Phytoalexins are antimicrobial and often antioxidative substances synthesized de novo by plants that accumulate rapidly at areas of pathogen infection. They are broad spectrum inhibitors and are chemically diverse with different types characteristic of particular plant species. Phytoalexins tend to fall into several classes including terpenoids, glycosteroids and alkaloids; however, researchers often find it convenient to extend the definition to include all phytochemicals that are part of the plant's defensive arsenal.
Phytoalexins produced in plants act as toxins to the attacking organism. They may puncture the cell wall, delay maturation, disrupt metabolism or prevent reproduction of the pathogen in question. Their importance in plant defense is indicated by an increase in susceptibility of plant tissue to infection when phytoalexin biosynthesis is inhibited. Mutants incapable of phytoalexin production exhibit more extensive pathogen colonization as compared to wild type. As such, host-specific pathogens capable of degrading phytoalexins are more virulent than those unable to do so.
When a plant cell recognizes particles from damaged cells or particles from the pathogen, the plant launches a two-pronged resistance: a general short-term response and a delayed long-term specific response.
As part of the induced resistance, the short-term response, the plant deploys reactive oxygen species such as superoxide and hydrogen peroxide to kill invading cells. In pathogen interactions, the common short-term response is the hypersensitive response, in which cells surrounding the site of infection are signaled to undergo apoptosis, or programmed cell death, in order to prevent the spread of the pathogen to the rest of the plant.
Long-term resistance, or systemic acquired resistance (SAR), involves communication of the damaged tissue with the rest of the plant using plant hormones such as jasmonic acid, ethylene, abscisic acid or salicylic acid. The reception of the signal leads to global changes within the plant, which induce genes that protect from further pathogen intrusion, including enzymes involved in the production of phytoalexins. Often, if jasmonates or ethylene (both gaseous hormones) is released from the wounded tissue, neighboring plants also manufacture phytoalexins in response. For herbivores, common vectors for disease, these and other wound response aromatics seem to act as a warning that the plant is no longer edible. Also, in accordance with the old adage, "an enemy of my enemy is my friend," the aromatics may alert natural enemies of the plant invaders to the presence thereof.
Recent research 
Allixin (3-hydroxy-5-methoxy-6-methyl-2-pentyl-4H-pyran-4-one), a non-sulfur-containing compound having a γ-pyrone skeleton structure, was the first compound isolated from garlic as a phytoalexin, a product induced in plants by continuous stress. This compound has been shown to have unique biological properties, such as anti-oxidative effects, anti-microbial effects, anti-tumor promoting effects, inhibition of aflatoxin B2 DNA binding, and neurotrophic effects. Allixin showed an anti-tumor promoting effect in vivo, inhibiting skin tumor formation by TPA in DMBA initiated mice. Herein, allixin and/or its analogs may be expected useful compounds for cancer prevention or chemotherapy agents for other diseases.
Role of natural phenols in the plant defense against fungal pathogens 
Natural phenols play a role in the plant defense against fungal pathogens.
In Vitis vinifera grape, trans-resveratrol is a phytoalexin produced against the growth of fungal pathogens such as Botrytis cinerea and delta-viniferin is another grapevine phytoalexin produced following fungal infection by Plasmopara viticola. Pinosylvin is a pre-infectious stilbenoid toxin (i.e. synthesized prior to infection), contrary to phytoalexins which are synthesized during infection. It is present in the heartwood of Pinaceae. It is a fungitoxin protecting the wood from fungal infection.
Sakuranetin is a flavanone, a type of flavonoid. It can be found in Polymnia fruticosa and rice, where it acts as a phytoalexin against spore germination of Pyricularia oryzae. In Sorghum, the SbF3'H2 gene, encoding a flavonoid 3'-hydroxylase, seems to be expressed in pathogen-specific 3-deoxyanthocyanidin phytoalexins synthesis, for example in Sorghum-Colletotrichum interactions.
Stilbenes are produced in Eucalyptus sideroxylon in case of pathogens attacks. Such compounds can be implied in the hypersensitive response of plants. High levels of polyphenols in some woods can explain their natural preservation against rot.
See also 
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- "The role of grape polyphenols on trans-resveratrol activity against Botrytis cinerea and of fungal laccase on the solubility of putative grape PR proteins, F. Favaron, M. Lucchetta, S. Odorizzi, A.T. Pais da Cunha and L. Sella, Journal of Plant Pathology (2009), 91 (3), pages 579–588, 2009 579" (PDF). Retrieved 2011-01-22.
- Timperio, Anna Maria; d’Alessandro, Angelo; Fagioni, Marco; Magro, Paolo; Zolla, Lello (2012). "Production of the phytoalexins trans-resveratrol and delta-viniferin in two economy-relevant grape cultivars upon infection with Botrytis cinerea in field conditions". Plant Physiology and Biochemistry 50 (1): 65–71. doi:10.1016/j.plaphy.2011.07.008. PMID 21821423.
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- Signals Regulating Multiple Responses to Wounding and Herbivores Guy L. de Bruxelles and Michael R Roberts
- The Myriad Plant Responses to Herbivores Linda L. Walling
- THE CHEMICAL DEFENSES OF HIGHER PLANTS GERALD A. ROSENTHAL
- Induced Systemic Resistance (ISR) Against Pathogens in the Context of Induced Plant Defences MARTIN HEIL
- Notes from the Underground Donald R. Strong and Donald A. Phillips
- Relationships Among Plants, Insect Herbivores, Pathogens, and Parasitoids Expressed by Secondary Metabolites Loretta L. Mannix