The Virulence Factors of the Bacterial Wilt Pathogen Ralstonia solanacearum

One of the most important virulence factors is a heterogeneous polymer of N-acetylated extracellular polysaccharide I (EPS I) [8]. EPS I-deficient mutants are nearly avirulent and do not colonize plant xylem vessels as efficient as wild type [4,9]. It has been suggested that EPS I directly causes wilting by physically blocking the vascular system and thereby alters water movement [8]. It has also been hypothesized that EPS protects R. solanacearum from plant antimicrobial defenses by cloaking bacterial surface features that could be recognized by hosts [4,9]. Interestingly, it has been recently found that R. solanacearum EPS I plays different roles in resistant and susceptible hosts [10]. In susceptible tomato plants, the wild-type and EPS I-deficient mutant induced generally similar defense responses; but in resistant tomato plants, the wild-type induced significantly greater defense responses than the EPS I-deficient mutants, suggesting that the EPS I itself is a specific elicitor of plant defense responses [10]. The Type III Secretion System