The role of water stress in plant disease resistance and the impact of water stress on the global transcriptome and survival mechanisms of the phytopathogen <i>Pseudomonas syringae</i>
Leaf water status may play a significant role in determining the outcome of bacterial-plant interactions based on changes in leaf hydration as well as the molecular and physiological changes that occur in bacterial cells when responding to such stresses. In this study, we characterized localized changes in leaf hydration of Arabidopsis thaliana that occurred in response to challenge with the ubiquitous bacterial plant pathogen Pseudomonas syringae. We found that A. thaliana resistance to P. syringae pv. tomato DC3000 cells expressing avrRpm1 involved virtually complete cessation of vascular water movement into the infection site coupled with water loss through the stomata. Interestingly, suppression of bacterial growth during AvrRpm1-mediated resistance was prevented by physically blocking leaf water loss through the stomata or by incubating plants at high relative humidity. Collectively, these results indicate that gene-for-gene resistance in A. thaliana requires at least a localized loss of water for effective suppression of P. syringe growth, suggesting that this water loss either directly starves the invading bacteria of water or induces other components of the hypersensitive response that suppress bacterial growth. To understand the responses of P. syringae to low water availability and if these responses vary among strains, we performed a transcriptome analysis of the closely related strains P. syringae pv. tomato DC3000 and pv. syringae B728a exposed to water stress. We found significant differences in gene expression between the two strains. For example, certain osmoadaptation genes were induced more strongly in B728a than in DC3000, including those for alginate biosynthesis and osmoprotectant transporters. In addition, many genes involved with pathogenicity and virulence, including genes in the HrpL regulon, were suppressed in DC3000 but not altered or even induced in B728a. Lastly, to better understand the ecological importance of the major compatible solute trehalose in P. syringae, we examined three putative trehalose biosynthetic operons in DC3000. Mutants lacking any of the operons were more sensitive to the effects of osmotic stress than wild-type DC3000. In addition, mutants lacking all three operons exhibited decreased survival compared to that of the wild type when grown on host and non-host plants. These results indicate that trehalose biosynthesis plays an important role in protecting P. syringae cells from osmotic stress and contributes to survival of this important pathogen when growing in association with host and non-host plants.