The mechanisms by which plants recognize and distinguish pathogenic bacteria from beneficial and saprophytic bacteria is unknown. Humans and other animals are faced with beneficial, saprophytic, and pathogenic bacteria as well. Common elements found in human and plant bacterial pathogens (e. g., hrp genes) suggest that studies of plant-pathogen models may provide insights into bacterial human diseases. We are studying one specific plant-microbe interaction-that of host tolerance to a bacterial pathogen. The objective of this proposal is to characterize the influence of microbial factors from the black rot pathogen, Xanthomonas campestris pv campestris, on the expression of the RXCI tolerance gene from Arabidopsis thaliana. Data obtained from these experiments will provide novel insights into mechanisms by which plants perceive a pathogen and alter gene expression to generate a resistance response. These studies will provide information on the biochemical and genetic mechanisms that a pathogenic organism and its plant host have evolved to tolerate each other. This information may also provide insight into recognition and signaling events in human bacterial diseases.