Genetic Mapping and Characterization of Net Blotch Dominant Resistance and Dominant Susceptibility Loci in Barley

Abstract

Net blotch is an economically important foliar disease of barley that occurs in two distinct forms: Spot Form Net blotch (SFNB) and Net Form Net Blotch (NFNB) caused by the necrotrophic fungal pathogens Pyrenophora teres f. maculata (Ptm) and Pyrenophora teres f. teres (Ptt), respectively. The recent emergence and the identification of both Ptm and Ptt isolates virulent on popular ND malting barley varieties have warranted the identification of new resistance sources. Association mapping was conducted on 2,062 diverse barley accessions phenotyped at the seedling stage with four diverse P. teres f. maculata isolates and genotyped with the 9k Illumina barley iSelect chip. A total of 138 significant marker-trait associations (MTA; −log10P value > 3.0) corresponding to 27 resistance loci were identified of which 21 loci were novel and six corresponded to previously characterized SFNB resistance QTL. Further, two higly resistant lines PI67381 and PI84314 were crossed with the two susceptible cultivars Tradition and Pinnacle grown in the Upper Midwestern US to develop three bi-parental recombinant inbred line (RIL) mapping populations of Tradition x PI67381, Pinnacle x PI67381 and Pinnacle x PI84314. These RIL populations were phenotyped with six diverse Ptm isolates and genotyped using PCR-GBS. MapDisto and Qgene were used to analyze the data and a total of twelve QTL were identified on chromosome 2H, 3H, 4H, 6H and 7H, of which nine were previously reported and the remaining three are considered novel. These resistances and the markers delimiting the QTL are being utilized to develop prebreeding lines by introgressing SFNB resistance into the cultivars Pinnacle and Tradition utilizing marker assisted selection. The barley line CI5791 exhibits a high level of resistance to diverse Ptt isolates collected from around the world. A forward genetics approach and an exome capture-mediated mapping-by-sequencing identified a candidate HvWRKY6 transcription factor gene required for NFNB resistance on chr 3H. We hypothesize that the HvWRKY6 gene function as a component of a conserved basal defense mechanism, which regulates the expression of other defense response genes that restrict lesion growth. The resistance/susceptibility loci identified in this study will facilitate the development of net blotch resistant cultivars.