Inter-Chromosomal Transfer of Immune Regulation during Infection of Barley with the Powdery Mildew Pathogen

Plant immune systems exemplify multi-tiered signaling networks. In the interaction between barley and its powdery mildew fungus, Blumeria graminis f. sp. hordei (Bgh), host ML (mildew) immune receptors sense Bgh effectors and trigger defense networks comprising thousands of genes. We sought to address regulation of immunity in this system by utilizing expression quantitative trait locus (eQTL) analysis. Trans eQTLs are regulatory regions that control the expression dynamics of hundreds to thousands of genes in response to particular environmental stimuli. We identified two clusters of trans eQTLs; within these clusters reside powdery mildew resistance loci derived from Hordeum laevigatum (MlLa) and H. vulgare cv. Algerian (Mla). These eQTLs regulate the expression of 961 and 3,296 genes during penetration of host epidermal cells and development of haustoria, respectively. Intriguingly, during haustorial development, Mla repurposes more than 30% (299) of the genes regulated by MlLa during penetration. Moreover, 292 of the 299 genes switch the allele responsible for higher expression when alternate regulatory control occurs between stages of infection. We postulate that master regulators respond to pathogen attack by activating a conserved set of genes.