What the Wild Things Do: Mechanisms of Plant Host Manipulation by Bacterial Type III-Secreted Effector Proteins

Abstract

Phytopathogenic bacteria possess an arsenal of effector proteins that enable them to subvert host recognition and manipulate the host to promote pathogen fitness. The type III secretion system (T3SS) delivers type III-secreted effector proteins (T3SEs) from bacterial pathogens such as Pseudomonas syringae, Ralstonia solanacearum, and various Xanthomonas species. These T3SEs interact with and modify a range of intracellular host targets to alter their activity and thereby attenuate host immune signaling. Pathogens have evolved T3SEs with diverse biochemical activities, which can be difficult to predict in the absence of structural data. Interestingly, several T3SEs are activated following injection into the host cell. Here, we review T3SEs with documented enzymatic activities, as well as T3SEs that facilitate virulence-promoting processes either indirectly or through non-enzymatic mechanisms. We discuss the mechanisms by which T3SEs are activated in the cell, as well as how T3SEs modify host targets to promote virulence or trigger immunity. These mechanisms may suggest common enzymatic activities and convergent targets that could be manipulated to protect crop plants from infection.

Keywords: Pseudomonas syringae; Ralstonia; Xanthomonas; biochemical activity; effector-triggered immunity; host activation; host targets; type III secreted effector; virulence promotion.

Figure 1

Schematic overview of bacterial type III-secreted effector (T3SE) activities in the plant intracellular space. The T3SEs discussed in this review are arranged in functional categories based on their characterized target(s) in terms of biological process or subcellular location. For each T3SE, a black border indicates that the T3SE negatively regulates its target in some manner, while T3SEs with a green border function as positive regulators. Note that the GALA family of T3SEs interacts with several target proteins that are primarily nuclear-localized but may be found in other subcellular locations as well. Limited data are available for the localization of AvrE-type T3SEs, but AvrE1 is known to associate with the plasma membrane.