Abstract
One of the most important micronutrients for bacterial growth is iron, whose bioavailability in soil is limited. Consequently, rhizospheric bacteria such as Pseudomonas fluorescens employ a range of mechanisms to acquire or compete for iron. We investigated the transcriptomic and proteomic effects of iron limitation on P. fluorescens Pf-5 by employing microarray and iTRAQ techniques, respectively. Analysis of this data revealed that genes encoding functions related to iron homeostasis, including pyoverdine and enantio-pyochelin biosynthesis, a number of TonB-dependent receptor systems, as well as some inner-membrane transporters, were significantly up-regulated in response to iron limitation. Transcription of a ribosomal protein L36-encoding gene was also highly up-regulated during iron limitation. Certain genes or proteins involved in biosynthesis of secondary metabolites such as 2,4-diacetylphloroglucinol (DAPG), orfamide A and pyrrolnitrin, as well as a chitinase, were over-expressed under iron-limited conditions. In contrast, we observed that expression of genes involved in hydrogen cyanide production and flagellar biosynthesis were down-regulated in an iron-depleted culture medium. Phenotypic tests revealed that Pf-5 had reduced swarming motility on semi-solid agar in response to iron limitation. Comparison of the transcriptomic data with the proteomic data suggested that iron acquisition is regulated at both the transcriptional and post-transcriptional levels.
Highlights
Pseudomonas fluorescens Pf-5 is a soil bacterium that was first described for its capacity to protect cotton seedlings from infection by Rhizoctonia solani and Pythium ultimum [1], [2]
A study with Pseudomonas putida conducted by Molina et al [32], showed that overnight cultures supplemented with either FeCl2 or FeCl3 had different growth biomass, which might suggest different levels of iron bioavailability or chemistry of the two iron sources used
Unlike observations reported for P. putida [32], we observed no notable difference between the growth rates and final cell densities of Pf-5 cultures supplemented with equal amounts of FeCl2 versus FeCl3 under our conditions (Figure S1)
Summary
Pseudomonas fluorescens Pf-5 is a soil bacterium that was first described for its capacity to protect cotton seedlings from infection by Rhizoctonia solani and Pythium ultimum [1], [2]. The potential of Pf-5 to control other plant pathogenic fungi and bacteria was demonstrated [3], [4]. Iron is abundant in soil but, under aerobic conditions, it exists primarily in the insoluble ferric oxide form [11], which is not available for microbial growth. The production and secretion of siderophores is thought to confer a competitive advantage on the producing organism, since siderophore-bound iron is not available to surrounding organisms lacking specific uptake systems for the ferric-siderophore complexes. The production of siderophores by biological control bacteria in the rhizosphere may suppress plant disease by sequestering iron in a form that is not available to pathogens, resulting in their iron starvation [15]
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