Abstract
During the course of a study on the functional biodiversity of the mycobiota inhabiting rainforests in Thailand, a fungal strain was isolated from a plant sample and shown to represent an undescribed species, as inferred from a combination of morphological and molecular phylogenetic methods. Molecular phylogenetic analyses, based on four DNA loci, revealed a phylogenetic tree with the newly generated sequences clustering in a separate branch, together with members of the Sulcatisporaceae (Pleosporales, Ascomycota). The Thai specimen morphologically resembled Neobambusicola strelitziae in having pycnidial conidiomata with phialidic conidiogenous cells that produce both fusoid-ellipsoid macroconidia and subcylindrical microconidia. However, the new fungus, for which the name Pseudobambusicola thailandica is proposed, differs from N. strelitziae in having conidiomata with well-defined necks, the presence of globose to subglobose thick-walled cells adjacent to conidiomata and the production of chlamydospores in culture. When cultures of P. thailandica, growing on water agar, were confronted with Caenorhabditis elegans nematodes, worms approaching the fungal mycelia were killed. This observation gave rise to a study of its secondary metabolites and six novel and two known compounds were isolated from submerged cultures of P. thailandica. The structures of metabolites 1–6, for which the trivial names thailanones A–F are proposed, were elucidated using a combination of spectral methods, including extensive 1 and 2D NMR analysis and high resolution mass spectrometry. Compounds 4 and 8 showed strong nematicidal and weak antifungal activity, whereas all other tested compounds showed moderate to weak nematicidal activity but no significant effects in the serial dilution assay against various fungi and bacteria. Compounds 1 and 8 also inhibited growth of the pathogenic basidiomycete Phellinus tremulae in a plate diffusion assay.
Highlights
Fungi are regarded as prolific sources of secondary metabolites with prominent and selective biological activities that can serve as a basis for development of new antimicrobials, agrochemical pesticides and other useful compounds (Bills and Gloer 2016, Karwehl and Stadler 2017)
Colonies were sub-cultured on potato carrot agar (PCA; potatoes 20 g; carrots 20 g; agar 20 g; distilled water 1 l) and oatmeal agar (OA; oatmeal 30 g; agar 18 g; distilled water 1 l) as described previously (Hernández-Restrepo et al 2017)
The combined dataset consisted of 35 taxa with 3126 characters of which 396 bp corresponded to ITS, 853 bp to large subunit of the nrRNA gene (LSU), 904 bp to rpb2 and 973 bp to tef1
Summary
Fungi are regarded as prolific sources of secondary metabolites with prominent and selective biological activities that can serve as a basis for development of new antimicrobials, agrochemical pesticides and other useful compounds (Bills and Gloer 2016, Karwehl and Stadler 2017). Many novel compounds with, for example, antimicrobial (Helaly et al 2016, 2017, Richter et al 2016, Kuephadungphan et al 2017), cytotoxic (Surup et al 2017) and antioxidative (Kuhnert et al 2015) effects were isolated in the authors’ laboratory from tropical fungi. Fungi represent a rich source of nematicidal compounds because they are both prey and natural antagonists of nematodes. Understanding the chemical basis for fungi nematode interactions offers natural biocontrol strategies (Anke et al 1995). Little has been done to screen for metabolites in nematophagous fungi or, for that measure, nematicidal metabolites in other fungi since the first studies of this kind during the 1990s (Stadler et al 1993a, b, 1994)
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