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.

The NEL Family of Bacterial E3 Ubiquitin Ligases

“…Many Gram-negative bacteria that are pathogens or symbionts of plants or animals use type III secretion systems (T3SSs) to inject proteins known as effectors into the cytosol of eukaryotic host cells [ 1 , 2 ]. These systems are molecular syringes, evolutionarily related to flagella, that use a single-step secretion mechanism to cross the inner and outer bacterial membranes and the host cell membrane [ 3 ].…”

Varietas Delectat: Exploring Natural Variations in Nitrogen-Fixing Symbiosis Research

“…Although MTI/PTI at the cell surface is very effective, microbes evolved virulence factors and apparatus for their delivery to the host cells to modify or attenuate the original immune responses. The type III secretion system (T3SS) of Gram-negative bacteria including pathogens and symbionts is a complex multiprotein secretion apparatus that actively exports effector proteins (T3 effectors) with diverse biochemical activities ( Schreiber et al, 2021 ) through the lumen of these tubular structures and directly into the eukaryotic host cells. In rhizobia, proteins that are either extracellular components of or secreted by the T3SS apparatus are termed as Nops, nodulation outer proteins, and are produced upon NF induction ( Staehelin and Krishnan, 2015 ).…”

Action Mechanisms of Effectors in Plant-Pathogen Interaction

“…To date, type III effectors in bacteria [ 23 , 24 ], LysM effectors in fungus [ 25 ], and RxLR effectors in oomycetes [ 26 , 27 ] have been extensively studied and many details about their biological functions are known so that the mechanism of action of effectors is gradually being unveiled. Ray et al [ 28 ] have provided a systematic review of the interaction between virus effectors and plants, so we do not repeat these details further here.…”