Update on Bacterial Type III Secretion Type III Protein Secretion in Plant Pathogenic Bacteria

Many gram-negative plant and animal pathogenic bacteria employ a type III secretion system (T3SS) to subvert and colonize their respective host organisms. The T3SS injects effector proteins directly into the cytosol of eukaryotic cells and thus allows the manipulation of host cellular activities to the benefit of the pathogen. In plant pathogenic bacteria, T3SSs are encoded by hrp (for hypersensitive response and pathogenicity) genes, which are so named because they are required for bacteria to cause disease in susceptible plants and to elicit the hypersensitive response in resistant plants (Lindgren et al., 1986). The hypersensitive response is a rapid local cell death at the infection site that restricts bacterial multiplication and is triggered by individual effector proteins in plants carrying a corresponding resistance gene (Dangl and Jones, 2001). hrp genes were found in almost all major gram-negative bacterial plant pathogens (e.g. Pseudomonas syringae, Xanthomonas spp., Ralstonia solanacearum, and Erwinia spp.), illustrating a central role of the T3SS in mediating diverse plantbacteria interactions (Alfano and Collmer, 2004; He et al., 2004; Büttner and Bonas, 2006). In this Update, we highlight some basic as well as recent experiments that have collectively yielded molecular insights into general principles and unique properties of T3SSs in plant pathogenic bacteria. Environmental conditions that influence the level of hrp gene expression during the infection have been reviewed recently (Tang et al., 2006) and will not be discussed here.