Field Analysis of SDS Resistance in Soybean Transgenic with the RLK from Rhg1 and Rfs2

Soybean (Glycine max (L. Merr.)) resistance to any population of  Heterodera glycines (I.), or Fusarium virguliforme (Akoi, O’Donnell, Homma & Lattanzi) required a functional allele at Rhg1/Rfs2. H. glycines, the soybean cyst nematode (SCN) was an ancient, endemic, pest of soybean whereas F. virguliforme causal agent of sudden death syndrome (SDS), was a recent, regional, pest. This study examined the role of a receptor like kinase (RLK) GmRLK18-1 (gene model Glyma_18_02680 at 1,071 kbp on chromosome 18 of the genome sequence) within the Rhg1/Rfs2 locus in causing resistance to SCN and SDS. A BAC (B73p06) encompassing all the DNA from Peking introgressed into Forrest and so the Rhg1/Rfs2  locus was sequenced from a resistant cultivar and compared to the sequences of two susceptible cultivars from which 800 SNPs were found but only 18 SNPs between susceptible cultivars. Sequence alignments inferred that the resistance allele was an introgressed region of about 59 kbp at the center of which the GmRLK18-1 was the most polymorphic gene and encoded protein. Analyses of  plants that were either heterozygous at or transgenic and so hemizygous with the resistance allele of GmRLK18-1 at a new location were made. Those plants infested with either H. glycines or F. virguliforme showed that the allele for resistance was dominant. In the absence of Rhg4 the RLK was sufficient to confer nearly complete resistance to both root and leaf symptoms of SDS caused by F.virguliforme and provided partial resistance to three different populations of nematodes (mature female cysts were reduced by 30-50%). In the presence of Rhg4 the plants with the transgene were nearly classed as fully resistant to SCN as well as SDS (females reduced to 11% of the susceptible control). A reduction in the rate of early seedling root development was also shown to be caused by the resistance allele of the GmRLK18-1.  Field trials of transgenic plants in 2010, 2011, 2012 and 2013 showed an increase in resistance to SDS, an increase in resistance to SCN. The inference that soybean has adapted part of an existing pathogen recognition and defense cascade (H.glycines; SCN and insect herbivory)  to a new pathogen (F. virguliforme; SDS) was supported by immunolocalization and ligand binding of the protein. Therefore there are broad implications for crop improvement. Stable resistance to many pathogens might be achieved by manipulation the genes encoding a small number of pathogen recognition proteins.