Iron‐regulated metabolites produced by Pseudomonas fluorescens WCS374r are not required for eliciting induced systemic resistance against Pseudomonas syringae pv. tomato in Arabidopsis

Abstract

The plant growth-promoting rhizobacterium Pseudomonas fluorescens WCS374r produces several iron-regulated metabolites, including the fluorescent siderophore pseudobactin (Psb374), salicylic acid (SA), and pseudomonine (Psm), a siderophore that contains a SA moiety. After purification of Psb374 from culture supernatant of WCS374r, its structure was determined following isoelectrofocusing and tandem mass spectrometry, and found to be identical to the fluorescent siderophore produced by P. fluorescens ATCC 13525. To study the role of SA and Psm production in colonization of Arabidopsis thaliana roots and in induced systemic resistance (ISR) against Pseudomonas syringae pv. tomato (Pst) by strain WCS374r, mutants disrupted in the production of these metabolites were obtained by homologous recombination. These mutants were further subjected to transposon Tn5 mutagenesis to generate mutants also deficient in Psb374 production. The mutants behaved similar to the wild type in both their Arabidopsis rhizosphere-colonizing capacity and their ability to elicit ISR against Pst. We conclude that Psb374, SA, and Psm production by P. fluorescens WCS374r are not required for eliciting ISR in Arabidopsis.

Ferric-Pyoverdine Recognition by Fpv Outer Membrane Proteins of Pseudomonas protegens Pf-5

“…5). WCS374 produces the secondary siderophore pseudomonine (41), and a pseudomonine biosynthesis gene cluster is also present in the A506 genome (49). Pyoverdine-deficient mutants of WCS374 and A506 did not cross-feed the Pf-5 ⌬pvdI-pchC mutant, however, indicating that the pyoverdines, rather than pseudomonine or another secondary siderophore that may be produced by these strains, were responsible for the cross-feeding.…”

“…d Structures predicted from bioinformatic analysis of the NRPS sequences in the pyoverdine gene clusters in the genomes of these strains. e Strains known to produce secondary siderophores, in addition to pyoverdines: P. protegens Pf-5 and CHA0, enantio-pyochelin (4); P. aeruginosa PAO1 and 7NSK2, pyochelin (7,48); P. fluorescens SBW25, ornicorrugatin (7); P. fluorescens WCS374 and A506, pseudomonine (41,49). Pf-5 can utilize enantio-pyochelin as an iron source, but it cannot utilize pyochelin (50) and lacks the TBDPs for ornicorrugatin and pseudomonine (E. W. Davis II, S. L. Hartney, and J. E. Loper, unpublished data).…”

Rhizobacterial salicylate production provokes headaches!

“…Nevertheless, presence of two potential Fur boxes in the promoter region of the pmsCEAB operon as well as repression of this gene cluster by iron has been confirmed (Mercado-Blanco et al 2001). Availability of iron and diverse substrates, as well as temperature affect bacterial production of SA and SA-based siderophores (Leeman et al 1996;De Meyer and Höfte 1997;Press et al 1997;Audenaert et al 2002;Ran et al 2005a;Djavaheri et al 2012). Despite that production of SA by bacteria can be easily confirmed in vitro, SA synthesis in vivo, in the ecological niche where they naturally live or can be artificially introduced, is more difficult to assess.…”

“…Deletions affecting pmsC reduced SA production in E. coli, whereas deletion of pmsB enterely abolished it (Mercado-Blanco et al 2001). Final evidence of the involvement of pmsB as the key gene for SA biosynthesis in strain WCS374 came after marker-exchange mutant analysis (Djavaheri et al 2012). Remarkably, the Psm biosynthesis gene cluster as well as genes putatively involved in the transport of this siderophore have identical organization in P. fluorescens WCS374, a rhizosphere biocontrol strain (Djavaheri et al 2012), and P. entomophila L48, an entomopathogenic bacterium (Matthijs et al 2009).…”

“…Final evidence of the involvement of pmsB as the key gene for SA biosynthesis in strain WCS374 came after marker-exchange mutant analysis (Djavaheri et al 2012). Remarkably, the Psm biosynthesis gene cluster as well as genes putatively involved in the transport of this siderophore have identical organization in P. fluorescens WCS374, a rhizosphere biocontrol strain (Djavaheri et al 2012), and P. entomophila L48, an entomopathogenic bacterium (Matthijs et al 2009). Recently, the gene cluster for the biosynthesis and uptake of Psm (or a Psm-like compound) has been reported to be present in the genomes of the biocontrol strains Pseudomonas spp.…”