Type III protein secretion in plant pathogenic bacteria.

2 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 (hypersensitive response [HR] and pathogenicity) genes, which are so named because they are required for bacteria to cause disease in susceptible plants and to elicit the HR in resistant plants (Lindgren et al., 1986). The HR 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 subspecies (spp.), Ralstonia solanacearum, Erwinia spp.), illustrating a central role of the T3SS in mediating diverse plant-bacteria 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. Delivery of effector proteins from the cytoplasm of Gram-negative bacteria to the plant cell interior requires the T3SS to transport proteins across multiple physical barriers: the two bacterial membranes separated by a peptidoglycan layer and the plasma membrane of the plant cell, which is surrounded by a thick cell wall (Fig. 1A). It is widely believed that the T3SS provides a continuous channel for effector proteins to travel from the bacterial cytoplasm directly into the cytoplasm of eukaryotic cells. Central to this belief is the observation that T3SSs in different bacteria invariably assemble filamentous supramolecular structures (He et al., 2004). Although the first T3SS-associated filamentous structure was discovered in the plant pathogen P. syringae (Roine et al., 1997), the most elegant and seminal work in the characterization of T3SS supramolecular structures was carried out in the mammalian pathogen Salmonella enterica. Work of Kubori and colleagues (1998) revealed that the T3SS of S. enterica consists of two pairs of rings that interact with the cytoplasmic and the outer …