PhD Thesis, Chintamani Thapa
Date
2019-07-29
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Journal Title
Journal ISSN
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Publisher
ORCID
Type
Thesis
Degree Level
Doctoral
Abstract
The first part of this thesis focuses on the investigation of the metabolism of cruciferous phytoalexins by Colletotrichum species (C. dematium, C. higginsianum and C. lentis). In the second part the isolation and characterization of phytotoxic and other secondary metabolites from the cruciferous phytopathogen C. higginsianum was explored.
Members of the Colletotrichum species are known to infect numerous plant species including crucifer species. Crucifers produce phytoalexins as a response to abiotic or biotic stresses, while some crucifer pathogenic fungi are able to metabolise such phytoalexins to less toxic metabolites. In this thesis, the metabolism of three important cruciferous phytoalexins (camalexin, brassinin and rapalexin A) by three Colletotrichum species (C. dematium, C. higginsianum and C. lentis) with different host ranges was investigated. All three species metabolized camalexin slowly and the metabolic products appeared to be different for each species. C. dematium metabolized camalexin into three metabolites: indole-3-carbonitrile, N-(2-hydroxyethyl)-indole-3-carboxamide and N-(2-hydroxyethyl)-indole-3-thiocarboxamide. The antifungal activity of these metabolites was significantly lower than that of camalexin. Chrysogine, a fungal metabolite, was produced in larger amounts when C. higginsianum was incubated with camalexin. The metabolism of brassinin was slow and yielded multiple uncharacterized products. The metabolism of rapalexin A, by C. dematium and C. higginsianum yielded two previously undescribed metabolites, N-Acetyl-S-(8-methoxy-4H-thiazolo[5,4-b]indol-2-yl)-L-cysteine and 4-hydroxy-3-(4-methoxy-1H-indol-3-yl)-2-thioxothiazolidine-4-carboxylic acid. The formation of these products suggested that Colletotrichum species metabolised isothiocyanates via the mercapturic acid pathway. To prove this hypothesis, the metabolism of two other isothiocyanates (benzyl and phenyl isothiocyanate) was investigated. Similar adducts of isothiocyanate with L-cysteine were observed. C. lentis was unable to metabolise rapalexin A.
C. higginsianum is a host specific pathogen of crucifers and causes anthracnose disease which lead to significant yield losses. In order to isolate phytotoxic metabolites from the fungus, culture extracts were obtained by growing fungus in different liquid culture medium were tested for phytotoxicity against the host plant. Only the culture extracts obtained from Potato Dextrose Broth (PDB) medium displayed phytotoxicity. The phytotoxicity-guided isolation of the crude extracts lead to the isolation of several metabolites from the culture: N1-(1-hydroxyethyl)phthalimide, tyrosol, higginsianin A, 6-(2-hydroxyethyl)-2-((1E,3E)-nona-1,3-dien-1-yl)tetrahydro-2H-pyran-3-ol, lovastatin and chrysogine. Among these metabolite 6-(2-hydroxyethyl)-2-((1E,3E)-nona-1,3-dien-1-yl)tetrahydro-2H-pyran-3-ol displayed mild phytotoxicity against the host Chinese cabbage.
Description
Keywords
Colletotrichum, metabolism, crucifers, phytoalexins, detoxification, brassinin, camalexin, rapalexin A
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Chemistry
Program
Chemistry