Transformation of the host-selective toxin destruxin B by wild crucifers: probing a detoxification pathway.

The destruxin B detoxification pathway present in Sinapis alba is also present in three unrelated species, Camelina sativa, Capsella bursa-pastoris, and Eruca sativa, suggesting a conservation of this pathway across crucifers. The chemical structure of a destruxin B metabolite, (6'-O-malonyl)hydroxydestruxin B beta-D-glucopyranoside, was also establised. Considering that Camelina sativa and Capsella bursa-pastoris detoxify destruxin B and produce the phytoalexins camalexins, these wild crucifers appear to represent unique and perhaps useful sources of blackleg resistance in strategic plant breeding.

Unprecedented Vilsmeier formylation: expedient syntheses of the cruciferous phytoalexins sinalexin and brassilexin and discovery of a new heteroaromatic ring system.

[reaction: see text]. A very concise first synthesis of sinalexin was achieved by regioselective formylation of 1-methoxyindoline-2-thione under Vilsmeier conditions followed by unprecedented ammonia workup. Similar formylation of indoline-2-thione yielded brassilexin and a novel pentacyclic heteroaromatic compound resulting from condensation of the Vilsmeier adduct of indoline-2-thione. Both sinalexin and brassilexin displayed strong antifungal activity against several pathogens of crucifers.

In planta sequential hydroxylation and glycosylation of a fungal phytotoxin: Avoiding cell death and overcoming the fungal invader.

To facilitate plant colonization, some pathogenic fungi produce phytotoxic metabolites that damage tissues; plants may be resistant to a particular pathogen if they produce an enzyme(s) that catalyzes detoxification of this metabolite(s). Alternaria blackspot is one of the most damaging and significant fungal diseases of brassica crops, with no source of resistance known within the Brassica species. Destruxin B is the major phytotoxin produced by the blackspot-causing fungus, Alternaria brassicae (Berkley) Saccardo. We have established that a blackspot-resistant species (Sinapis alba) metabolized (14)C-labeled destruxin B to a less toxic product substantially faster than any of the susceptible species. The first metabolite, hydroxydestruxin B ((14)C-labeled), was further biotransformed to the beta-d-glucosyl derivative at a slower rate. The structures of hydroxydestruxin B and beta-d-glucosyl hydroxydestruxin B were deduced from their spectroscopic data [NMR, high resolution (HR)-MS, Fourier transform infrared (FTIR)] and confirmed by total chemical synthesis. Although these hydroxylation and glucosylation reactions occurred in both resistant (S. alba) and susceptible (Brassica napus, Brassica juncea, and Brassica rapa) species, hydroxylation was the rate limiting step in the susceptible species, whereas glucosylation was the rate limiting step in the resistant species. Remarkably, it was observed that the hydroxydestruxin B induced the biosynthesis of phytoalexins in blackspot-resistant species but not in susceptible species. This appears to be a unique example of phytotoxin detoxification and simultaneous phytoalexin elicitation by the detoxification product. Our studies suggest that S. alba can overcome the fungal invader through detoxification of destruxin B coupled with production of phytoalexins.

Phytoalexins from crucifers: synthesis, biosynthesis, and biotransformation.

Phytoalexins play a significant role in the defense response of plants. These secondary metabolites, which are synthesized de novo in response to diverse forms of stress, including fungal infection, are part of the plants' chemical and biochemical defense mechanisms. Phytoalexins from crucifers are structurally and biogenetically related, but display significantly different biological activities. Here, we review work reporting the chemical structures, synthesis, biosynthesis and metabolism of cruciferous phytoalexins, as well as their biological activity towards different microorganisms.

Sinalbins A and B, phytoalexins from Sinapis alba: elicitation, isolation, and synthesis.

The chemical structure and synthesis of sinalbin A is described. This cruciferous phytoalexin is produced by white mustard (Sinapis alba) after treatment with biotic and abiotic elicitors. In addition, a related metabolite, named sinalbin B, is present in extracts from elicited plants, but not in those from non-elicited controls. Sinalbin B, which was also synthesized, appears to be both a phytoalexin and a biosynthetic precursor of sinalbin A.

Wasalexins A and B, new phytoalexins from wasabi: isolation, synthesis, and antifungal activity.

The chemical structure determination of two phytoalexins from wasabi (Wasabia japonica, syn. Eutrema wasabi), a plant resistant to virulent isolates of the blackleg fungus [Leptosphaeria maculans (Desm.) Ces. et de Not., asexual stage Phoma lingam (Tode ex Fr.) Desm.], as well as their synthesis and antifungal activity towards isolates of P. lingam and P. wasabiae is reported.