Identification and Characterization of the Net Form Net Blotch Susceptibility Locus on Barley Chromosome 6H

Net form net blotch (NFNB), caused by the necrotrophic fungal pathogen Pyrenophora teres f. teres (Ptt), is a destructive foliar disease of barley. Resistance to Ptt has been mapped in diverse barley lines from around the world and many of the reported resistance QTL lie near the centromere on barley chromosome 6H. The recessive resistance genes rpt.r and rpt.k conferring resistance to NFNB isolates 15A and 6A in barley lines Rika and Kombar, respectively, were previously mapped to the chromosome 6H region. A Rika × Kombar high-resolution map was constructed by screening 2,976 recombinant gametes and the rpt.r/rpt.k region was saturated with STS markers designed from barley ESTs identified by synteny analysis with Brachypodium distachyon. This high-resolution mapping delimited the rpt.r/rpt.k locus to ~0.43 cM. Within the delimited region, two genes from the barley cultivar Morex, designated rpt.cg1 and rpt.cg2, were found to have high homology to a leucine rich repeat (LRR) gene family containing a predicted N-terminal transmembrane domain and an imperfect LRR near the C-terminus. Allele analysis of 45 diverse barley cultivars showed a direct correlation between susceptibility to NFNB isolates 6A and 15A and the presence of Rika and Kombar alleles of candidates Spt1 and Spt2 (previously known as rpt.k and rpt.r, respectively). Preliminary expression and mutant analysis supports the hypothesis that these genes are strong Spt1 and Spt2 candidates. Our hypothesis is that Spt1 and Spt2 genes act as dominant susceptibility factors that interact with necrotrophic effectors in an inverse gene-for-gene manner.