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

BMC Plant Biology (Impact Factor: 3.81). 03/2013; 13(1):43. DOI: 10.1186/1471-2229-13-43
Source: PubMed


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.

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.

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.

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    • "Some alleles of the Rhg gene product encode hybrid enzymes being fused translationally to a kinase domain (Fig. 10). The proteins appear to dimerize in hetero-and homodimers (Afzal et al., 2013). In theory this might explain the temperature dependence of the resistance reaction (Palmateer et al., 2000). "
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    ABSTRACT: Novel tools to improve resistance to sudden death syndrome (SDS) and the underlying Fusarium root rot (FRR) caused by Fusarium virguliforme (Aoki) have been developed for soybean [Glycine max (L.) Merr.]. Eighteen resistance loci have been identified and confirmed over the past two decades (named Rfs1to Rfs18). To select the beneficial alleles of 8 to 10 loci per cross needed for optimal resistance is a difficult task for plant breeders. Resistance mechanisms to FRR provide only partial protection. Crops contend with many Fusaria, a group with a wide host range and flexible hemibiotrophic lifestyle. Full resistance is absent among the leguminacea, brassicacea, cucurbitacea, and solanacea. This review focuses on the use of plant genomics resources to aid breeding selection for resistance to SDS. The SDS is a combination of two diseases. The first includes rotted roots and toxin-restricted root development. Resistances include variations in infection severity, infection frequency, and rot severity. The second is caused by toxins translocated from infected roots to the shoots. Leaf scorch, supra-petiolar abscission, pod abortion and early plant maturity are consequences of many toxin to target interactions. Breeding for combined FRR and SDS resistance has begun using a set of exciting new tools for pathogen quantification in roots. Resistance genes were proven, including GmRLK18-1 (Glyma_18_02680) Rfs2, and MIPs1a (EC Rfs3. The new tools provide an opportunity for new breeding initiatives. This review aims to inform these new programs of the core discoveries from the past 20 yr, to incorporate best practices from old and new initiatives.
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