The Leucine-Rich Repeat Domain At RHG1/RFS2 Underlying Resistance to Two Soybean Pathogens: Biochemical and Genetic Opportunities for Crop Improvement.

The protein encoded by GmRLK18-1 (Glyma_18_02680 on chromosome 18) was a receptor like kinase (RLK) encoded within the soybean (Glycine max L. Merr.) Rhg1/Rfs2 locus. The gene underlies part of resistance to the soybean cyst nematode (SCN) Heterodera glycines (I.) and causal agent of sudden death syndrome (SDS) Fusarium virguliforme (Aoki) in transgenic plants suggesting the protein was involved in pathogen sensing.  The aims here were to evaluate the LRRs ability to; homo-dimerize; bind larger proteins; and bind to small CLE  peptides. Western analysis suggested homo-dimers could form after protein extraction from root membranes. Equally, the purified LRR domain, from residue 131-485, formed a mixture of monomers and homo-dimers in vitro. Cross-linking experiments in vitro showed the H274N region was close (<11.1 A) to the highly conserved cysteine residue C196 on the second homo-dimer subunit. Binding constants of 20-142 nM for nine different CLE peptides found in plant and nematode secretions. Effects on plant phenotypes including wilting, stem bending and resistance to infection by SCN were observed when roots were treated with 50 pM of the peptides. Far-Western analyses followed by MS showed methionine synthase and cyclophilin also bound strongly to the LRR domain. A second LRR from GmRLK08-1 (Rhg4 region) did not show these strong interactions.suggesting the RHG1 LRR domain was integrating several specific signals associated with pathogen egress. The avidly bound CLE peptides GmTGIF, GmCLE34,  GmCLE3 and HgCLE were involved in root growth inhibition and here were also shown to alter stem morphology and resistance to SCN.