ABSTRACT
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.
Subject(s)
Host-Pathogen Interactions , Phytochemicals , Plants/metabolism , Ecosystem , Molecular Structure , Plants/chemistry , Plants/immunologyABSTRACT
In general, the chemodiversity of phytoalexins, elicited metabolites involved in plant defense mechanisms against microbial pathogens, correlates with the biodiversity of their sources. In this work, the phytoalexins produced by four wild cruciferous species (Brassica tournefortii, Crambe abyssinica (crambe), Diplotaxis tenuifolia (sand rocket), and Diplotaxis tenuisiliqua (wall rocket)) were identified and quantified by HPLC with photodioarray and electrospray mass detectors. In addition, the production of indole glucosinolates, biosynthetic precursors of cruciferous phytoalexins, was evaluated. Tenualexin, (=2-(1,4-dimethoxy-1H-indol-3-yl)acetonitrile), the first cruciferous phytoalexin containing two MeO substituents in the indole ring, was isolated from D. tenuisiliqua, synthesized, and evaluated for antifungal activity. The phytoalexins cyclobrassinin and spirobrassinin were detected in B. tournefortii and C. abyssinica, whereas rutalexin and 4-methoxybrassinin were only found in B. tournefortii. D. tenuifolia, and D. tenuisiliqua produced 2-(1H-indol-3-yl)acetonitriles as phytoalexins. Because tenualexin appears to be one of the broad-range antifungals occurring in crucifers, it is suggested that D. tenuisiliqua may have disease resistance traits important to be incorporated in commercial breeding programs.
Subject(s)
Acetonitriles/pharmacology , Antifungal Agents/pharmacology , Brassicaceae/chemistry , Fungi/drug effects , Glucosinolates/pharmacology , Indoles/pharmacology , Acetonitriles/chemistry , Acetonitriles/isolation & purification , Antifungal Agents/chemistry , Antifungal Agents/isolation & purification , Brassicaceae/classification , Dose-Response Relationship, Drug , Glucosinolates/chemistry , Glucosinolates/isolation & purification , Indoles/chemistry , Indoles/isolation & purification , Microbial Sensitivity Tests , Molecular Conformation , Molecular Structure , Structure-Activity RelationshipABSTRACT
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.
Subject(s)
Amino Acids/metabolism , Isothiocyanates/metabolism , Sesquiterpenes/chemical synthesis , Amino Acids/chemistry , Isothiocyanates/chemistry , Molecular Structure , Sesquiterpenes/chemistry , Sesquiterpenes/metabolismABSTRACT
Although isocyanides are not rare amongst terrestrial microbes and marine organisms, despite tens of thousands of natural products isolated from plants, isocyanides are still missing. Isocyalexin A is the first isocyanide of plant origin. Isocyalexin A was isolated from UV-irradiated rutabaga roots and shown to be a new cruciferous phytoalexin. Its chemical structure was proven by analysis of NMR spectroscopic data and chemical synthesis.
Subject(s)
Antifungal Agents/pharmacology , Brassica napus/chemistry , Fungi/drug effects , Indoles/pharmacology , Nitriles/pharmacology , Plant Roots/chemistry , Sesquiterpenes/pharmacology , Antifungal Agents/chemical synthesis , Antifungal Agents/chemistry , Dose-Response Relationship, Drug , Indoles/chemical synthesis , Indoles/chemistry , Isothiocyanates/chemical synthesis , Isothiocyanates/chemistry , Isothiocyanates/pharmacology , Magnetic Resonance Spectroscopy , Microbial Sensitivity Tests , Molecular Structure , Nitriles/chemical synthesis , Nitriles/chemistry , Sesquiterpenes/chemical synthesis , Sesquiterpenes/chemistry , Structure-Activity Relationship , PhytoalexinsABSTRACT
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.
Subject(s)
Brassicaceae/chemistry , Indole Alkaloids/chemistry , Sesquiterpenes , Indole Alkaloids/isolation & purification , Indole Alkaloids/metabolism , Indole Alkaloids/pharmacology , Molecular Structure , PhytoalexinsABSTRACT
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.
Subject(s)
Brassicaceae/metabolism , Glucosinolates/metabolism , Indoles/metabolism , Sesquiterpenes/metabolism , Sulfides/metabolism , Brassicaceae/radiation effects , Deuterium/metabolism , Glucosinolates/chemical synthesis , Glucosinolates/chemistry , Indoles/chemical synthesis , Indoles/chemistry , Plant Leaves/metabolism , Plant Leaves/radiation effects , Sesquiterpenes/chemistry , Sulfides/chemistry , Thiocarbamates/chemistry , Thiocarbamates/metabolism , Ultraviolet Rays , PhytoalexinsABSTRACT
The chemical structures, syntheses, metabolism and biological activities of the cruciferous phytoalexins discovered to date, with particular focus on the latest results dealing with their biosynthesis and detoxification are reviewed.