Rpp2 Encodes a TIR-NBS-WH-LRR Protein That Confers Resistance to Phakopsora pachyrhizi in Soybean

The obligate biotrophic fungus Phakopsora pachyrhizi Syd. & P. Syd., the causal agent of soybean rust, is among the most formidable pathogens of soybean (Glycine max [L.] Merr.). The pathogen is now established in all major soybean growing areas of the world and presents a significant impediment to global soybean production. Most soybean germplasm is susceptible, enabling the fungus to penetrate and colonize the leaf tissue, causing tan-colored necrotic lesions to form at the site of infection. Severe infection reduces photosynthesis and causes premature defoliation, which ultimately decreases crop yield and seed quality. Eight genetic loci, Rpp1/Rpp1b to Rpp7 and Rpp6907, that confer race-specific resistance to P. pachyrhizi (Rpp) have been identified. Rpp2 was identified and characterized in the soybean accession PI 230970 and fine-mapped to a 188.1 kb interval on chromosome 16, a region predicted to contain several toll/interleukin-1 receptor nucleotide-binding leucine-rich repeat (TIR-NLR) genes. To identify Rpp2, we constructed a bacterial artificial chromosome (BAC) library from the resistant soybean accession PI 230970. Sequencing BACs that span the Rpp2 locus identified fourteen candidate genes with homology to the TIR-NLR family of resistance genes with integrated winged-helix (WH) domains. Of these, seven are predicted to encode full-length R proteins. Co-silencing the Rpp2 candidate genes compromised resistance in soybean accession PI 230970. Gene expression analysis suggests that a single gene, Rpp2C7_PI, which shares greatest homology to Rpp2C6_Wms82 (Glyma.16G136600) in the Williams 82 reference genome, is responsible for Rpp2-mediated resistance.