Publication Date:
2023-02-08
Description:
Plant pathogens remain a significant threat to crop yield and crop quality. Their control with chemical pesticides is faced with public resistance because of associated human and environmental toxicities. There is therefore a need for more novel and ‘friendlier’ agrochemicals, such as natural products, for the control of plant disease. Fungi have a long history in crop protection as a source of bioactive compounds. This is exemplified by strobilurins, terrestrial fungal secondary metabolites, which have a large market share of current agricultural fungicides. However, efforts to discover new natural products from terrestrial fungi are limited by re-isolation of known compounds. This is in part, because a subset of biosynthetic gene clusters (BGCs) remain silent and are not transcribed in laboratory monoculture conditions. This study sought to exploit the biosynthetic potential of fungi for discovery of new anti-phytopathogenic natural products. Towards this aim, chemical investigations were performed: (i) on fungal isolates from a less studied ecosystem, Windebyer Noor – including an unexploited habitat, sea foam, (ii) on fungal isolates from less studied genera, and (iii) after employing biosynthetic gene activating strategies, such as co-cultivation and variation of cultivation media. Molecular networking-based metabolomics was employed to facilitate dereplication of fungal metabolites, and to prioritize fungal extracts for isolation of new compounds with anti-phytopathogenic activity. A unique and direct co-cultivation approach involving 21 fungal strains (out of 123 fungal isolates obtained from the Windebyer Noor) against four phytopathogens (two bacteria: Pseudomonas syringae and Ralstonia solanacearum, and two fungi: Magnaporthe oryzae and Botrytis cinerea) on solid agar was employed. Organic extracts of the mono- and co-cultures were monitored by comparison of their anti-phytopathogenic activities and metabolome by untargeted metabolomics, using MS/MS-based molecular networking (MN). The phytopathogens established interspecies communications with marine fungi, resulting in the production of 9 new compound clusters exclusively produced in the co-cultures. Overall, 62 compounds belonging to terpenes, alkaloids, peptides, and polyketides were annotated, in addition to several new putative metabolites with potential anti-phytopathogenic activities. This new combined approach of direct co-cultivation, metabolomics and anti-phytopathogenic activity assessments, allowed the prioritization of two co-cultures, Cosmospora sp. against M. oryzae and Acremonium sp. against P. syringae, for future isolation of natural products. Another new co-cultivation approach was employed between sediment-derived isolates obtained from the Windebyer Noor. Strains were categorized as ‘strong’ or ‘weak’ based on their anti-phytopathogenic potencies. Intra- and inter-categorial co-cultivations were monitored by their anti-phytopathogenic activities and chemical profiles. The co-culture of Plenodomus influorescens (strong) and Pyrenochaeta nobilis (weak), representing less-studied genera, was selected for chemical investigation. From the dichloromethane subextract of this prioritized co-culture, five natural products were isolated; two 12-membered macrolides, dendrodolide E and its new analog dendrodolide N, two azaphilones spiciferinone and its new analog 8a-hydroxy-spiciferinone and a known bis-naphtho-γ-pyrone type mycotoxin, cephalochromin. Structures of these compounds were elucidated by 1D and 2D NMR, HRMS and [α]D analyses. Cephalochromin was over expressed in P. influorescens in response to the interspecies crosstalk with P. nobilis, and further exhibited strong anti-phytopathogenic activity against Xanthomonas campestris and Phytophthora infestans. Fungal isolates derived from sea foam, an unexploited habitat, obtained from the Windebyer Noor, were investigated for their anti-phytopathogenic potential. The metabolome of six representative strains was studied using metabolomics, incorporating molecular networking. In total, 49 metabolites were putatively annotated to known bioactive compounds with several derivatives representing new natural products. Penicillium sp. isolate 31.68F1B showed four strain-specific molecular families with strong inhibitory activity against the phytopathogen M. oryzae (IC50: 2.2 μg/mL), and was prioritized for future isolation of novel natural products. This study has successfully highlighted the enormous potential of marine fungi for the discovery of anti-phytopathogenic compounds. Furthermore, the discovery of novel natural products and a novel approach in strain selection for co-cultivation in this study presents optimism for control of pathogens in the agrochemical industry. It has discovered new natural products and a new approach in strain selection for co-cultivation.
Type:
Thesis
,
NonPeerReviewed
Format:
text
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