Genetic dissection of CLE12 and CLE13 small peptide-mediated autoregulation of nodulation in pea

Legume root nodulation with nitrogen-fixing bacteria requires precise control via root-shoot-root autoregulation of nodulation (AON). Post-translationally modified root-derived CLAVATA3/Embryo Surrounding Region-Related (CLE) peptides signal through shoot acting leucine-rich repeat receptors (CLAVATAs) to regulate nodule number, and this pathway is a target to optimise nodulation. We characterise the AON system in the crop model pea (Pisum sativum L.) and address key gaps in our understanding of AON; the role of parallel signalling pathways, shoot receptor complexes and downstream targets. We use novel mutant combinations, overexpression, grafting, gene expression and careful analysis of infection and nodule organogenesis using GFP-labelled rhizobium. These studies provide evidence that, in pea, both PsCLE12 and PsCLE13 require arabinosylation via PsRDN1. Perception of PsCLE12 and PsCLE13 in the shoot to suppress the mature nodules in the root requires the pea CLAVATA1 orthologue PsNARK and PsCLV2. However, we found little evidence that PsCLE12 and PsCLE13 suppress infection thread development or that they act via PsTML1 and/or PsTML2, root acting suppressors of nodulation, indicating a role for additional CLE signals. Grafting and double mutant studies suggest that PsNARK may act together with PsCLV2, but also independently, to influence nodulation, providing in planta support for shoot receptor complexes that control AON.