Abstract
Most cereal species are attacked by rust fungi, many of which cause diseases of global significance for cereal production. Cereal rusts display a narrow host specificity and most cereal rust species can only colonize one or a few closely related grass species. Cultivated barley (Hordeum vulgare) and wild bulbous barley (H. bulbosum) are two closely related grass species that diverged about six million years ago. The two species are colonized by separate rust pathogens that show a strict host specificity. The leaf rust pathogen of cultivated barely (Puccinia hordei) can only infect cultivated barley but not wild bulbous barley, while the opposite is true for P. hordei-bulbosi. We used this barley-rust pathosystem to study the genetic and molecular basis of host specificity to rusts. We identified two orthologous lectin receptor-like kinases as quantitative determinants of host specificity. Both genes conferred resistance to adapted as well as to non-adapted rusts, but the effect against non-adapted rusts was much stronger. Our results show that quantitative host and non-host resistance can be conferred by the same genes.