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Systemic regulation of nodulation and root development

Team SILEG / Florian Frugier


Forward genetic screens allowed us to identify mutants affected both in root and nodule development, referred to as compact root architecture (cra ; Laffont et al., 2010).

One of these mutants, cra2 , affects antagonistically nodule and lateral root development and is defective in a CLAVATA1-like LRR-RLK (Huault et al., 2014). Graft experiments showed that the regulation of symbiotic nodulation was determined by the activity of the receptor in shoots, implying a systemic regulation of root nodulation. The MtCRA2-dependent pathway is required for the action of C-terminally Encoded Peptides 1 (MtCEP1) signaling peptides, either to regulate positively nodulation and negatively lateral root formation (Mohd-Radzman et al., 2016). This CEP/CRA2 systemic positive regulation of nodulation acts independently from another systemic negative pathway previously identified, and named autoregulation of nodulation, relying on the Super Numeric Nodules" (SUNN) receptor and on CLAVATA3-like (CLE) signaling peptides (Laffont et al., 2019). More recently, we established that a coordination between these two systemic pathways exists to regulate dynamically the number of nodules formed on the root system depending on environmental conditions, first through the regulation of a common effector module involving a microRNA, the miR2111, produced in shoots and acting in roots on TML (Too Much Love) 1 and TML2 targets regulating negatively nodulation (Gautrat et al., 2019; Gautrat et al., 2020); and second, by the coordinated production of specific CEP and CLE signaling peptides depending on environmental conditions, and notably of MtCEP7 and MtCLE13 in response to rhizobium or Nod factors through the cytokinin/CRE1 receptor pathway and the NIN transcription factor (Laffont et al., 2020).

A working model for a dynamic fine tuning of nodulation by SUNN and CRA2 systemic pathways. CRA2, Compact Root Architecture 2 LRR-RLK ; SUNN, Super Numeric Nodules LRR-RLK ; CLE12, CLE13 : CLAVATA3-like peptides 12 and 13 ; CEP1, C-terminally Encoded Peptides 1 ; AON, Autoregulation Of Nodulation.