Plant & Animal Genomes XIV Conference

January 14-18, 2006
Town & Country Convention Center
San Diego, CA

Suppression Of Plant Defenses By A Pseudomonas syringae Type III Effector Protein

Robert B. Abramovitch^1,2 , Gregory B. Martin^1,2

¹  Boyce Thompson Institute for Plant Research, Tower Road, Ithaca, New York, 14853, USA

²  Department of Plant Pathology, Cornell University, Ithaca, New York, 14853, USA

Pseudomonas syringae pv. tomato DC3000 is a model plant pathogen for examining the molecular basis of plant disease susceptibility and immunity. DC3000 uses a type III secretion system to directly deliver bacterial effector proteins into the plant cell. Effector proteins are believed to play a key role in DC3000 pathogenesis of tomato. The complete genome of DC3000 was sequenced, revealing this pathogen carries over twenty type III effector proteins. Many of these effectors have no similarity to known proteins and their function in DC3000 pathogenesis is unknown. Therefore, identifying functions and biochemical activities for type III effectors is a key step in translating discoveries from the DC3000 genome into understanding the basis of DC3000 pathogenesis. In previous work, we observed that the AvrPtoB type III effector promotes pathogen virulence by suppressing programmed cell death (PCD) associated with plant immunity. To identify targets of AvrPtoB, we performed a yeast two-hybrid screen and isolated tomato ubiquitin (Ub) as an AvrPtoB interactor. AvrPtoB is ubiquitinated in vitro when incubated with rabbit reticulocyte lysates, and is observed as a smear when expressed in vivo. Together, these data suggested AvrPtoB might exhibit E3 Ub ligase activity. Indeed, AvrPtoB E3 activity is observed in the presence of recombinant E1 Ub activating enzyme and specific E2 Ub conjugating enzymes. Quantitative decreases in AvrPtoB anti-PCD activity and DC3000 virulence are correlated with decreases in AvrPtoB ubiquitination. Therefore, AvrPtoB employs E3 ubiquitin ligase activity as a novel virulence mechanism to suppress plant defenses.