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Appl. Environ. Microbiol. doi:10.1128/AEM.00860-08
Copyright (c) 2008, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Examination of global gene expression suggests an important role for nutrient acquisition in early pathogenesis in a Pseudomonas aeruginosa-plant infection model

Program in Cell and Molecular Biology, Center for Rhizosphere Biology, Department of Horticulture and Landscape Architecture, Department of Microbiology, Immunology, and Pathology, Colorado State University, Fort Collins, CO 80523

^* To whom correspondence should be addressed. Email: j.vivanco{at}colostate.edu.

	Abstract

Although Pseudomonas aeruginosa is an opportunistic pathogen that does not often naturally infect alternate hosts such as plants, the plant-P. aeruginosa model has become a widely recognized system for identifying new virulence determinants and studying pathogenesis of this organism. Here we examine how both host factors and P. aeruginosa PAO1 gene expression are affected in planta after infiltration into incompatible and compatible cultivars of tobacco (Nicotiana tabacum L.). Nicotiana tabacum has a resistance gene (N) against tobacco mosaic virus; and although resistance to PAO1 infection correlated to the presence of a dominant N-gene, our data suggests that it is not a factor in resistance against PAO1. We did observe that the resistant tobacco cultivar had higher basal levels of salicylic acid, and a stronger salicylic acid response upon infiltration of PAO1. Salicylic acid acts as a signal to activate defense responses in plants, limiting the spread of the pathogen and preventing access to nutrients. It has also been shown to have direct virulence modulating effects on P. aeruginosa. We also examined host effects on the pathogen by analyzing global gene expression profiles of bacteria removed from the intracellular fluid of the two plant hosts. We discovered that the availability of micronutrients, particularly sulfate and phosphates, are important for in planta pathogenesis, and that the amounts of these nutrients made available to the bacteria may in turn have an effect on virulence gene expression. Indeed, there are several reports suggesting that P. aeruginosa virulence is influenced in mammalian hosts by the availability of micronutrients such as iron and nitrogen and by levels of O₂.