatula-Sinorhizobium meliloti symbiosis. Here, we reveal through transcript degradome and small RNA sequencing analysis that knockdown of MtSKI3 impairs the miR172-directed endonucleolytic cleavage of the mRNA encoding Nodule Number Control 1 (MtNNC1), an APETALA2 transcription factor that negatively modulates nodulation. Knockdown of MtNNC1 enhances nodule number, bacterial infection, and the induction of MtENOD40 upon inoculation with S. meliloti, whereas overexpression of an miR172-resistant form of MtNNC1 significantly reduces nodule formation. This work identifies miR172 cleavage of MtNNC1 and its control by MtSKI3, a component of the 3´-to-5´mRNA degradation pathway, as a regulatory hub controlling indeterminate nodulation.