Stem rust caused by Puccinia graminis f. sp. tritici was among the most devastating diseases of barley in the northern Great Plains of the U.S. and Canada before the deployment of the stem rust-resistance gene Rpg1 in 1942. Since then, Rpg1 has provided durable protection against stem rust losses in widely grown barley cultivars (cvs.). Extensive efforts to clone Rpg1 by synteny with rice provided excellent flanking markers but failed to yield the gene because it does not seem to exist in rice. Here we report the map-based cloning and characterization of Rpg1. A high-resolution genetic map constructed with 8,518 gametes and a 330-kb bacterial artificial chromosome contig physical map positioned the gene between two crossovers 0.21 centimorgan and 110 kb apart. The region including Rpg1 was searched for potential candidate genes by sequencing low-copy probes. Two receptor kinase-like genes were identified. The candidate gene alleles were sequenced from resistant and susceptible cvs. Only one of the candidate genes showed a pattern of apparently functional gene structure in the resistant cvs. and defective gene structure in the susceptible cvs. identifying it as the Rpg1 gene. Rpg1 encodes a receptor kinase-like protein with two tandem protein kinase domains, a novel structure for a plant disease-resistance gene. Thus, it may represent a new class of plant resistance genes.
Seedling Spot Blotch Resisance Gene, Rcs5
The Rcs5 gene confers seedling resistance to spot blotch caused by the fungus Cochliobolus sativus. Spot blotch is a common and economically important foliar disease of barley in the Midwestern United States. Genetic mapping localized the resistance gene between the markers MWG622 and KAJ154 on the short arm of chromosome 1(7H). A BAC clone physical contig was constructed consisting of 4 clones 053N3, 612G14, 452P9, and 808M17. The BAC clones 612G14 and 808M17 were sequenced by the Arizona Genomics Institute. Sequence assembly resulted in 25 contigs ranging from 1kb to 28.5kb for 808M17 and 7 contigs ranging from 0.6kb to 63.7kb for 612G14. The assembly of 612G14 and 808M17 left an unsequenced gap of ~15kb within the Rcs5 region, which was filled by sequencing the NotI subclone, TBD001. The resulting sequence was analyzed by a gene prediction program (FGENESH), which yielded 40 putative genes, 22 with protein homology. Of the 22 protein hits, 19 had Triticeae EST's in the NCBI database. Five genes are predicted within the sequence contig covering the Rcs5 region: a lipase-like gene, an FBOX domain, an expressed gene with unknown homology, a ribosomal-like protein, and a p450-monooxygenase. The latter gene, a p450-monooxygenase, may be important in detoxifying infected tissue since C. sativus is known to produce several toxic compounds that may play an important role in infection or pathogenesis. The five genes were hybridized to arrayed cDNA libraries and are being analyzed for expression and gene structure between resistant and susceptible cultivars.
The barley stem rust-resistance gene Rpg1 is a novel disease-resistance gene with homology to receptor kinases.
R. Brueggeman, N. Rostoks, D. Kudrna, A. Kilian, F. Han, J. Chen, A. Druka, B. Steffenson, and A. Kleinhofs
Genetically engineered stem rust resistance in barley using the Rpg1 gene.
Henriette Horvath, Nils Rostoks, Robert Brueggeman, Brian Steffenson, Diter von Wettstein and Andris Kleinhofs
