Transcriptional responses of Pseudomonas syringae to growth in epiphytic versus apoplastic leaf sites

Abstract

Some strains of the foliar pathogen Pseudomonas syringae are adapted for growth and survival on leaf surfaces and in the leaf interior. Global transcriptome profiling was used to evaluate if these two habitats offer distinct environments for bacteria and thus present distinct driving forces for adaptation. The transcript profiles of Pseudomonas syringae pv. syringae B728a support a model in which leaf surface, or epiphytic, sites specifically favor flagellar motility, swarming motility based on 3-(3-hydroxyalkanoyloxy)alkanoic acid surfactant production, chemosensing, and chemotaxis, indicating active relocation primarily on the leaf surface. Epiphytic sites also promote high transcript levels for phenylalanine degradation, which may help counteract phenylpropanoidbased defenses before leaf entry. In contrast, intercellular, or apoplastic, sites favor the high-level expression of genes for GABA metabolism (degradation of these genes would attenuate GABA repression of virulence) and the synthesis of phytotoxins, two additional secondary metabolites, and syringolin A. These findings support roles for these compounds in virulence, including a role for syringolin A in suppressing defense responses beyond stomatal closure. A comparison of the transcriptomes from in planta cells and from cells exposed to osmotic stress, oxidative stress, and iron and nitrogen limitation indicated that water availability, in particular, was limited in both leaf habitats but was more severely limited in the apoplast than on the leaf surface under the conditions tested. These findings contribute to a coherent model of the adaptations of this widespread bacterial phytopathogen to distinct habitats within its host.endophyte | epiphyte | phyllosphere

Physiological and Transcriptional Responses to Osmotic Stress of Two Pseudomonas syringae Strains That Differ in Epiphytic Fitness and Osmotolerance

“…M, the gene is missing from the B728a microarray. c This gene showed increased transcript abundance in response to osmotic stress in a subsequent transcriptome analysis (19). d This gene showed increased transcript abundance in response to a matric stress (polyethylene glycol [molecular weight, 8,000]), which represents a distinct form of water stress (62).…”

“…On the basis of the role of T6SSs in protein transfer during antagonistic bacterial cell-cell interactions (46,47), osmoinduction of the core T6SS machinery may be relevant to the interaction of B728a with other epiphytic organisms rather than to B728a osmoadaptation per se and may contribute to epiphytic fitness by antagonizing other microbes. Although osmoinduction may signal the presence of the cell in a phyllosphere habitat (19), the ecological benefit of osmotic stress as a T6SS-inducing signal is not clear.…”

“…The microarray data were analyzed as described previously (19). Briefly, linear models for microarray data analysis (29) were applied to share information across genes when estimating error variances.…”

“…This prediction is supported by the identification of a P. syringae locus required for both fitness on leaves and osmotolerance in culture (17), the contribution of alginate to P. syringae fitness on leaves (18), the preferential survival of highly aggregated over nonaggregated P. syringae cells on drying leaf surfaces (15), and the strong induction of water stress tolerance genes in P. syringae cells on and in leaves (19). Interestingly, following leaf infiltration by P. syringae, endophytic population sizes were directly correlated with the availability of water in the leaf intercellular spaces (20), indicating a need to tolerate low water availability even at endophytic sites.…”

Hemin transported protein of Xanthomonas axonopodis pv. glycines functions on leaf colonization and virulence on soybean

“…glycines cells recovered from infected soybean plants. For in planta experiment, Xag cells in soybean were isolated from the leaves according to the method described by Yu et al (2013). Briefly, the bacterial cells at 1 × 10 8 cfu/mL were introduced by vacuum infiltration into soybean leaves.…”