bIncreasing evidence indicates that despite exposure to harsh environmental stresses, Salmonella enterica successfully persists on plants, utilizing fresh produce as a vector to animal hosts. Among the important S. enterica plant colonization factors are those involved in biofilm formation. S. enterica biofilm formation is controlled by the signaling molecule cyclic di-GMP and represents a sessile lifestyle on surfaces that protects the bacterium from environmental factors. Thus, the transition from a motile, planktonic lifestyle to a sessile lifestyle may represent a vital step in bacterial success. This study examined the mechanisms of S. enterica plant colonization, including the role of diguanylate cyclases (DGCs) and phosphodiesterases (PDEs), the enzymes involved in cyclic di-GMP metabolism. We found that two biofilm components, cellulose and curli, are differentially required at distinct stages in root colonization and that the DGC STM1987 regulates cellulose production in this environment independent of AdrA, the DGC that controls the majority of in vitro cellulose production. In addition, we identified a new function for AdrA in the transcriptional regulation of colanic acid and demonstrated that adrA and colanic acid biosynthesis are associated with S. enterica desiccation tolerance on the leaf surface. Finally, two PDEs with known roles in motility, STM1344 and STM1697, had competitive defects in the phyllosphere, suggesting that regulation of motility is crucial for S. enterica survival in this niche. Our results indicate that specific conditions influence the contribution of individual DGCs and PDEs to bacterial success, perhaps reflective of differential responses to environmental stimuli. E nteric human pathogens, such as Salmonella enterica, are frequently studied in the context of an animal host, but it has become increasingly apparent that a significant portion of their life cycle occurs on plants (for a review, see reference 1). S. enterica is exposed to numerous environmental stresses during colonization of animal or plant hosts; stresses common in plant niches include plant defense responses and fluctuations in temperature, humidity, and nutrient availability. Despite these obstacles, S. enterica is ubiquitous in the plant environment, is commonly isolated from water used for irrigation or pesticide application (2), and persists for months on roots and leaves during crop production (3, 4). These abilities have led to an increased incidence of human disease, particularly through the consumption of contaminated fresh produce (5, 6). We predict that the bacterium's ability to transition from a potentially motile lifestyle (in water) to a more sessile lifestyle (on plants) would impact its success on plants.In support of this idea, several S. enterica attachment factors contribute to colonization of roots: proteinaceous curli (previously called thin, aggregative fimbriae), O-antigen capsule, and cellulose (a polymer of ␤-1,4 glucan chains) (7,8). The transcription factor CsgD (previously referre...

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