Complex interactions among plants and microbes have far-reaching impacts on plant health and productivity. On the positive side, certain species of microbes function as biocontrol agents that directly or indirectly assist plants to resist attack by other disease-causing microbes. Other microbes are pathogenic; they are directly responsible for causing plant disease that may lead to catastrophic consequences in human terms. Unfortunately, comparatively little is known about the molecular basis of either beneficial or detrimental plant-microbe interactions, and it is very difficult to unravel these microscopic interactions in the field. Thus, there is a significant need to develop model systems where these details can be studied in carefully controlled laboratory studies.

Our group uses state-of-the-art tools and technologies to study a bacterial plant pathogen, Pseudomonas syringae. Some close relatives - other members of the genus Pseudomonas - cause diseases in other species of plants and even animals. In contrast, other relatives live in close association with plants and release compounds help plants resist attacks by fungi. Still others have unique beneficial effects such as the capabilities to break down certain kinds of toxic pollutants. Our research sheds important new light into all of these diverse application areas.

The particular strain of P. syringae we study causes "speck" disease in both tomato (see figure below) and Arabidopsis thaliana, a small non-agricultural species favored for laboratory study. The complete DNA sequence of this strain of P. syringae is known, as is that of A. thaliana; the DNA sequence of tomato will be available soon. The availability of the DNA sequence for both the pathogen and host, and the relative ease of observing the disease process in the laboratory, make P. syringae an ideal model system for understanding the molecular basis of plant-microbe interactions.

Tomato speck disease caused by Pseudomonas syringae on fruit (left) and leaves (right). Related strains of P. syringae cause disease in a wide variety of food and horticultural crops.

The molecular processes inside the plant host and bacterial pathogen cells, as well as the "chemical communication" between these cells, can all be monitored using the powerful technologies. These processes are governed by proteins that are, in turn, encoded by genes defined by the DNA sequence - the genome. Our group focuses on using a combination of computational and laboratory methods to identify genes in the P. syringae genome and determine how the expression of these genes is controlled. Roughly speaking, we "reverse engineer" the genetic "circuits" in the bacterial cell related to interactions with its environment, including interactions with plant cells and plant-associated compounds. The end results of our research are used by other groups as starting points for detailed studies of plant defense mechanisms.