Chemical Study of the Blackleg Fungus: Metabolites, Phytotoxins and Phytoalexins

by Chumala, Paulos Barbe

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.