Exploring the Role of Water Limitation in Plant Defense and in the Ecology of Foliar Bacterial Pathogens
Does plant defense involve starving bacteria for water?
How do plant defense responses ultimately limit the growth of bacterial pathogens? Using bacterial bioreporters that report on the water status of bacterial cells in planta, we have shown that the bacterial pathogen Pseudomonas syringae pv. tomato experiences a rapid decrease in water availability following invasion of resistant Arabidopsis thaliana plants. Moreover, this decrease was large enough to halt the growth of the bacteria in simulations performed with cultured cells. This discovery suggests that localized changes in water status could be at least one factor limiting bacterial growth during a successful defense response. Is this phenomenon unique to this pathosystem or is it common to other hosts and bacterial pathogens? How does it happen, that is, what physiological changes in the plant alter water availability to bacteria at the infection site? These questions are the focus of current investigations.
Do bacteria use plant-derived compounds to enhance their growth when starved for water in plants?
In a battle between plants and bacteria for water, the bacteria have their own cache of defenses. One of these is the ability to accumulate compatible solutes, allowing restoration of homeostasis with their external environment and thus growth. Although this accumulation can result from bacterial synthesis of compatible solutes, it can also result from the uptake of compounds from the environment. In fact, such uptake is preferred over synthesis because it requires less energy. Our studies indicate that P. syringae is best protected from low water availability by compounds that are present in plants.
During P. syringae invasion of plants, to what extent do the bacteria actually access and use plant-derived compounds for protection from water stress? We are identifying the complete repertoire of genes that are involved in the P. syringae response to water stress in an effort to form a comprehensive picture of how this organism adapts to this stress. As one component of these efforts, we are identifying the P. syringae pv. tomato transporters that take up plant-derived protective compounds. Knowledge of these transporters will help us evaluate how important uptake of these plant compounds is to P. syringae pathogenicity and fitness. Worldwide agricultural production is increasingly facing crop exposure to saline- and drought-stressed conditions due to the extensive use of irrigation and global climate change. Like in bacteria, exposure to these stresses promotes accumulation of these protective compounds in plants. Thus, these results are important to understanding if the ecology or virulence of foliar bacterial pathogens is likely to be altered on plants exposed to drought or salinity stress or on plants bred or engineered for enhanced tolerance to these stresses.