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Development of an Arabidopsis - Pseudomonas syringae co-culture system to investigate mechanisms of plant immunity against bacterial pathogens

Qing Yan: Oregon state University

<div>Plants recognize microbial features termed pathogen-associated molecular patterns (PAMPs) and activate defense responses termed pattern-triggered immunity (PTI). To overcome PTI, bacterial pathogens such as <em>Pseudomonas syringae</em> use a type III secretion system (T3SS) to deliver immune-suppressing effector proteins into host cells. Although PTI elicits massive changes in plant cell physiology, the exact mechanism(s) of how PTI limits bacterial growth is(are) largely unknown. Here we report the development of an infection system using Arabidopsis suspension cells and <em>P. syringae</em> that allows for well-defined analyses of signaling events between host and pathogen during infection. Using this co-culture system, we found that <em>P. syringae</em> grows to high levels in the presence of plant cells in a T3SS-dependent manner, and that this growth can be effectively suppressed by pre-treating the plant cells with PAMPs. To investigate the underlying mechanism of this growth inhibition, we performed a metabolomics analysis and found several compounds that are known inducers of T3SS genes were exuded in lower amounts by PAMP-treated plant cells. Furthermore, exogenous application of these metabolites effectively suppressed PAMP-mediated inhibition of <em>P. syringae</em> growth in this co-culture system. These data support the hypothesis that decreased metabolite release from defense-elicited plant cells is a contributing to limiting pathogen growth during PTI.</div>