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

Correspondence to: Gregory B. Martin^1,2 Correspondence and requests for materials should be addressed to G.B.M. (Email: gbm7@cornell.edu).

Abstract

Many bacterial pathogens of plants and animals use a type III secretion system to deliver diverse virulence-associated 'effector' proteins into the host cell¹. The mechanisms by which these effectors act are mostly unknown; however, they often promote disease by suppressing host immunity². One type III effector, AvrPtoB, expressed by the plant pathogen Pseudomonas syringae pv. tomato, has a carboxy-terminal domain that is an E3 ubiquitin ligase³. Deletion of this domain allows an amino-terminal region of AvrPtoB (AvrPtoB_1–387) to be detected by certain tomato varieties leading to immunity-associated programmed cell death⁴. Here we show that a host kinase, Fen, physically interacts with AvrPtoB_1–387 and is responsible for activating the plant immune response. The AvrPtoB E3 ligase specifically ubiquitinates Fen and promotes its degradation in a proteasome-dependent manner. This degradation leads to disease susceptibility in Fen-expressing tomato lines. Various wild species of tomato were found to exhibit immunity in response to AvrPtoB_1–387 and not to full-length AvrPtoB. Thus, by acquiring an E3 ligase domain, AvrPtoB has thwarted a highly conserved host resistance mechanism.

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