Conserved hinge regions in SYMRK enable release of Malectin-like Domain for symbiont passage during rhizobia–legume symbiosis

Symbiosis Receptor Kinase (SYMRK), a malectin-like-domain/leucine-rich-repeat receptor-like-kinase (MLD-LRR-RLK), is the upstream most component in the Common-Symbiosis-Signalling-Pathway. We highlight two Proline residues that were distinctly acquired by SYMRK orthologues in its hinge-regions to constitute a signalling module for allowing progress of symbionts across transcellular barriers during rhizobia-legume symbiosis. Within the Ectodomain hinge (EctoD-hinge) all MLD-LRR-RLKs have a conserved W1xnGDPCxnW2x4C motif, where SYMRK orthologues within legumes have a distinct signature with a Proline preceding W2 enabling cleavage of SYMRK ectodomain for releasing MLD. Within the kinase hinge (KD-hinge) at gatekeeper+1 position, a conserved Glutamate in MLD-LRR-RLKs is replaced by Proline in all SYMRK orthologues that enabled its dual-specific kinase activity for ensuring ectodomain cleavage. Substitution of either Proline restricted cortical progression of symbionts forming infection patches in the nodule apex without affecting epidermal invasion and nodule organogenesis. This halt was entirely overcome by ectopic expression of free MLD demonstrating the released MLD to have an active role in progress of symbionts. Overall, we show that conservation of distinct Prolines in hinge-regions of SYMRK orthologues in legumes generates a signalling module involving dimerization and optimal phosphorylation of SYMRK for releasing MLD as an active transducer of symbiosis signalling.