Title: Erwinia Virulence Gene Regulation and Inhibition
PI: Amy Charkowski, University of Wisconsin

Erwinia carotovora and Erwinia chrysanthemi cause rot and wilt diseases on vegetable and ornamental crops. In the field, these diseases are promoted by environmental conditions such as rain and hot weather or damaging hail. Erwinia diseases are also commonly found in greenhouses on numerous high value ornamental crops and in vegetable storages, where they can lead to the total loss of a crop. In Wisconsin, these pathogens cause significant losses each year, particularly in the potato and ornamental plant industries. As with nearly all bacterial plant pathogens, there are currently no effective chemical control methods for Erwinia diseases in the field, greenhouse or storage. Transgenic plants have been constructed that are resistant to soft rot Erwinia, but they have not been accepted by commercial producers and this is not a realistic control option for the ornamental industry, where multiple varieties of hundreds of species are produced and it is cost prohibitive to make them all transgenic. Local producers recognize the severity of this problem. For example, losses in potato storages due to Erwinia and other storage pathogens were so severe over the past decade, that the Wisconsin potato and vegetable growers are raising two million dollars to build a vegetable storage research facility for Wisconsin researchers to aid in finding new control measures for this disease.

The most studied virulence factor in Erwinia are the plant cell wall degrading enzymes secreted by the bacteria through the Out system, which macerate tubers and promote lesion formation in stems and stolens. Many additional factors involved in Erwinia-plant interactions have also been described, including genes encoding iron uptake, the type III secretion system, anti-microbial peptide degradation enzymes, a peptide synthase, antibiotic synthesis genes, multidrug efflux systems, and adhesins. Most of these virulence factors are present in both E. chrysanthemi and E. carotovora and many are also found in other plant and animal pathogens. The E. chrysanthemi 3937 genome was recently sequenced, which has allowed our lab and others to more quickly identify genes important for bacterial pathogenicity.

Recently we discovered that E. chrysanthemi 3937 is able to form pellicles, a type of biofilm that forms at the air-liquid interface in cultures, and that the type III secretion system (T3SS), motility, bacterial cellulose synthesis, and energy taxis (aerotaxis) all contribute to pellicle formation. The T3SS and motility were already known to be important for bacterial pathogenicity and recently we found that energy taxis genes are also required for pathogenicity. Thus, although we have not yet determined if bacterial aggregation itself is important for virulence, the pellicle assay has allowed us to quickly screen for mutations that affect aggregation and, so far, all of the mutants we have found that are affected in pellicle formation are also reduced in virulence.

The goal of our work is to better understand how bacterial pathogens cause disease and to use this knowledge to develop better control measures for bacterial plant pathogens. The objectives of this proposal are to determine how energy taxis contributes to virulence and to use the E. chrysanthemi 3937 pellicle assay to identify virulence protein inhibitors.

This project is funded by USDA - Hatch.