The CC-NB-LRR-type Rdg2a Resistance Gene Evolved Through Recombination and Confers Immunity to the Seed-Borne Barley Leaf Stripe Pathogen in the Absence of Hypersensitive Cell Death

Abstract

Leaf stripe disease on barley is caused by the seed-transmitted hemi-biotrophic fungus Pyrenophora graminea. Race-specific resistance to leaf stripe is controlled by two known Rdg (resistance to Drechslera graminea) genes: the H. spontaneum-derived Rdg1a, mapped to chromosome 2HL and Rdg2a, identified in H. vulgare, mapped on chromosome 7HS. Both resistance genes have been extensively used in classical breeding. The positional cloning and molecular characterization of the Rdg2a locus is described here. BAC and cosmid libraries, respectively, derived from barley cvs. Morex (susceptible to leaf stripe) and Thibaut (the donor of the Rdg2a allele) were used for physical mapping of Rdg2a. At the Rdg2a locus, three sequence-related coiled-coil, nucleotide-binding site and leucine-rich repeat (CC-NB-LRR) encoding genes were identified. Sequence comparisons suggested that paralogs of this resistance locus evolved through recent gene duplication and were subjected to frequent sequence exchange. Transformation of the leaf stripe susceptible cv. Golden Promise with two Rdg2a candidates identified a member of the CC-NB-LRR gene family that conferred resistance against the Dg2 leaf stripe isolate, towards which the Rdg2a gene is effective. Histological analysis demonstrated that Rdg2a-mediated leaf stripe resistance prevents pathogen colonisation in the embryos without any detectable hypersensitive cell death response, indicating an unusual resistance mechanism for a CC-NB-LRR protein.