Involvement of Vitamin B6Biosynthesis Pathways in the Insecticidal Activity of Photorhabdus luminescens

Photorhabdus luminescens is a Gram-negative entomopathogenic bacterium which symbiotically associates with the entomopathogenic nematodeHeterorhabditis bacteriophora .P. luminescens is highly virulent to many insects and nonsymbiotic nematodes, includingCaenorhabditis elegans . To understand the virulence mechanisms ofP. luminescens , we obtained virulence-deficient and -attenuated mutants againstC. elegans through a transposon-mutagenized library. From the genetic screening, we identified thepdxB gene, encoding erythronate-4-phosphate dehydrogenase, as required forde novo vitamin B6 biosynthesis. Mutation inpdxB caused growth deficiency ofP. 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 B6 . Supplementation with three other B6 vitamers (pyridoxal, pyridoxine, and pyridoxamine) also restored the growth of thepdxB mutant, suggesting the existence of a salvage pathway for vitamin B6 biosynthesis inP. luminescens . Moreover, supplementation with PLP restored the virulence-deficient phenotype againstC. elegans . Combining these results with the fact thatpdxB mutation also caused attenuation of insecticidal activity, we concluded that the production of appropriate amounts of vitamin B6 is critical forP. luminescens pathogenicity.IMPORTANCE The Gram-negative entomopathogenic bacteriumPhotorhabdus luminescens symbiotically associates with the entomopathogenic nematodeHeterorhabditis bacteriophora .P. luminescens is highly virulent to many insects and nonsymbiotic nematodes, includingCaenorhabditis elegans . We have obtained several virulence-deficient and -attenuatedP. luminescens mutants againstC. elegans through genetic screening. From the genetic analysis, we present the vitamin B6 biosynthetic pathways inP. luminescens that are important for its insecticidal activity. Mutation inpdxB , encoding erythronate-4-phosphate dehydrogenase and required for thede novo vitamin B6 biosynthesis pathway, caused virulence deficiency againstC. elegans and growth deficiency ofP. luminescens in nutrient-poor medium. Because such phenotypes were restored under nutrient-rich conditions or by supplementation with B6 vitamers, we showed the presence of the two vitamin B6 synthetic pathways (de novo and salvage) inP. luminescens and also showed that the ability to produce an appropriate amount of vitamin B6 is critical forP. luminescens pathogenicity.