C. BACTERIAL EPS AND THE HOST RESPONSE
VI. Conclusions
Many genes are activated during pathogenesis, and their respective gene products are known in several cases. Reactions of the gene products with plants are not clearly understood, but they probably are secreted as those in Yersinia and other human and animal pathogens (Kado, 1994).
Susceptible cabbage cultivars can be infected at all stages of the crop cycle but are most vulnerable in the seedling stage when leaf surfaces are wet. The hydrophilic environment of the leaf clearly changes as the leaf dries. Irrigation water and rain leach substances from leaf surfaces, and as moisture evaporates, solute concentrations in the surface water film increase. The bacteria must respond quickly to these changes in order to colonize the leaf surface. The yellow pigment, xanthomonadin, likely protects cells from deleterious UV-irradiation, enabling bacteria to survive epiphytically, and mutants for pigment production show reduced ability to survive (Poplawsky and Chun, 1997, 1998). Bacteria show chemotaxis toward guttation fluids and invasion is commonly through the hydathodes. Once inside the tissues, the bacteria multiply on nutrients provided by vascular fluids and proceed into the major veins. As the bacteria deplete nutrients in the xylem and approach stationary phase, additional nutrients must be supplied from the plant. Nutrients are supplied by tissue degradation with subsequent release of polysaccharides needed for cellular protection and bacterial growth.
Pathogenicity factors probably are produced in response to changes in the environment as the bacteria adapt to the internal components of the plant (Dow and Daniels, 1994).
Diffusible factors are involved in mUltiple signalling systems that up-regulate specific sets of genes as the bacteria reach stationary phase. Pathogenicity factors include EPS and extracellular enzymes, whose syntheses are regulated by complex mechanisms.
EPS production may be independently regulated by different pathways, because mutations in three different classes of genes (rpjF, hrpXc, pigB) result in EPS-deficient
mutants (Barber et aI., 1997; Kamoun and Kado, 1990a, b; Poplawsky et aI., 1997).
Starvation stimulates hrp-gene expression and may result in greater symptom development in compatible hosts (Dow and Daniels, 1994). Reciprocally, high nitrogen levels in the vascular tissue may repress activity of hrp genes and shift the host-pathogen interaction in favor of the plant, which becomes more resistant to colonization (McElhaney et al., 1998). Hrp genes do not appear to interact with the rpj regulon, however, indicating that different signaling mechanisms are in place.
The complexities of the host-pathogen interactions of Xcc with crucifers are currently being deciphered and integrated into a general model with detailed studies of gene regulation. Thus, a knowledge of this bacterial pathosystem, albeit presently incomplete, provides us with the essential details from which some meaningful generalizations can later be derived for similar bacterial pathogens that invade foliage and vascular tissues.
Acknowledgements
I wish to thank MJ. Daniels,
c.r.
Kado, J.E. Leach, S.S. Patil, and T. Oku for providing unpublished data and insightful comments.References
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