Polyketide Synthase–Terpenoid Synthase Hybrid Pathway Regulation of Trap Formation through Ammonia Metabolism Controls Soil Colonization of Predominant Nematode-Trapping Fungus

Abstract

Polyketide synthase−terpenoid synthase (PKS−TPS) hybrid pathways for biosynthesis of unique sesquiterpenyl epoxy-cyclohexenoids (SECs) have been found to be widely distributed in plant pathogenic fungi. However, the natural and ecological functions of these pathways and their metabolites still remain cryptic. In this study, the whole PKS−TPS hybrid pathway in the predominant nematode-trapping fungus Arthrobotrys oligospora was first proposed according to all the intermediates and their derivatives from all the A. oligospora mutants with a deficiency in each gene involved in SEC biosynthesis. Most mutants displayed significantly increased trap formation which was correlated with alteration of the ammonia level. Further analysis revealed that the main metabolites involved in ammonia metabolism were largely increased in most mutants. However, significantly retarded colonization in soil were observed in most mutants compared to the wild-type strain due to significantly decreased antibacterial activities. Our results suggested that A. oligospora used the PKS−TPS hybrid pathway for fungal soil colonization via decreasing fungal nematode-capturing ability. This also provided solid evidence that boosting fungal colonization in soil was the secondary metabolite whose biosynthesis depended on a PKS−TPS hybrid pathway.

Acetylation of Sesquiterpenyl Epoxy-Cyclohexenoids Regulates Fungal Growth, Stress Resistance, Endocytosis, and Pathogenicity of Nematode-Trapping Fungus Arthrobotrys oligospora via Metabolism and Transcription

“…The metabolites extracted from WT and mutant strains of A. oligospora were analyzed by UPLC–MS according to the methods described in the literature. − …”

“…For detecting conidial production, the WT and mutant strains were inoculated on YMA medium at 28 °C for 14 days. Then, the spores were washed with ddH 2 O with 2% Tween 80 and filtered through a funnel and the spore yields were calculated as described previously . For determining the spore germination rate, 2 × 10 4 spores were spread on a water agar (WA) plate and the germinated spores were counted at 4, 8, and 12 h, respectively.…”

“…Arthrobotrys oligospora is a predominant nematode-trapping fungus (NTF) that can produce adhesive three-dimensional networks mainly induced by ammonia to trap and kill nematodes when nutrients are scarce. , The outstanding abilities to survive in diverse ecological systems have made the fungus an ideal natural agent against plant parasitic nematodes. − Distinct from other NTF, Arthrobotrys oligospora can produce a unique class of a polyketide synthase–terpenoid synthase (PKS–TPS)-derived sesquiterpenyl epoxy-cyclohexenoids (SECs). , The class of SECs with significant morphological regulatory activity was responsible for fungal survival and colonization in soil and played a key role in the immense distribution of A. oligospora in diverse ecological systems. − …”

“…Previous studies revealed that a gene cluster AOL_s 00215 g (215 g ) containing a 6-methylsalicylic acid (6-MSA) PKS gene 283 was involved in the biosynthesis of SECs in A. oligospora (Figure A) . In the 215 g gene cluster, 9 genes including 274, 276, 277, 278, 279, 280, 281, 282, and 283 have been characterized for the biosynthesis of SECs through the construction of mutant strains with disruption of each gene combined with analysis of metabolic profiles of these mutants. − Genes 283, 281, and 282 were responsible for the biosynthesis of the first, second, and third PKS precursors, respectively, for SECs, and gene 276 was responsible for the biosynthesis of the first PKS–TPS hybrid precursor .…”

“…oligospora (Figure A) . In the 215 g gene cluster, 9 genes including 274, 276, 277, 278, 279, 280, 281, 282, and 283 have been characterized for the biosynthesis of SECs through the construction of mutant strains with disruption of each gene combined with analysis of metabolic profiles of these mutants. − Genes 283, 281, and 282 were responsible for the biosynthesis of the first, second, and third PKS precursors, respectively, for SECs, and gene 276 was responsible for the biosynthesis of the first PKS–TPS hybrid precursor . Genes 277, 278, 279, and 280 were involved in the biosynthesis of the farnesyl-cyclohexenol backbone of SECs and the epoxy formation. − Gene 274 was responsible for the introduction of the hydroxyl group in the epoxy farnesyl-cyclohexenol backbone .…”