Components of Brachypodium distachyon resistance to nonadapted wheat stripe rust pathogens are simply inherited

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Abstract
Phytopathogens have a limited range of host plant species that they can successfully parasitise ie. that they are adapted for. Infection of plants by nonadapted pathogens often results in an active resistance response that is relatively poorly characterised because phenotypic variation in this response often does not exist within a plant species, or is too subtle for genetic dissection. In addition, complex polygenic inheritance often underlies these resistance phenotypes and mutagenesis often does not impact upon this resistance, presumably due to genetic or mechanistic redundancy. Here it is demonstrated that phenotypic differences in the resistance response of Brachypodium distachyon to the nonadapted wheat stripe rust pathogen Puccinia striiformis f. sp. tritici (Pst) are genetically tractable and simply inherited. Two dominant loci were identified on B. distachyon chromosome 4 that each reduce attempted Pst colonisation compared with sib and parent lines without these loci. One locus (Yrr1) is effective against diverse Australian Pst isolates and present in two B. distachyon mapping families as a conserved region that was reduced to 5 candidate genes by fine mapping. A second locus, Yrr2, shows Pst race-specificity and encodes a disease resistance gene family typically associated with host plant resistance. These data indicate that some components of resistance to nonadapted pathogens are genetically tractable in some instances and may mechanistically overlap with host plant resistance to avirulent adapted pathogens. Plant pathogens are specialists and can colonise only a limited number of plant species (hosts). Pathogen infection of a plant that is not a host of the disease often results in an active plant defense response. This poorly characterised defense response is durable as phytopathogens rarely successfully colonise new hosts. The ability to transfer this resistance to host plants would be highly beneficial in protecting crops from disease. However, this resistance has been difficult to genetically dissect as all members of a plant species are resistant. Here we show that some accessions of the model grass Brachypodium distachyon show some variation in their ability to suppress colonisation by the wheat stripe rust pathogen Puccinia striiformis. Brachypodium is not a host species of wheat stripe rust disease and no accessions are fully susceptible, however, some allow more pathogen growth than others. We have exploited these relatively subtle phenotypic differences to genetically dissect this difference in resistance and identified two Brachypodium loci that contribute increased resistance to the nonadapted wheat stripe rust pathogen.
Funding Information
  • Two Blades Foundation
  • Biotechnology and Biological Sciences Research Council Doctoral Training Programme (BB/F017294/1)
  • Institute Strategic Programme
  • Gatsby Charitable Foundation (GB)
  • Human Frontiers Science Program (LT000218/2011)
  • CSIRO OCE Postdoctoral Fellowship

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