Involvement of Vitamin B6 Biosynthesis Pathways in the Insecticidal Activity of Photorhabdus luminescens.

Photorhabdus luminescens is a Gram-negative entomopathogenic bacterium which symbiotically associates with the entomopathogenic nematode Heterorhabditis bacteriophora. P. luminescens is highly virulent to many insects and nonsymbiotic nematodes, including Caenorhabditis elegans. To understand the virulence mechanisms of P. luminescens, we obtained virulence-deficient and -attenuated mutants against C. elegans through a transposon-mutagenized library. From the genetic screening, we identified the pdxB gene, encoding erythronate-4-phosphate dehydrogenase, as required for de novo vitamin B-6 biosynthesis. Mutation in pdxB caused growth deficiency of P. luminescens in nutrient-poor medium, which was restored under nutrient-rich conditions or by supplementation with pyridoxal 5=-phosphate (PLP), an active form of vitamin B-6. Supplementation with three other B-6 vitamers (pyridoxal, pyridoxine, and pyridoxamine) also restored the growth of the pdxB mutant, suggesting the existence of a salvage pathway for vitamin B-6 biosynthesis in P. luminescens. Moreover, supplementation with PLP restored the virulence-deficient phenotype against C. elegans. Combining these results with the fact that pdxB mutation also caused attenuation of insecticidal activity, we concluded that the production of appropriate amounts of vitamin B-6 is critical for P. luminescens pathogenicity. IMPORTANCE The Gram-negative entomopathogenic bacterium Photorhabdus luminescens symbiotically associates with the entomopathogenic nematode Heterorhabditis bacteriophora. P. luminescens is highly virulent to many insects and nonsymbiotic nematodes, including Caenorhabditis elegans. We have obtained several virulence-deficient and -attenuated P. luminescens mutants against C. elegans through genetic screening. From the genetic analysis, we present the vitamin B-6 biosynthetic pathways in P. luminescens that are important for its insecticidal activity. Mutation in pdxB, encoding erythronate-4-phosphate dehydrogenase and required for the de novo vitamin B-6 biosynthesis pathway, caused virulence deficiency against C. elegans and growth deficiency of P. luminescens in nutrient-poor medium. Because such phenotypes were restored under nutrient-rich conditions or by supplementation with B-6 vitamers, we showed the presence of the two vitamin B-6 synthetic pathways (de novo and salvage) in P. luminescens and also showed that the ability to produce an appropriate amount of vitamin B-6 is critical for P. luminescens pathogenicity.