The Arabidopsis Thaliana-Pseudomonas Syringae Interaction

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. 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 . In the late 1980s, several strains belonging to pathovars tomato, maculicola, pisi, and atropurpurea of Pseudomonas syringae were discovered to infect the model plant Arabidopsis thaliana (reviewed by Crute et al., 1994). The establishment of the Arabidopsis-P. syringae pathosystem triggered a period of highly productive research that has contributed to the elucidation of the fascinating mechanisms underlying plant recognition of pathogens, signal transduction pathways controlling plant defense responses, host susceptibility, and pathogen virulence and avirulence determinants. In this chapter we trace the discovery of this pathosystem, overview the most salient aspects of this interaction, and point out the gaps in our knowledge. At the end of this chapter we will also provide a glossary of relevant pathology-related technical terms (Appendix I), a list of people who are studying this interaction so readers can seek help if they have further questions about the Arabidopsis-P. syringae interaction (Appendix II), and several experimental procedures commonly used in the study of the Arabidopsis-P. syringae interaction (Appendix III).