Role of secondary metabolites in the biocontrol activity of Pseudomonas corrugata and Pseudomonas mediterranea

In this study, the Pseudomonas corrugata strain CFBP 5454 and the P. mediterranea strain CFBP 5447 were shown to produce diffusible compounds that inhibit the in vitro growth of plant pathogenic fungi and bacteria and antifungal volatile compounds. In addition, both bacterial strains were found to produce cyanide. Mutant derivatives in LuxR transcriptional regulators, i.e. P. corrugata GL2 (pcoR mutant) and GLRFIA (rfiA mutant), and P. mediterranea PSMER (pmeR mutant) and PSRFIA (rfiA mutant) impaired in cyclic lipopeptide (CLP) production, showed a diffusible compound-mediated reduced activity, depending on the biocontrol strain, challenge microorganism and culture medium. The volatile compound-mediated activity and cyanide production were not affected in the mutants. Genome analysis of the P. corrugata strain CFBP 5454 led to the identification of putative genes involved in the hydrogen cyanide (HCN) biosynthesis. HCN is a volatile organic compound (VOC), and as in other Pseudomonas, the HCN cluster consisted of three contiguous structural genes, hcnABC, which together encoded a membrane-bound HCN synthase complex, which was sufficient for cyanogenesis. The putative hcnA gene was insertionally inactivated. A genomic mutant was characterized, and the role of this compound in biocontrol activity was investigated. A qualitative test for the detection of HCN production confirmed that in the hcnA mutant strain, metabolite production is completely abolished. In vitro experiments on the phytopathogenic fungus Botrytis cinerea also showed that HCN production is mainly involved in conidia germination inhibition.