Biotransformation of cruciferous phytoalexins by pathogenic fungi
Okanga, Francis Inyangala
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Studies on the biotransformation of the cruciferous phytoalexin cyclobrassinin, phytoalexin analogues methyl tryptamine dithiocarbamate, methyl tryptamine carbamate, Nb-carbomethoxyindole-3-methanamine and cyclobrassinin homologue by the cruciferous phytopathogenic fungi Phoma lingam, Alternaria brassicae and Rhizoctonia solani were undertaken. These studies were aimed at understanding the mechanism of phytoalexin detoxification by the pathogens. After an optimum period of incubation of the fungus and phytoalexin/analogue, cultures were filtered, the broth extracted and analyzed by TLC and HPLC. Flash column chromatography and/or preparative TLC resulted in isolation of the metabolites which were characterized by spectroscopic techniques (1H NMR, 13C NMR, FTIR and MS). Incubation of cyclobrassinin with P. lingam isolates yielded dioxibrassinin, 3-methylenaminoindole-2-thione and brassilexin as metabolites. Cyclobrassinin was metabolized by R. solani to yield three products identified as 2-mercaptoindole-3-carboxaldehyde, brassicanal A and 5-hydroxybrassicanal A. The biotransformation of methyl tryptamine dithiocarbamate by P. lingam isolates yielded Nb-acetyltryptamine, indole-3-acetic acid, indole-3-carboxylic acid, methyl indole-3-acetate, methyl 2-oxotryptamine dithiocarbamate, methyl tryptamine dithiocarbamate-S-oxide oxindole-3-acetic acid, methyl 3a-hydroxy-3,3a,8,8a-tetrahydropyrrolo[2,3-' b']indol-1(2H)-yl carbodithioate, tryptamine and tryptophol. The fungal pathogen A. brassicae biotransformed methyl tryptamine dithiocarbamate to yield two products identified as Nb-acetyltryptamine and tryptamine. Incubation of methyl tryptamine carbamate with 'P. lingam' isolates gave indole-3-acetic acid, indole-3-carboxylic acid and tryptophol as metabolites. Cyclobrassinin homologue yielded one metabolite identified as cyclobrassinin homologue metabolite on incubation with P. lingam isolates. For each compound and fungal isolate, further biotransformation of metabolites were carried out in order to map out the sequence and hence the biotransformation pathway(s). Bioassays were conducted to compare the antifungal activity of the parent compound and it's metabolites. The results of the biotransformation studies showed that the fungal transformations of phytoalexins/analogues were detoxification processes.