Pseudomonas syringae is a Gram-negative, rod-shaped bacterium with polar flagella (Figure 1; Agrios, 1997). Strains of P. syringae collectively infect a wide variety of plants. Different strains of P. syringae, however, are known for their diverse and host-specific interactions with plants (Hirano and Upper, 2000). A specific strain may be assigned to one of at least 40 pathovars based on its host range among different plant species (Gardan et al., 1999) and then further assigned to a race based on differential interactions among cultivars of the host plant. Understanding the molecular basis of this high level of host specificity has been a driving force in using P. syringae as a model for the study of host-pathogen interactions. In crop fields, infected seeds are often an important source of primary inoculum in P. syringae diseases, and epiphytic bacterial growth on leaf surfaces often precedes disease development (Hirano and Upper, 2000). P. syringae enters the host tissues (usually leaves) through wounds or natural openings such as stomata, and in a susceptible plant it multiplies to high population levels in intercellular spaces. Infected leaves show water-soaked patches, which eventually become necrotic. Depending on P. syringae strains, necrotic lesions may be surrounded by diffuse chlorosis. Some strains of P. syringae also cause cankers and galls (Agrios, 1997). In resistant plants, on the other hand, P. syringae triggers the hypersensitive response (HR), a rapid, defense-associated death of plant cells in contact with the pathogen (Klement, 1963; Klement et al., 1964; Bent, 1996; Greenberg, 1996; Dangl et al., 1996; Hammond-Kosack and Jones, 1997). In this situation, P. syringae fails to multiply to high population levels and causes no disease symptoms. Open in a separate window Figure 1. A transmission electron microscope image of Pseudomonas syringae pv. tomato DC3000. Note that DC3000 produces polar flagella (15 nm in diameter) and a few Hrp pili (8 nm in diameter). The flagella and Hrp pili are indicated with arrows. Flagella enable bacteria to swim toward or away from specific chemical stimuli. Hrp pili are involved in type III secretion of avirulence and virulence proteins.