A. Vivian, D.L. Arnold
doi: 10.4454/jpp.v82i3.1168
Bacterial pathogens often show great discrimination among potential host plants with regard to their abilities to cause disease. The basis of this race/cultivar specificity has been shown to reside in the matching of specific resistance (R) genes in the plant with determinants called avirulence (avr) genes in the pathogen: recognition in this way between host and pathogen enables the plant to react defensively to limit the invasion of the pathogen (Keen, 1990). Recent advances have established that the recognition between host and pathogen occurs inside plant cells (Gopalan et al., 1996; Leister et al., 1996; Scofield et al., 1996; Tang et al., 1996; Van den Ackerveken et al., 1996). A specialised bacterial secretion system, the type III Hrp protein secretion system, specified by a set of hrp genes (hypersensitive response and pathogenicity), is required both for signalling between the pathogen and the plant involving matching avr/R genes and also for pathogenicity (Hueck, 1998). There is circumstantial evidence that avr gene products are transferred into plant cells by the Hrp system and by implication other proteins that are required for pathogenicity. The term effector protein has been suggested to encompass all proteins that traverse the Hrp pathway into the plant, irrespective of whether they function as elicitors or as virulence factors (van Dijk et al., 1999). A range of effector genes and their products have been shown to depend for their effects in planta on a functional Hrp pathway and these include the products of avr vir (virulence) and hop (Hrp-dependent outer protein) genes (Alfano and Collmer, 1996; Mudgett and Staskawicz, 1998). This review will seek to present our current knowledge of these effector genes among the Gram-negative genera, Pseudomonas, Xanthomonas, Ralstonia and Erwinia and to consider their probable role(s) in the disease process.