Antibiotics of Pseudomonas protegens FD6 are essential for biocontrol activity

Pseudomonas protegens (formerly Pseudomonas fluorescens) FD6, which was isolated from the canola rhizosphere, is a biocontrol agent that protects against the fungal pathogens Botrytis cinerea and Monilinia fructicola. The complete sequence of the 6.7-Mb Pseudomonas protegens FD6 genome was determined. Genomic analysis of strain FD6 led to the identification of twelve putative gene clusters of secondary metabolites, including the antibiotics 2,4-diacetylphloroglucinol (2,4-DAPG), pyoluteorin (PLT) and pyrrolnitrin (PRN). To assess the role of 2,4-DAPG and PLT in the biocontrol activity of P. protegens FD6, two mutants of P. protegens FD6, including strains ΔphlC and ΔpltD, were constructed using homologous recombination technology. The results showed that no significant differences were observed in the antagonistic action of the ΔphlC mutant compared with wild-type FD6. However, the ΔpltD mutant no longer exhibited antifungal activity. HPLC analysis indicated that ΔphlC mutant could not biosynthesize 2,4-DAPG, whereas PLT production in the ΔphlC mutant was significantly increased by 67-fold compared with wild-type FD6. Biosynthesis of PLT in the ΔpltD mutant was not detected, whereas the level of 2,4-DAPG was vastly decreased over sixteenfold compared with strain FD6. The major antagonistic activity produced by P. protegens FD6 corresponded to 2,4-diacetylphloroglucinol and pyoluteorin. There is an inverse interaction between 2,4-DAPG and PLT.