Activation of plant immunity by components of the type III secretion system from Xanthomonas campestris pv. vesicatoria

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Plants defend themselves against bacterial pathogens by the elicitation of basal defense reactions which are often activated upon recognition of pathogen-associated molecular patterns (PAMPs) by corresponding plant receptors. Specialized Gram-negative bacterial pathogens, however, suppress PAMP-triggered immunity (PTI) by the actions of effector proteins. In many cases, effector proteins are translocated into plant cells by a type III secretion (T3S) system, which is evolutionarily related to the bacterial flagellum. Notably, the main component of the extracellular flagellar filament is a well studied bacterial PAMP. In contrast, components of translocation-associated T3S systems have not yet been reported to contribute to the elicitation of PTI. To investigate a potential PAMP activity of the translocation-associated T3S system, we performed infection studies with mutant strains of the plant-pathogenic bacterium Xanthomonas campestris pv. vesicatoria, which lack the T3S translocon and thus the predicted transport channel for effector proteins in the plant plasma membrane. Our preliminary studies suggest that the pretreatment of host and non-host plants (e.g. Arabidopsis) with translocon-deficient X. campestris pv. vesicatoria strains induces the activation of specific defense genes and suppresses subsequent infections by pathogenic strains. The observed PTI reactions were significantly reduced in the absence of the extracellular T3S pilus or the complete T3S system, suggesting that extracellular components or substrates of the T3S system act as PAMPs. Aim of the proposed project is the characterization of the PAMP activity of extracellular components or substrates of the T3S system from X. campestris pv. vesicatoria and the identification of plant genes which are involved in the induced PTI responses. Infection experiments with various pathogens will reveal whether the T3S system-associated PAMP confers a general resistance against microbial invaders. PTI elicitation by bacterial culture supernatants and purified candidate PAMPs will be analysed to identify the T3S system-associated PAMP. To characterize the induced defense responses, we will measure PTI reactions such as the production of reactive oxygen species and ethylene, the activation of mitogen-activated protein kinases, the deposition of callose into the plant cell wall and the induction of defense genes. For the identification of plant genes, which are involved in the PTI response to the T3S system-associated PAMP, different Arabidopsis accessions will be tested for their responsiveness to the T3S system-associated PAMP. Candidate genes will be identified by mapping approaches, possibly in a potential second funding period. Taken together, the proposed experiments will help to characterize plant defense strategies in response to the bacterial T3S system and might reveal a potential use of translocon-deficient X. campestris pv. vesicatoria strains as general biocontrol agents.