Unveiling the first indole-fused thiazepine: structure, synthesis and biosynthesis of cyclonasturlexin, a remarkable cruciferous phytoalexin.

As a mechanism of defense against pathogens and other types of stress, watercress plants produce a variety of elicited chemical defenses generally known as phytoalexins. Herein the chemical structure, synthesis, biosynthesis and antifungal activity of cyclonasturlexin, the most intriguing indolyl phytoalexin isolated to date, are reported.

Divergent reactivity of an indole glucosinolate yields Lossen or Neber rearrangement products: the phytoalexin rapalexin A or a unique ß-d-glucopyranose fused heterocycle.

Transformation of 1-t-Boc-4-methoxyindole-3-glucosinolate under acidic conditions yielded the potent phytoalexin rapalexin A, providing its first biomimetic synthesis via Lossen type rearrangement, while a novel 1-thioimidocarbonyl-ß-d-glucopyranose heterocyclic system was obtained under basic conditions via Neber type rearrangement.

Brassinin oxidase, a fungal detoxifying enzyme to overcome a plant defense -- purification, characterization and inhibition.

Blackleg fungi [Leptosphaeria maculans (asexual stage Phoma lingam) and Leptosphaeria biglobosa] are devastating plant pathogens with well-established stratagems to invade crucifers, including the production of enzymes that detoxify plant defenses such as phytoalexins. The significant roles of brassinin, both as a potent crucifer phytoalexin and a biosynthetic precursor of several other plant defenses, make it critical to plant fitness. Brassinin oxidase, a detoxifying enzyme produced by L. maculans both in vitro and in planta, catalyzes the detoxification of brassinin by the unusual oxidative transformation of a dithiocarbamate to an aldehyde. Purified brassinin oxidase has an apparent molecular mass of 57 kDa, is approximately 20% glycosylated, and accepts a wide range of cofactors, including quinones and flavins. Purified brassinin oxidase was used to screen a library of brassinin analogues and crucifer phytoalexins for potential inhibitory activity. Unexpectedly, it was determined that the crucifer phytoalexins camalexin and cyclobrassinin are competitive inhibitors of brassinin oxidase. This discovery suggests that camalexin could protect crucifers from attacks by L. maculans because camalexin is not metabolized by this pathogen and is a strong mycelial growth inhibitor.

Determination of the enantiomeric purity of the phytoalexins spirobrassinins by 1H NMR using chiral solvation.

A simple and inexpensive method for enantiomeric discrimination of the phytoalexins spirobrassinin (1), 1-methoxyspirobrassinin (2) and synthetic analog 1-methylspirobrassinin (6) using the chiral solvating agent 2,2,2-trifluoro-1-(9-anthryl)ethanol in C(6)D(6) is described. Using this method the enantiomeric composition of each sample can be determined accurately by (1)H NMR and the compounds can be recovered readily by chromatography.

Phytoalexins from Thlaspi arvense, a wild crucifer resistant to virulent Leptosphaeria maculans: structures, syntheses and antifungal activity.

Phytoalexins are inducible chemical defenses produced by plants in response to diverse forms of stress, including microbial attack. Our search for phytoalexins from cruciferous plants resistant to economically important fungal diseases led us to examine stinkweed or pennycress (Thlaspi arvense), a potential source of disease resistance to blackleg. We have investigated phytoalexin production in leaves of T. arvense under abiotic (copper chloride) and biotic elicitation by Leptosphaeria maculans (Desm.) Ces. et de Not. [asexual stage Phoma lingam (Tode ex Fr.) Desm.], and report here two phytoalexins, wasalexin A and arvelexin (4-methoxyindolyl-3-acetonitrile), their syntheses and antifungal activity against isolates of P. lingam/L. maculans, as well as the isolation of isovitexin, a constitutive glycosyl flavonoid of stinkweed, having antioxidant properties but devoid of antifungal activity.

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.

Vital staining of plant cell suspension cultures: evaluation of the phytotoxic activity of the phytotoxins phomalide and destruxin B.

A cell suspension culture assay to determine the phytotoxicity of the fungal toxins phomalide, a host-selective toxin produced by the fungus Phoma lingam (Tode ex Fr.) Desm., perfect stage Leptosphaeria maculans (Desm.) Ces. et de Not., and destruxin B, the major host-selective toxin produced by the fungus Alternaria brassicae (Berk.) Sacc., was carried out with three Brassica spp. It was established that phomalide was significantly less phytotoxic to Cutlass (Brassica juncea), the cultivar resistant to L. maculans, than to Westar (B. napus), the cultivar susceptible to L. maculans, at concentrations ≤2×10-5 M. Similar to phomalide, destruxin B, at concentrations ≤5×10-5 M, decreased the viability of cells of the cultivar resistant to A. brassicae (Ochre, Sinapis alba) less than the viability of cells of the susceptible cultivar (Westar, B. napus). Considering the high selectivity of phomalide and its direct correlation with plant disease resistance, phomalide may have great potential application in breeding programs screening/selecting for blackleg resistance in brassicas.