Genetic mapping and pathology of a barley leaf rust resistance gene from Hordeum bulbosum

Barley leaf rust, caused by Puccinia hordei, is one of the most widely distributed and economically significant foliar disease of barley. In this study, the disease resistance of two Hordeum bulbosum introgression lines (ILs) ‘182Q20’ and ‘200A12’ to P. hordei was characterized, and the resistance gene conferred by the H. bulbosum introgression of IL ‘200A12’ was mapped. Disease resistance of ILs ‘182Q20’ and ‘200A12’ was evaluated at seedling and post-seedling stages under greenhouse conditions by using a conventional method by assessing uredinia counts and disease symptoms, and a novel quantitative method by assessing fungal growth in the infected leaf tissues. The study showed that at seedling stage both introgression lines ‘182Q20’ and ‘200A12’ had partial resistance to P. hordei, and very susceptible host responses were observed for both introgression lines when plants were inoculated with high levels of urediniospores. While at post-seedling stages, the disease resistance of the both introgression lines, especially ‘200A12’, was much more effective than at seedling stage. In addition, as plant development progressed the disease resistance of the two introgression lines was more effective. At adult plant stage, nearly complete resistance was observed for the two introgression lines. The disease resistance of IL ‘200A12’ is conferred by a homozygous H. bulbosum introgression at the distal end of barley chromosome 1HL. An F₂ mapping population of 1368 plants, derived from IL ‘200A12’ and its barley genetic background ‘Emir’, was developed to map the resistance gene of IL ‘200A12’. A total of 19 F₂ interspecific recombinant lines were identified, and selfed to obtain F₃ homozygous recombinant lines. The phenotyping of these F₃ homozygous recombinant lines was also carried out with both conventional visual assessment method and novel fungal biomass assay. With the help of these two assessment methods, the F₃ homozygous recombinant lines were readily classified into susceptible or resistant group. Coupling with the genotyping results of F₃ homozygous recombinant lines based on 47 polymorphic markers within the H. bulbosum region, the H. bulbosum introgression containing resistance locus was narrowed from 0.75 cM to 0.15 cM, and located at the proximal end of the original introgression. The quantitative fungal biomass was proven useful for evaluation of disease resistance, especially partial resistance, of barley plants to P. hordei, and provided more accurate results than conventional visual assessment method. Overall, this study has proven that the two H. bulbosum introgression lines, ‘182Q20’ and ‘200A12’, was partially resistant to P. hordei at seedling stage, but more effective at post-seedling stages. The H. bulbosum introgression of ‘200A12’ containing resistance locus has been narrowed to a 0.15 cM genomic region. The closely linked markers to this confined genomic region will be used as effective molecular tools for barley breeding programs to accelerate the utilization of this valuable resistance gene of IL ‘200A12’