The Coiled-Coil NLR Rph1, Confers Leaf Rust Resistance in Barley Cultivar Sudan.

Unraveling and exploiting mechanisms of disease resistance in cereal crops is currently limited by their large repeat-rich genomes and the lack of genetic recombination or cultivar (cv)-specific sequence information. We cloned the first leaf rust resistance gene Rph1 (Rph1 a) from cultivated barley (Hordeum vulgare) using "MutChromSeq," a recently developed molecular genomics tool for the rapid cloning of genes in plants. Marker-trait association in the CI 9214/Stirling doubled haploid population mapped Rph1 to the short arm of chromosome 2H in a physical region of 1.3 megabases relative to the barley cv Morex reference assembly. A sodium azide mutant population in cv Sudan was generated and 10 mutants were confirmed by progeny-testing. Flow-sorted 2H chromosomes from Sudan (wild type) and six of the mutants were sequenced and compared to identify candidate genes for the Rph1 locus. MutChromSeq identified a single gene candidate encoding a coiled-coil nucleotide binding site Leucine-rich repeat (NLR) receptor protein that was altered in three different mutants. Further Sanger sequencing confirmed all three mutations and identified an additional two independent mutations within the same candidate gene. Phylogenetic analysis determined that Rph1 clustered separately from all previously cloned NLRs from the Triticeae and displayed highest sequence similarity (89%) with a homolog of the Arabidopsis (Arabidopsis thaliana) disease resistance protein 1 protein in Triticum urartu In this study we determined the molecular basis for Rph1-mediated resistance in cultivated barley enabling varietal improvement through diagnostic marker design, gene editing, and gene stacking technologies.

Exploring and exploiting the boundaries of host specificity using the cereal rust and mildew models.

Individual plants encounter a vast number of microbes including bacteria, viruses, fungi and oomycetes through their growth cycle, yet few of these pathogens are able to infect them. Plant species have diverged over millions of years, co-evolving with few specific pathogens. The host boundaries of most pathogen species can be clearly defined. In general, the greater the genetic divergence from the preferred host, the less likely that pathogen would be able to infect that plant species. Co-evolution and divergence also occur within pathogen species, leading to highly specialized subspecies with narrow host ranges. For example, cereal rust and mildew pathogens (Puccinia and Blumeria spp.) display high host specificity as a result of ongoing co-evolution with a narrow range of grass species. In rare cases, however, some plant species are in a transition from host to nonhost or are intermediate hosts (near nonhost). Barley was reported as a useful model for genetic and molecular studies of nonhost resistance due to rare susceptibility to numerous heterologous rust and mildew fungi. This review evaluates host specificity in numerous Puccinia/Blumeria-cereal pathosystems and discusses various approaches for transferring nonhost resistance (NHR) genes between crop species to reduce the impact of important diseases in food production.