Nematicidal Key Precursors for the Biosynthesis of Morphological Regulatory Arthrosporols in the Nematode-Trapping Fungus Arthrobotrys oligospora.

Arthrobotrys oligospora is the first recognized nematode-trapping fungus and by far the most abundant in the environment. Our recent study revealed the polyketide synthase (PKS) gene AOL_s00215g283 in A. oligospora involved in the production of many secondary metabolites and the trap formation of the fungus. Here we report that the disruption of two genes in the upstream flanking region of the gene AOL_s00215g283, AOL_s00215g281 and AOL_s00215g282, which putatively encoded one amidohydrolase and one cytochrome P450 monooxygenase, respectively, both resulted in significant nematicidal activity of the cultural broths of the mutants and loss of morphological regulatory arthrosporols. Chemical investigation revealed the huge accumulation of 6-methylsalicylic acid in the cultural broth of the mutant ΔAOL_s00215g281 and the high production of m-cresol in the mutant ΔAOL_s00215g282, respectively. Further bioassay revealed that 6-methylsalicylic acid and m-cresol displayed significant nematicidal activity toward root-knot nematodes Meloidogyne incognita with IC values of 300 and 100 μg/mL, respectively. The mutant ΔAOL_s00215g282 displayed a more complex metabolite profile than the mutant ΔAOL_s00215g281, suggesting that m-cresol was a more versatile key precursor than 6-methylsalicylic acid. These findings not only demonstrated that the gene AOL_s00215g283 encodes the 6-methylsalicylic acid synthase and the gene AOL_s00215g281 encodes the decarboxylase for 6-methylsalicylic acid but also provided evidence for the potential functions of the precursors in fungal complex biosynthetic pathways and had more implications for the establishment of efficient fungal biocontrol agents.