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.
Highlights
Many bacterial strains from the genus Pseudomonas are capable of suppressing a range of plant diseases caused by soil-borne plant pathogenic fungi, due to their ability to biosynthesize antimicrobial metabolites
Mutants r26 and k36 exhibited similar expression levels of genes sup5, sup6 and orf8 during stationary and mid-exponential phase in both nutrient media. These results indicate that the insertional mutations in genes gcd and pqqE, which result in an inactive glucose dehydrogenase, decreased the transcription of genes sup5, sup6 and orf8 which are responsible for the biocontrol activity of Ps. fluorescens strain X
The biocontrol ability of Ps. fluorescens strain X depends on glucose dehydrogenase, its co-factor pyrroloquinoline quinone (PQQ), and the proteins encoded by two additional genes
Summary
Many bacterial strains from the genus Pseudomonas are capable of suppressing a range of plant diseases caused by soil-borne plant pathogenic fungi, due to their ability to biosynthesize antimicrobial metabolites. Antibiotics, cyclic lipopeptides (CLPs) with antimicrobial activity, siderophores and hydrogen cyanide are the main secondary metabolites to which the biological control is attributed [1]. Regulation of the biosynthesis of these antimicrobial metabolites has been extensively studied. A wide range of environmental as well as endogenous factors control the transcription of several genes involved in the biosynthesis of antimicrobial metabolites [2,3]. Glucose is one of the environmental factors which affect the biosynthesis of secondary metabolites such as oomycin A [4], kanosamine [5], DAPG [6], pyoluteorin and pyochelin [7], prodigiosin [8], pyrrolnitrin and phenazine [9]. It has been proposed that it is gluconic acid, not glucose, that regulates the production of antimicrobial metabolites [10]. Gluconic acid has been suggested as having a direct inhibitory effect on phytopathogenic fungi sensitive to lower concentrations of the acid [11]
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