ROLE OF THE TYPE III PROTEIN SECRETION SYSTEM IN BACTERIAL INFECTION OF PLANTS
In the past two decades, research into the molecular basis of bacterial pathogenesis has led to the conclusion that although different bacteria may use unique mechanisms to subvert hosts, a few strategies are common. One striking example is the discovery that many plant and animal bacterial pathogens contain members of a family of protein secretion systems classified as type III. The type III secretion system (TTSS) supramolecular structures in both mammalian and plant pathogenic bacteria have been characterized. The central importance of the TTSSs in mammalian and plant bacterial pathogenesis is underscored by the finding that a defect in this system often leads to a complete loss of bacterial pathogenicity. Plant pathogenic bacteria encounter a unique eukaryotic cell type that is enveloped by a cell wall; they multiply predominantly in the intercellular space outside of the plant cell wall and are therefore extracellular pathogen. Plant basal defense is associated with the induction of certain general defense genes, the production of antimicrobial phytoalexins, and the fortification of plant cell walls, which involves the localized deposition of callose (β-1, 3-glucan) and other compounds in the plant cell wall. Recent studies show that certain virulent Pseudomonas syringae strains have found ways to break down gene-for-gene resistance and expand the host range at the cultivar level. TTSS effectors appear to be highly evolved microbial molecules that have adapted to carry out precise functions on specific host proteins.
