Abstract
Bacterial AvrE-family Type-III effector proteins (T3Es) contribute significantly to the virulence of plant-pathogenic species of Pseudomonas, Pantoea, Ralstonia, Erwinia, Dickeya and Pectobacterium, with hosts ranging from monocots to dicots. However, the mode of action of AvrE-family T3Es remains enigmatic, due in large part to their toxicity when expressed in plant or yeast cells. To search for targets of WtsE, an AvrE-family T3E from the maize pathogen Pantoea stewartii subsp. stewartii, we employed a yeast-two-hybrid screen with non-lethal fragments of WtsE and a synthetic genetic array with full-length WtsE. Together these screens indicate that WtsE targets maize protein phosphatase 2A (PP2A) heterotrimeric enzyme complexes via direct interaction with B’ regulatory subunits. AvrE1, another AvrE-family T3E from Pseudomonas syringae pv. tomato strain DC3000 (Pto DC3000), associates with specific PP2A B’ subunit proteins from its susceptible host Arabidopsis that are homologous to the maize B’ subunits shown to interact with WtsE. Additionally, AvrE1 was observed to associate with the WtsE-interacting maize proteins, indicating that PP2A B’ subunits are likely conserved targets of AvrE-family T3Es. Notably, the ability of AvrE1 to promote bacterial growth and/or suppress callose deposition was compromised in Arabidopsis plants with mutations of PP2A genes. Also, chemical inhibition of PP2A activity blocked the virulence activity of both WtsE and AvrE1 in planta. The function of HopM1, a Pto DC3000 T3E that is functionally redundant to AvrE1, was also impaired in specific PP2A mutant lines, although no direct interaction with B’ subunits was observed. These results indicate that sub-component specific PP2A complexes are targeted by bacterial T3Es, including direct targeting by members of the widely conserved AvrE-family.
Highlights
Phytopathogenic bacteria encode effector molecules that remodel host cells in numerous ways [1,2,3]
AvrE-family T3Es, which are widely distributed among plant-pathogenic bacteria, suppress host defense responses and contribute to water-soaking, which is perhaps the most common symptom of bacterial diseases and likely results in the release of nutrients from host cells to promote pathogen growth
We report here that two AvrE-family T3Es, WtsE from the maize pathogen Pantoea stewartii subsp. stewartii and AvrE1 from the tomato and Arabidopsis pathogen Pseudomonas syringae pv. tomato, each target protein phosphatase 2A (PP2A) complexes in susceptible hosts via direct interaction/association with specific B’ regulatory subunits
Summary
Phytopathogenic bacteria encode effector molecules that remodel host cells in numerous ways [1,2,3]. The AvrE-family of T3Es are well conserved in a wide-spectrum of plant pathogenic bacteria, including the genera Pseudomonas, Pantoea, Erwinia, Dickeya, Ralstonia and Pectobacterium, with hosts ranging from monocots to dicots, including maize, Arabidopsis, tomato, apple and pear. Carotovorum (Pcc), is required and sufficient for disease-associated cell death in host plants [18, 24]. DspE, the only known type-III effector encoded by the broad-host-range bacterial soft-rot pathogen Pectobacterium carotovorum subsp. Despite their importance for the virulence of bacteria infecting both model and crop plants, the mechanism by which AvrE-family T3Es perturb host cells is poorly understood
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