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Phytoalexins are antimicrobial substances, some of which are antioxidative as well. They are defined not by their having any particular chemical structure or character, but by the fact that they are defensively synthesized de novo by that produce the compounds rapidly at sites of pathogen infection. In general phytoalexins are broad spectrum inhibitors; they are chemically diverse, and different chemical classes of compounds are characteristic of particular plant taxon. Phytoalexins tend to fall into several chemical classes, including , glycomics, and ; however, the term applies to any that are induced by microbial infection.
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 such as jasmonic acid, ethylene, abscisic acid, or salicylic acid. The reception of the signal leads to global changes within the plant, which induce expression of genes that protect from further pathogen intrusion, including enzymes involved in the production of phytoalexins. Often, if jasmonates or ethylene (both gaseous hormones) are released from the wounded tissue, neighboring plants also manufacture phytoalexins in response. For herbivores, common disease vector for , 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.
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 plant pathology-specific 3-deoxyanthocyanidin phytoalexin synthesis, for example in Sorghum-Colletotrichum interactions.
6-Methoxymellein is a dihydroisocoumarin and a phytoalexin induced in carrot slices by ultraviolet C, that allows resistance to Botrytis cinerea and other .
Danielone is a phytoalexin found in the papaya fruit. This compound showed high antifungal activity against Colletotrichum gloesporioides, a pathogenic fungus of papaya.
Stilbenoid are produced in Eucalyptus sideroxylon in case of pathogen attacks. Such compounds can be implied in the hypersensitive response of plants. High levels of in some woods can explain their natural preservation against rot.
are phytoalexins produced by Avena sativa in its response to Puccinia coronata var. avenae f. sp. avenae, the oat crown rust. (Avenanthramides were formerly called avenalumins.)
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