Glucosinolate response in winter oilseed rape Brassica napus ssp. oleifera to Verticillium dahliae (non-pathogenic) and V. longisporum comb. nov., Karapapa, Bainbridge and Heale, 1997 (pathogenic)

摘要

Glucosinolates are a class of about 100 naturally occurring thioglucosides characteristic of the Cruciferae and related families in the order Capparales. These thioglucosides differ in the molecular structure of the side-chain R constituted of aliphatic, aromatic and heteroaromatic residues. The hydrolysis of glucosinolates involves enzymatic attack by myrosinases on the thioglucosidic bond to yield D-glucose and an unstable thiohydroximate-O-sulphonate; the latter rearranges non-enzymatically to form sulphate and one of a wide range of products which are bio-active compounds. Aliphatic and aromatic glucosinolates can give rise to volatile isothiocyanates (at pH: 5-7), or nitriles (at pH: 3), thiocyanates and epithionitriles. Indole glucosinolatesgive rise to unstable isothiocyanates (at pH: 7), or to indolylnitrile and elemental sulphur (at pH: 4). The glucosinolate-myrosinase system has been assumed to act as part of the general host defence mechanisms (5); firstly, based on studies investigating the plant glucosinolate response to microbes, insects and wounding, and secondly, the in vitro activity of glucosinolate breakdown products towards several pathogens. Indole-glucosinolate synthesis, and in some cases, aliphatic glucosinolate synthesis, are induced by Brassica pests and pathogens, including Leptosphaeria maculans (15), Plasmodiophora brassicae (6, 17), Alternaria brassicae (8), Delia floralis (1, 2, 9), Psylliodes chrysocephala (13), as well as by artificial damage (3, 9). On the contrary, concentrations of the aromatic glucosinolate gluconasturtiin (GLNS) were found to be in significantly higher amounts in healthy control roots as compared with roots infested with Delia floralis (1). In vitro experiments have demonstrated the antifungal activity of enzymatically-hydrolysed, glucosinolate products, against a large number of different fungi (7, 14). Verticillium wilt of oilseed rape is caused by a fungus recently characterized as a near-diploid species viz V. longisporum comb. nov.(11 and see this Symposium). All V. longisporum isolates (except for 4 strains shown to be recombinants), tested against winter oilseed rape cultivars were highly virulent, whereas V. dahliae isolates were nonpathogenic (11). The main aim of this study was to examine how a compatible, pathogenic, isolate (161) of V. longisporum might overcome, or avoid the glucosinolate defence system of oilseed rape, as compared with an incompatible, non-pathogenic, typical isolate (130) of V. dahliae (i.e. an haploidspecies). The glucosinolate levels and profiles of inoculated (using 161 or 130) and uninoculated (control), oilseed rape plants of cv. Cobra, were investigated employing HPLC analysis of glucosinolates in roots, hypocotyls, cotyledons and leaves at 3, 10 and 15 d post-inoculation. The toxicity of the breakdown products of 6 individual glucosinolates in the presence of the enzyme myrosinase (EC 3.2.3.1), on growth of isolates of both V. longisporum and V. dahliae, as well as the individual toxicity of the known GLNS breakdown products (phenylethyl isothiocyanate and propenyl nitrile), were investigated on amended media.