Chemical study of the blackleg fungus : metabolites, phytotoxins and phytoalexins
Date
2005-02
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ORCID
Type
Degree Level
Doctoral
Abstract
New isolates Mayfair 2 and Laird 2 of the blackleg fungus [Phoma lingam, perfect
stage Leptosphaeria maculans (Desm.) Ces. et de Not.] cause blackleg on the
traditionally resistant brown mustard (Brassica juncea). Mayfair 2/Laird 2 isolates of the
blackleg fungus produce a diverse array of secondary metabolites in liquid cultures in
minimal media. The isolation (FCC, RP-FCC, prep TLC) and characterization (NMR,
MS, X-ray) of these metabolites resulted in the identification of four groups of
compounds: polanrazines, sesquiterpenes, phomapyrones, and depsilairdin, where eleven
of them were new. Phytotoxic assays conducted with these metabolites on B. napus
(resistant), B. juncea (susceptible), and S. alba (resistant) indicated depsilairdin (122) to
be a host-selective toxin. Studies on the biosynthetic origin of phomapyrone A suggested
phomapyrones to be of polyketide origin.
Sixteen Thlaspi isolates of blackleg fungus originating from Saskatchewan were
characterized on the basis of their secondary metabolite profiles, and it was determined
that two of them were Polish type isolates and the remaining fourteen were weakly
virulent type isolates.
The investigation of Thlaspi arvense (stinkweed) for phytoalexins led to the
isolation of two phytoalexins: arvelexin (142) and wasalexin A (38). The structures of
these phytoalexins as well as that of isovitexin, a secondary metabolite of T. arvense,
were established based on spectroscopic methods and by synthesis. Arvelexin, as a
phytoalexin, was reported for the first time here, but wasalexin A was previously reported
from wasabi (Wasabia japonica, syn. Eutrema wasabi). Arvelexin (142) exhibited similar
antifungal activity as indole-3-acetonitrile against L. maculans/P. lingam.
The secondary metabolites potentially involved in mediating the interactions
between Brassica species and the new blackleg isolates Mayfair 2 and Laird 2 were
analysed in infected plant tissues by HPLC. The host-selective toxins phomalairdenone A
(55) and depsilairdin (122), and phytoalexins, brassinin (35), rutalexin (137) and
spirobrassinin (138) were detected in B. napus, whereas brassilexin (139) and sinalexin
(140) were detected in S. alba, and brassilexin (139) was detected in B. juncea.
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Degree
Doctor of Philosophy (Ph.D.)
Department
Chemistry
Program
Chemistry