Default to plant cell death & defense

Xanthomonas effector suppression & activation of plant cell death

Many phytopathogenic T3SEs have been shown to target and suppress plant immune system, however such perturbation of the plant immune system can also trigger plant defense-like responses. Such a phenotype is difficult to interpret when elucidating the biochemical function of T3SEs. We faced this exact challenge while studying XopX, a highly conserved core effector from Xanthomonas. In our work, we discovered that XopX contributes to Xe virulence by suppressing specific aspects of plant immunity (i.e. reactive oxygen accumulation), but simultaneously activates plant defense responses, including PTI gene transcription, ethylene production, and plant cell death (PCD). A similar pattern of “dual behavior” (i.e. suppression of plant immunity coupled to activation of plant defense and PCD) was previously identified for the Pseudomonas core T3SE AvrE1 and led to the model that plants can respond to T3SE virulence function with a “default to death and defense” strategy. Our study adds XopX to this emerging class of T3SEs and highlights important considerations for evaluating how individual T3SEs contribute to the outcome of plant-microbe interactions.

We are currently characterizing the biochemical activity of XopX in planta. Preliminary data from transcript profiling and protein-protein interaction screens suggest that XopX is perturbing a core plant cellular process during Xe infection. Results from these studies are the basis of new work to uncover how XopX regulates both plant defense and disease symptom development during Xe-tomato interactions.