Independent evolution of powdery mildew avirulence effectors (AVRA) and their surveillance by the allelic barley Mildew Locus A (MLA) immune receptors.
I. SAUR (1), X. Lu (1), B. Kracher (1), S. Bauer (1), T. Maekawa (1), P. Schulze-Lefert (1) (1) Department of Plant-Microbe Interactions, Max Planck Institute for Plant Breeding Research, Germany

In barley, the allelic nucleotide-binding oligomerization-like receptors encoded at the mildew locus A (MLAs) confer isolate-specific resistance to the powdery mildew fungus Blumeria graminis f. sp. hordeii (Bgh) through the recognition of Bgh effectors. MLAs are polymorphic at their LRR domains, which is thought to result from iterative cycles of effector/receptor adaption and direct interaction of MLA LRR domains with sequence-related Bgh avirulence (AVRA) effectors. To identify AVRA effectors we sequenced transcripts of 16 Bgh isolates and extracted infection phenotype-specific polymorphisms in loci encoding candidate-secreted effector proteins (CSEPs). The identified CSEPs trigger cell death in barley and Arabidopsis thaliana cells expressing cognate MLAs. Strikingly, the identified CSEPs are sequence-unrelated, evolved from different ancestral genes and we were unable to detect direct interaction between the identified AVRA proteins and cognate MLAs. We therefore propose that allelic MLAs indirectly detect Bgh AVRA effectors through AVRA host targets. Yet, all known indirect effector-recognition mechanisms are unable to explain the evolution of so many (>20) MLA variants. To understand the underlying mechanism, we now focus on the identification of AVRA host targets and could already demonstrate AVRA-mediated manipulations of one important host immune component. Our data gives first insights into the evolution of allelic NLRs in response to unrelated effectors.

Abstract Number: P17-601
Session Type: Poster