Metabolomics insights into the polyketide-lactones produced by Diaporthe caliensis sp. nov., an endophyte of the medicinal plant Otoba gracilipes

Plants of the genus Otoba have been the basis for the treatment of tropical diseases in indigenous communities of countries like Colombia. Despite the lack of knowledge about their bioactive principles, endophytic fungi derived from medicinal plants are a prolific source of innovative chemistry. We systematically investigated the secondary metabolite production of a previously undescribed species of Diaporthe, herein introduced as Diaporthe caliensis sp. nov., using different metabolomics approaches together with classical chemical screening. To get an outline of the chemical space produced by this fungus, an exploratory molecular networking (MN) analysis was undertaken. A major molecular family was found to contain the known 10-membered lactone phomol (1), together with other putative congeners as compound 3. After isolation by preparative high-performance liquid chromatography, we confirmed phomol (1) as the main reason for the antimicrobial activity of the crude extract. The unknown absolute configuration of 1 was determined by the synthesis of alpha-methoxy-alpha-trifluoromethylphenylacetyl (MTPA)-esters and chemical degradation experiments. Moreover, caliensolides A (2) and B (3) were isolated, and their structures were elucidated as novel butenolides structurally unrelated to 1. Overall, the initial MN analysis incorrectly clustered compounds 1 and 3 within a single molecular family, despite evident differences in chemical structures and biosynthetic origin. Contrariwise, the unsupervised substructure discovery algorithm MS2LDA provided a deeper understanding of the fragmentation patterns and correctly clustered the polyketide-lactones produced by D. caliensis sp. nov. Our findings encourage the exploration of Colombian fungal diversity, which as demonstrated here could result in the discovery of new natural products.IMPORTANCEThe integration of metabolomics-based approaches into the discovery pipeline has enabled improved mining and prioritization of prolific secondary metabolite producers such as endophytic fungi. However, relying on automated untargeted analysis tools might lead to misestimation of the chemical complexity harbored in these organisms. Our study emphasizes the importance of isolation and structure elucidation of the respective metabolites in addition to deep metabolome analysis for the correct interpretation of untargeted metabolomics approaches such as molecular networking. Additionally, it encourages the further exploration of endophytic fungi from traditional medicinal plants for the discovery of natural products.

Automated summary

This study systematically investigated the secondary metabolite production of a previously undescribed species of Diaporthe from medicinal plants and discovered several compounds, confirming phomol as the main antimicrobial component. Additionally, novel butenolides unrelated to phomol were identified. The study emphasizes the importance of correct interpretation of untargeted metabolomics approaches and encourages further exploration of endophytic fungi from traditional medicinal plants for the discovery of new natural products.