Orthologous receptor kinases quantitatively affect the host status of barley to leaf rust fungi.

Global food security depends on cereal crops with durable disease resistance. Most cereals are colonized by rust fungi, which are pathogens of major significance for global agriculture. Cereal rusts display a high degree of host specificity and one rust species or forma specialis generally colonizes only one cereal host. Exploiting the non-host status and transferring non-host resistance genes between cereal crop species has been proposed as a strategy for durable rust resistance breeding. The molecular determinants that define the host status to rusts, however, are largely unknown. Here, we show that orthologous genes at the Rphq2 locus for quantitative leaf rust resistance from cultivated barley and Rph22 from wild bulbous barley affect the host status to leaf rusts. Both genes encode lectin receptor-like kinases. We transformed Rphq2 and Rph22 into an experimental barley line that has been bred for susceptibility to non-adapted leaf rusts, which allowed us to quantify resistance responses against various leaf rust species. Rphq2 conferred a much stronger resistance to the leaf rust of wild bulbous barley than to the leaf rust adapted to cultivated barley, while for Rph22 the reverse was observed. We hypothesize that adapted leaf rust species mitigate perception by cognate host receptors by lowering ligand recognition. Our results provide an example of orthologous genes that connect the quantitative host with non-host resistance to cereal rusts. Such genes provide a basis to exploit non-host resistance in molecular breeding.