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Barley Bait-and-Switch: a receptor kinase bait switches on programmed cell death in response to Puccinia graminis
Robert Brueggeman: North Dakota State University; Shyam Solanki: North Dakota State University; Roshan Sharma Poudel: North Dakota State University; Jonathan Richards: North Dakota State University; Gazala Ameen: North Dakota State University
<div>The barley NLR genes, <i>Rpg5</i> and <i>HvRga1</i>, are required together for <i>rpg4</i>-mediated resistance to <i>P. graminis</i> f. sp. <i>tritici </i>(<i>Pgt</i>)<i>,</i> wheat stem rust, and have the head-to-head genome architecture of integrated decoy (ID) resistance mechanisms. The ID hypothesis explains why one NLR partner contains an atypical ID domain that mimics a pathogen’s virulence effector target. Once these host effector targets are fused to NLR receptors they function as IDs that recognize the presence of the pathogen switching on defense responses. Alleles of <i>Rpg5 </i>contain unique C-terminal IDs, with the resistance allele having a protein kinase (PK) ID and susceptible alleles containing a protein phosphatase 2C (PP2C) ID. The <i>Rpg5</i>-PK ID progenitor, HvApk1b, is the <i>Arabidopsis</i> AtAPK1b ortholog that is involved in stomatal aperture opening. Laser Capture Microdissection followed by qPCR determined that HvApk1b is also highly expressed in barley stomata. Thus, we hypothesize that the <i>Pgt</i> effector, Avr-4/5, manipulates HvApk1b during the infection process facilitating pathogen entry through otherwise closed stomata. Barley counter evolved the <i>Rpg5-PK</i> ID immunity receptor via duplication and translocation of the HvAPK1b target protein. Thus, <i>Pgt</i> evolved a virulence effector that facilitates stomata entry, yet, barley counter evolved a “bait” that betrays the pathogen switching on defense responses resulting in resistance to diverse <i>Puccinia graminis</i> formae speciales and races including TTKSK.</div>

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