Why Does Wheat Stripe Rust Not Infect Barley?

We have been investigating the genetic architecture underlying host species specificity of the obligate fungal plant pathogen Puccinia striiformis f. sp. tritici (PST), the causal agent of wheat stripe rust. Although PST principally infects wheat, it has the ability to marginally infect a range of grass species that include barley, rye, and Brachypodium. We have phenotyped the growth of PST at both the macroscopic and microscopic level in a panel of wild, landrace, and cultivated barley accessions. Susceptibility was found to be a relatively rare event; confirming barley as an inappropriate host to PST. This was juxtaposed to extensive susceptibility of barley to the appropriate pathogen, barley stripe rust (P. striiformis f. sp. hordei; PSH). Resistance to PSH has been well studied, but it is unclear what is determining the host specificity of PST. To address this, we leveraged the extensive genetic resources of barley to perform linkage and association mapping for resistance to PST. Mapping of inappropriate host resistance found that resistance was contributed by a few major effect loci. Strikingly, resistance co-localized with several powdery mildew loci including Mla, Mlf, and mlo. These same loci have previously been implicated in conditioning resistance to rice blast (Magnaporthe oryzae), suggesting that these loci may govern host status in several pathosystems. Our efforts are now focused on understanding the link between resistance to wheat stripe rust and barley powdery mildew by identifying the genes conditioning this resistance.