Barley-Puccinia rusts: a model system to study the genetics, evolution and mechanisms of nonhost immunity in plants

The genetic basis of nonhost resistance is one of the most intriguing questions in the field of infectious diseases. The inheritance is very hard to study, since it typically requires interspecific crosses between host and nonhost species. Until now, mutagenesis and transformation have lead to discovery of major key-genes in nonhost defence, but they do not make clear what role those key-genes (if any) play in natural differences in host status between plant species. Our work is one of the very few comprehensive forward genetics studies on nonhost plant resistance involving more than one heterologous pathogen species. Our system, barley- Puccinia rusts, focuses on natural variation in the plant species and is therefore evolutionary relevant. We found a quantitative inheritance, requiring a QTL analysis. We also showed that in each immune barley line, a different set of genes is involved in nonhost resistance. In contrast to what might be expected, nonhost immunity of barley to various heterologous rust species is not due to a few key-genes with general effectiveness to a whole class of similar pathogen species, i.e. rust fungi.The results from comparison ofQTLs conferring nonhost immunity in the three different mapping populations showed that in each population different sets of loci were implicated in resistance, with very few genes in common between the populations, suggesting a high diversity of genes conferring nonhost immunity to nonhost pathogens. These loci were significantly associated with QTLs for partial resistance to the pathogen P. hordei , and also with defense related genes.