Characterization of the Biocontrol Activity of Pseudomonas fluorescens Strain X Reveals Novel Genes Regulated by Glucose

Abstract

Pseudomonas fluorescens strain X, a bacterial isolate from the rhizosphere of bean seedlings, has the ability to suppress damping-off caused by the oomycete Pythium ultimum. To determine the genes controlling the biocontrol activity of strain X, transposon mutagenesis, sequencing and complementation was performed. Results indicate that, biocontrol ability of this isolate is attributed to gcd gene encoding glucose dehydrogenase, genes encoding its co-enzyme pyrroloquinoline quinone (PQQ), and two genes (sup5 and sup6) which seem to be organized in a putative operon. This operon (named supX) consists of five genes, one of which encodes a non-ribosomal peptide synthase. A unique binding site for a GntR-type transcriptional factor is localized upstream of the supX putative operon. Synteny comparison of the genes in supX revealed that they are common in the genus Pseudomonas, but with a low degree of similarity. supX shows high similarity only to the mangotoxin operon of Ps. syringae pv. syringae UMAF0158. Quantitative real-time PCR analysis indicated that transcription of supX is strongly reduced in the gcd and PQQ-minus mutants of Ps. fluorescens strain X. On the contrary, transcription of supX in the wild type is enhanced by glucose and transcription levels that appear to be higher during the stationary phase. Gcd, which uses PQQ as a cofactor, catalyses the oxidation of glucose to gluconic acid, which controls the activity of the GntR family of transcriptional factors. The genes in the supX putative operon have not been implicated before in the biocontrol of plant pathogens by pseudomonads. They are involved in the biosynthesis of an antimicrobial compound by Ps. fluorescens strain X and their transcription is controlled by glucose, possibly through the activity of a GntR-type transcriptional factor binding upstream of this putative operon.

Swarming motility in plant-associated bacteria

“…Thus, fl agellated bacteria would be expected to swim or to swarm under certain soil saturation levels. Recent studies demonstrated that displacement of Bradyrhizobium japonicum is achieved in 80% saturated soil (Covelli et al, 2013) while Pseudomonas fl uorescens strain X (Kremmydas et al, 2013) displayed a fast movement in 50% saturated soil (Fig. 1).…”

Gac‐mediated changes in pyrroloquinoline quinone biosynthesis enhance the antimicrobial activity of Pseudomonas fluorescens SBW25

“…The upregulation of the pqq and gcd genes in the gacS mutant was confirmed by qPCR (not shown). In Pseudomonas and other Gram‐negative bacteria, gluconic acid biosynthesis has been shown to be dependent on PQQ as an enzymatic cofactor of the membrane‐bound quinoprotein, glucose dehydrogenase (Gcd) (Goldstein, ; Kaur et al ., ; de Werra et al ., ; Kremmydas et al ., ; Ponraj et al ., ). Gcd acts as a glucose sensor and catalyses glucose into gluconic acid by periplasmic oxidation.…”

Genome Sequence of Pseudomonas chlororaphis Strain 189

“…Like PA23, P . chlororaphis 189 contained an array of biosynthetic pathways capable of producing metabolites involved in biocontrol, including hydrogen cyanide (6), phenazine (7), pyocin (8), pyrroloquinoline quinone (9), and cell wall degradative enzymes. Genes conferring the ability to produce surfactants and form biofilms (10) were also found.…”