PhD Exit Seminar – Bradley Laflamme (Desveaux and Guttman Labs)

A Systems-Level Study of Effector-Triggered Immunity in Arabidopsis thaliana

Abstract

Despite the continued prevalence of agricultural crop failures due to pathogen attack, any given plant host is resistant to most pathogen diversity. This phenomenon, often described as “broad-spectrum disease resistance,” is well-documented though poorly understood in terms of how various facets of defense contribute to its overall potency. Effector Triggered Immunity (ETI) is a major branch of plant innate immunity which entails the recognition of pathogen virulence factors, termed effectors, by intracellular Nucleotide-Binding Leucine Rich Repeat (NLR) proteins. We used the Arabidopsis thaliana-Pseudomonas syringae pathosystem to explore the contribution of ETI to broad-spectrum resistance. Though most P. syringae strains utilize a type III secretion system to translocate effectors into host cells, there is extensive genetic diversity across strains at effector-coding loci. We hypothesized that the effector sequence diversity of P. syringae, most of which has never been cloned and studied, contains hitherto unidentified ETI responses. We thus developed PsyTEC (P. syringae Type III Effector Compendium), a collection of 529 effectors from across 494 genetically diverse P. syringae strains, to screen for interactions between P. syringae effectors and the A. thaliana immune system, ultimately identifying 59 effectors spanning 19 protein families which elicit ETI. We found that the majority of P. syringae strains have the capacity to elicit ETI on A. thaliana, and that this broad-spectrum resistance to P. syringae diversity is conferred by a small repertoire of NLRs. Two NLRs, ZAR1 and CAR1, can potentially recognize 94.7% of strains due to their recognition of multiple broadly distributed effectors. We further explored ZAR1’s recognition of 5 P. syringae effectors and identified other genetic requirements for these ETI responses. We propose that ZAR1’s capacity for broad effector recognition is achieved by monitoring several cytoplasmic kinases for perturbation by effectors. This indirect monitoring enables a single NLR to guard against many virulence factors from diverse pathogen backgrounds. These systems-level insights into the ETI landscape of A. thaliana-P. syringae highlight its importance to the plant host’s broad-spectrum resistance strategy: ETI is ubiquitous in this system, and sophisticated mechanisms of indirect recognition enable immune responses to be disseminated through a small repertoire of NLRs.

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Join Zoom Meeting

https://utoronto.zoom.us/j/89335412013

Meeting ID: 893 3541 2013

Host: Darrell Desveaux (darrell.desveaux@utoronto.ca)

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