TYPE III SECRETION SYSTEM EFFECTOR PROTEINS: Double Agents in Bacterial Disease and Plant Defense

Abstract

Many phytopathogenic bacteria inject virulence effector proteins into plant cells via a Hrp type III secretion system (TTSS). Without the TTSS, these pathogens cannot defeat basal defenses, grow in plants, produce disease lesions in hosts, or elicit the hypersensitive response (HR) in nonhosts. Pathogen genome projects employing bioinformatic methods to identify TTSS Hrp regulon promoters and TTSS pathway targeting signals suggest that phytopathogenic Pseudomonas, Xanthomonas, and Ralstonia spp. harbor large arsenals of effectors. The Hrp TTSS employs customized cytoplasmic chaperones, conserved export components in the bacterial envelope (also used by the TTSS of animal pathogens), and a more specialized set of TTSS-secreted proteins to deliver effectors across the plant cell wall and plasma membrane. Many effectors can act as molecular double agents that betray the pathogen to plant defenses in some interactions and suppress host defenses in others. Investigations of the functions of effectors within plant cells have demonstrated the plasma membrane and nucleus as subcellular sites for several effectors, revealed some effectors to possess cysteine protease or protein tyrosine phosphatase activity, and provided new clues to the coevolution of bacterium-plant interactions.

Epigenetic Control of a Transcription Factor at the Cross Section of Two Antagonistic Pathways

“…2A) when plants were treated with either the pathogenic DC3000 or with the hrcC mutant strain lacking the ability to inject bacterial effector proteins into plant cells 28 (except for an apparent increase 48 hours after inoculation with hrcC, see Discussion). Transgenic plants expressing the ATX1-promotor driven reporter protein, GUS, were inoculated with DC3000, with the hrcC strain, or with water (mock-treatment).…”

Botrytis small RNA Bc-siR37 suppresses plant defense genes by cross-kingdom RNAi

“…13,4,10,29,40,23 In the plant-pathogen arms race, successful pathogens secrete dozens of effector proteins to modulate host physiology and to undermine PTI, the pathways of which has been well characterized for bacterial and oomycete pathogens. 1,16,17 Bacterial pathogens evolved molecular secretion systems for effector delivery, such as the type-III and type-IV secretion systems, 31 while the delivery and entry of oomycete and fungal effectors into host cells was just started to be understood. As the second layer of the immune response, plant species have evolved many resistance (R) genes.…”

Structural Analysis of Xanthomonas XopD Provides Insights into Substrate Specificity of Ubiquitin-like Protein Proteases

“…This pathogen is the causative agent of bacterial spot disease in tomato and pepper plants (16). Upon contact, Xanthomonas employs an hrp Ϫ (hypersensitive response and pathogenicity) type 3 secretion system to inject effector proteins into host plant cells (17,18). There are 20 -25 different proteins that make up a typical type 3 secretion system apparatus, which forms a syringe-like structure projecting across the inner bacterial membrane, the peptidoglycan layer, the outer bacterial membrane, and the host cell wall and plasma * This work was supported in part by NIAID Grant AI056404 from the National membrane to reach the host cell cytoplasm (18,19).…”

“…There are 20 -25 different proteins that make up a typical type 3 secretion system apparatus, which forms a syringe-like structure projecting across the inner bacterial membrane, the peptidoglycan layer, the outer bacterial membrane, and the host cell wall and plasma * This work was supported in part by NIAID Grant AI056404 from the National membrane to reach the host cell cytoplasm (18,19). The effector proteins are transported through this syringe-like channel in an unfolded state to reach the host cell interior (18), where they interact with specific targets to cause plant cell death (17). In planta, XopD exhibits an isopeptidase activity that reduces the amount of SUMO protein conjugates (15).…”