Di Lecce, Roberta (2022) Bioactive metabolites produced by pathogenic fungi of forest plants. [Tesi di dottorato]

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Pathogenic fungi are the main causes of forest plant disease. They induce alterations during developmental stages and the development of diseases, in particular canker, which determine huge economic and environmental losses, especially for plant nurseries and wood industries. Fungal-plant host interactions represent biochemically complex systems that are being studied also by metabolites involved. Pathogenic fungi are able to produce phytotoxins, i.e. secondary metabolites, which are involved at different stages of plant pathogenesis. Considering the strong economic and ecological impact of forest plant diseases, the study of the plant-pathogen interaction and the metabolites involved is of remarkable importance. The aim of this PhD project has been on the investigation of phytotoxins produced by forest plants fungal pathogens. Culture filtrates have been obtained from Hymenoscyphus fraxineus, responsible of ash dieback, from Fimetariella rabenhorstii and Stilbocrea macrostoma, two fungi associated to Quercus brantii decline, from Diplodia olivarum, an aggressive pathogen of different plants in Italy, from Alternaria argyroxiphii, responsible of leaf spot of mahogany (Khaya senegalensis), and from Diplodia sapinea, associated to canker of Tunisian pine (Pinus pinaster). They have been exhaustively extracted and, the corresponding phytotoxic extracts have been purified to obtain metabolites by bio-guided chromatographic processes. A new metabolite, named hyfraxinic acid, has been isolated and characterized from the culture filtrates of H. fraxineus together with four furano diterpenoids. Among them, the relative and the absolute configurations have been assigned to 1-deoxyviridiol by crystallographic analyses and by the application of the advanced Mosher’s method. The new compound has been showed the most toxic effect in the phytotoxic assay. A new chromenone and a new benzophenone, named rabenchromenone and rabenzophenone, have been isolated from the culture filtrate of F. rabenhostrii together with moniliphenone and coniochaetone A. The absolute configuration of rabenchromenone has been determined by application of electronic circular dichroism (ECD) spectroscopy supported by quantum mechanical calculations compared with the experimental spectrum. In the leaf puncture assay, rabenzophenone has proved to be the most toxic compound. From the culture filtrate of S. macrostoma, three well-known compounds have been identified as 2,5-dihydroxymethylfuran, 5-hydroxymethyl-2-furaldehyde and tyrosol. All metabolites have been phytotoxic in the biological assay, but in particular the phytotoxicity of the two furan derivatives has been concentration-dependent. A new cleistanthane nor-diterpenoid, named olicleistanone, has been isolated and characterized from the culture filtrate of D. olivarum, which has showed to be a racemate. Other four pimarane diterpenoids and an isocoumarin has been identified. In the bioassays, olicleistanone has been showed a zootoxic activity against Artemia salina L. From the culture filtrate of A. argyroxiphii, three new metabolites, named argyrotoxins A-C, have been isolated and characterized. The absolute configuration of argyrotoxin A has been determined by quantum chemical calculations and electronic circular dichroism experiments using a chiroptical probe. Other known compounds have been identified as porritoxinol, its phthalide, the phthalide carboxylic acid derivative, zinniol, alternariol and its 4-methyl ether. In the leaf puncture assay, argyrotoxin A and B have been phytotoxic on Phaseolus vulgaris L. leaves only. Two new trisubstituted furanones, named pinofuranoxins A and B, have been isolated and characterized from the culture filtrate of D. sapinea. Their relative and absolute configurations have been assigned by NOESY experiment and by computational and ECD analyses, respectively. Both compounds have presented phytotoxic, antifungal and zootoxic activities and in details pinofuranoxin A has been more active than pinofuranoxin B.

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