Metabolism of crucifer phytoalexins in Sclerotinia sclerotiorum: detoxification of strongly antifungal compounds involves glucosylation

Abstract

The strongly antifungal phytoalexins brassilexin and sinalexin were metabolized by the stem rot fungus Sclerotinia sclerotiorum to glucosyl derivatives, whereas the phytoalexins brassicanal A, spirobrassinin and 1-methoxyspirobrassinin, displaying lower antifungal activity, were transformed via non-glucosylating pathways. Significantly, these transformations led to metabolites displaying no detectable antifungal activity. The chemical characterization of all new metabolites as well as the chemistry of these processes and a facile chemical synthesis of 1-beta-D-glucopyranosylbrassilexin are reported. Overall, our results indicate that phytoalexins, strongly antifungal against S. sclerotiorum, are detoxified via glucosylation, which in turn suggests that S. sclerotiorum has acquired efficient glucosyltransferase(s) that can disarm some of the most active plant chemical defenses. Consequently, we suggest that these glucosylation reactions are potential metabolic targets to control S. sclerotiorum.

Cloning, purification and characterisation of brassinin glucosyltransferase, a phytoalexin-detoxifying enzyme from the plant pathogen Sclerotinia sclerotiorum

“…This enzyme, SsBGT1, was much less effective at glucosylating cyclobrassinin (5% activity relative to that of brassinin) than a cell-free extract of mycelia from phytoalexin-induced cultures of S. sclerotiorum (83% activity relative to that of brassinin) . Brassilexin was not a substrate of SsBGT1, although previous analyses of cell cultures have shown that, like brassinin, brassilexin is N-glucosylated (Pedras and Hossain, 2006). Similarly, 6-methoxycamalexin was glucosylated to a very low extent, contrary to previous results obtained in cell cultures (Pedras and Ahiahonu, 2002).…”

“…1G and H). Other phytoalexins such as sinalexin, camalexin and 6-methoxycamalexin are oxidised before glucosylation is carried out (Pedras and Hossain, 2006). Brassinin glucosyltransferase (BGT) activity in crude cell-free extracts of S. sclerotorium is induced by the phytoalexins camalexin and, to a much lesser extent by spirobrassinin, and by the brassinin analogues methyl tryptamine dithiocarbamate and methyl 1-methyltryptamine dithiocarbamate .…”

Phytoalexins from Brassicaceae: News from the front

“…5) (Sexton et al, 2009). Previously, S. sclerotiorum was found to detoxify cyclobrassinin (6), brassilexin (32) (Pedras and Hossain, 2006) and 6-methoxycamalexin (37) via glucosylation (Pedras and Ahiahonu, 2002), therefore it is likely that another glucosyl transferase(s) specific for glucosylation of these phytoalexins is(are) produced by S. sclerotiorum.…”

Catalytic Enantioselective Henry Reactions of Isatins: Application in the Concise Synthesis of (S)‐(−)‐Spirobrassinin

“…For example, (À)-4, as a representative cruciferous phytoalexin, is a useful model for the study of a plants chemical and biochemical defense mechanisms. [16] Herein, we present the first asymmetric synthesis of this target in four steps (Scheme 3). [19] A related approach to that of the non-asymmetric version reported by Kutschy, Monde, and co-workers [19b] was explored to find an asymmetric synthesis by taking advantage of highly enantioenriched adduct 3 a.…”

“…[14] The compound displays various biological properties including plant defense, [15] antifungal, [16] and antitumor activities, [17] as well as oviposition stimulation. [18] .…”