The Biochemistry and Molecular Genetics of Host Range Definition in «Pseudomonas syringae»

Abstract

Many Gram-negative bacterial pathogens of plant and animals harbor a conserved type III protein secretion system for the injection into the eukaryotic cells of host range determinants, or “effectors”. In Pseudomonas syringae, effectors are the products of avirulence and virulence genes, and the type III secretion apparatus is encoded by hrp/hrc genes, which are included in a large pathogenicity island with a tripartite mosaic structure. Individual effectors can have a dual role, depending on the plant host: they can either promote disease or elicit a defence response (hypersensitive response, HR) that diminishes virulence or restricts host range. There is strong evidence to support the belief that effectors act inside host plant cells and interact specifically with the products of plant resistance genes, to elicit the HR, or putative plant “susceptibility genes”, to elicit disease. Therefore, the capacity of a P. syringae strain to infect a given host, and hence host range, is determined by the sum of the individual activities (either positive or negative) of all the effectors that are injected into a host plant cell. Many P. syringae strains also produce other extracellular factors that could be involved in the interaction with the host, although only phytotoxins and certain hormones have been shown so far to increase virulence. Unlike effectors, however, their action does not appear to be host-specific.