Phytoalexins are antimicrobial secondary metabolites produced de novo by plants in response to stress, including microbial attack. In general, phytoalexins are important components of plant defenses against fungal and bacterial pathogens. The phytoalexins of crucifers are indole alkaloids derived from (S)-tryptophan, most of which contain a sulfur atom derived from cysteine. Beside their antimicrobial activity against different plant pathogenic species, cruciferous phytoalexins have shown anticarcinogenic effects on various human cell lines. This review focuses on the phytoalexins produced by cruciferous plants reported to date, with particular emphasis on their chemical synthesis, biosynthesis, metabolism by plant fungal pathogens and biological activities. A summary table containing all phytoalexins, their cultivated and wild cruciferous sources, their synthetic starting materials, biotransformation products and biological activities is provided.
A critical perspective on phytoanticipins, constitutive plant secondary metabolites with defensive roles against microbes is presented. This mini-review focuses on the chemical groups and structural types of defensive plant metabolites thus far not reviewed from the phytoanticipin perspective: i) fatty acid derivatives and polyketides, ii) terpenoids, iii) shikimates, phenylpropanoids and derivatives, and iv) benzylisoquinoline and pyrrolizidine alkaloids. The more traditional groups of phytoanticipins are briefly summarized, with particular focus on the latest results: i) benzoxazinoids, ii) cyanogenic glycosides, iii) glucosinolates and their metabolic products, and iv) saponins. Current evidence suggests that a better understanding of the functions of plant metabolites will drive their application to protect crops against microbial diseases.
Understanding defence pathways of plants is crucial to develop disease-resistant agronomic crops, an important element of sustainable agriculture. For this reason, natural plant defenses such as phytoalexins, involved in protecting plants against microbial pathogens, have enormous biotechnological appeal. Crucifers are economically important plants, with worldwide impact as oilseeds, vegetables of great dietetic value and even nutraceuticals. Notably, the intermediates involved in the biosynthetic pathways of unique cruciferous phytoalexins such as rapalexin A and isocyalexin A remain unknown. Toward this end, using numerous perdeuterated compounds, we have established the potential precursors of these unique phytoalexins and propose for the first time their detailed biosynthetic pathway. This pathway involves a variety of intermediates and a novel amino acid as the central piece of this complex puzzle. This work has set the stage for the discovery of enzymes and genes of the biosynthetic pathway of cruciferous phytoalexins of unique scaffolds.
Salt cress (Thellungiella salsuginea also known as T. halophila) is a wild cruciferous extremophile highly resistant to salt, drought, and cold. The recent discovery that salt cress produces the phytoalexins wasalexins A and B, and the phytoanticipins 1-methoxyglucobrassicin and 4-methoxyglucobrassicin in relatively higher amounts than other cruciferous species, prompted investigation of their biosynthetic relationships. Toward this end, perdeuterated 1-methoxybrassinin, l-Trp, glucobrassicin, 1-methoxyindolyl-3-acetaldoxime, brassinin, and methionine, as well as the corresponding natural abundance compounds, were administered to salt cress plants previously irradiated with UV-light (λ(max) 254 nm). Remarkably, administration of hexadeuterated glucobrassicin led to incorporation of several deuterium atoms into wasalexins A and B, 1-methoxyglucobrassicin and 4-methoxyglucobrassicin. This unprecedented discovery suggests that glucobrassicin is a biosynthetic precursor of wasalexins and methoxylated glucosinolates in salt cress.
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