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Open Access Research article

Homo-dimerization and ligand binding by the leucine-rich repeat domain at RHG1/RFS2 underlying resistance to two soybean pathogens

Ahmed J Afzal12, Ali Srour12, Abhishek Goil12, Sheeja Vasudaven13, Tianyun Liu4, Ram Samudrala4, Navneet Dogra5, Punit Kohli5, Ayan Malakar12 and David A Lightfoot12*

Author Affiliations

1 Department of Molecular Biology, Microbiology and Biochemistry and Center for Excellence the Illinois Soybean Center, Southern Illinois University at Carbondale, IL 62901, USA

2 Genomics Core Facility; Department of Plant Soil and Agricultural Systems, Southern Illinois University at Carbondale, Carbondale, IL 62901-4415, USA

3 Department of Molecular Biology and Biochemistry, University of California at Irvine, Irvine, CA 92697-4560, USA

4 Department of Microbiology Box 357242, University of Washington, Seattle, WA 98195-7242, USA

5 Department of Chemistry, Southern Illinois University at Carbondale, IL 62901, USA

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BMC Plant Biology 2013, 13:43  doi:10.1186/1471-2229-13-43

Published: 15 March 2013

Abstract

Background

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 locus underlies resistance to the soybean cyst nematode (SCN) Heterodera glycines (I.) and causal agent of sudden death syndrome (SDS) Fusarium virguliforme (Aoki). Previously the leucine rich repeat (LRR) domain was expressed in Escherichia coli.

Results

The aims here were to evaluate the LRRs ability to; homo-dimerize; bind larger proteins; and bind to small peptides. Western analysis suggested homo-dimers could form after protein extraction from roots. The purified LRR domain, from residue 131–485, was seen to form 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 peptides found in plant and nematode secretions were found. 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 bound strongly to the LRR domain. A second LRR from GmRLK08-1 (Glyma_08_g11350) did not show these strong interactions.

Conclusions

The LRR domain of the GmRLK18-1 protein formed both a monomer and a homo-dimer. The LRR domain bound avidly to 4 different CLE peptides, a cyclophilin and a methionine synthase. The CLE peptides GmTGIF, GmCLE34, GmCLE3 and HgCLE were previously reported to be involved in root growth inhibition but here GmTGIF and HgCLE were shown to alter stem morphology and resistance to SCN. One of several models from homology and ab-initio modeling was partially validated by cross-linking. The effect of the 3 amino acid replacements present among RLK allotypes, A87V, Q115K and H274N were predicted to alter domain stability and function. Therefore, the LRR domain of GmRLK18-1 might underlie both root development and disease resistance in soybean and provide an avenue to develop new variants and ligands that might promote reduced losses to SCN.

Keywords:
Receptor; Leucine-rich repeat; Ligand; Peptide; Cross-link; Predicted