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

plant Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...

s 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

taxa In biology, a taxon (back-formation from ''taxonomy''; : taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and ...

. Phytoalexins tend to fall into several chemical classes, including

terpenoid The terpenoids, also known as isoprenoids, are a class of naturally occurring organic compound, organic chemicals derived from the 5-carbon compound isoprene and its derivatives called terpenes, diterpenes, etc. While sometimes used interchangeabl ...

s, glyco

steroid A steroid is an organic compound with four fused compound, fused rings (designated A, B, C, and D) arranged in a specific molecular configuration. Steroids have two principal biological functions: as important components of cell membranes t ...

s, and

alkaloid Alkaloids are a broad class of natural product, naturally occurring organic compounds that contain at least one nitrogen atom. Some synthetic compounds of similar structure may also be termed alkaloids. Alkaloids are produced by a large varie ...

s; however, the term applies to any phytochemicals that are induced by microbial infection.

Function

Phytoalexins are produced in plants to act as toxins to the attacking organism. They may puncture the

cell wall A cell wall is a structural layer that surrounds some Cell type, cell types, found immediately outside the cell membrane. It can be tough, flexible, and sometimes rigid. Primarily, it provides the cell with structural support, shape, protection, ...

, 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 types. 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 In chemistry, a superoxide is a compound that contains the superoxide ion, which has the chemical formula . The systematic name of the anion is dioxide(1−). The reactive oxygen ion superoxide is particularly important as the product of t ...

and

hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscosity, viscous than Properties of water, water. It is used as an oxidizer, bleaching agent, and antiseptic, usua ...

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 Apoptosis (from ) is a form of programmed cell death that occurs in multicellular organisms and in some eukaryotic, single-celled microorganisms such as yeast. Biochemistry, Biochemical events lead to characteristic cell changes (Morphology (biol ...

, 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 hormone Plant hormones (or phytohormones) are signal molecules, produced within plants, that occur in extremely low concentrations. Plant hormones control all aspects of plant growth and development, including embryogenesis, the regulation of Organ (anat ...

s such as jasmonic acid,

ethylene Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon–carbon bond, carbon–carbon doub ...

, 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 vectors for

plant disease Plant diseases are diseases in plants caused by pathogens (infectious organisms) and environmental conditions (physiological factors). Organisms that cause infectious disease include fungi, oomycetes, bacteria, viruses, viroids, virus-like or ...

s, 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-4''H''-pyran-4-one), a non-sulfur-containing compound having a γ-pyrone skeletal 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 Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...

binding, and neurotrophic effects. Allixin showed an anti-tumor promoting effect in vivo, inhibiting skin

tumor A neoplasm () is a type of abnormal and excessive growth of tissue. The process that occurs to form or produce a neoplasm is called neoplasia. The growth of a neoplasm is uncoordinated with that of the normal surrounding tissue, and persists ...

formation by TPA in DMBA initiated mice. Herein, allixin and/or its analogs may be expected to be useful compounds for cancer prevention or chemotherapy agents for other diseases.

Role of natural phenols in the plant defense against fungal pathogens

Polyphenol Polyphenols () are a large family of naturally occurring phenols. They are abundant in plants and structurally diverse. Polyphenols include phenolic acids, flavonoids, tannic acid, and ellagitannin, some of which have been used historically as ...

s, especially isoflavonoids and related substances, play a role in the plant defense against fungal and other microbial pathogens. In ''

Vitis vinifera ''Vitis vinifera'', the common grape vine, is a species of flowering plant, native to the Mediterranean Basin, Mediterranean region, Central Europe, and southwestern Asia, from Morocco and Portugal north to southern Germany and east to northern ...

'' grape, ''trans''-

resveratrol Resveratrol (3,5,4′-trihydroxy-''trans''-stilbene) is a stilbenoid, a type of natural phenol or polyphenol and a phytoalexin produced by several plants in response to injury or when the plant is under attack by pathogens, such as bacterium, ba ...

is a phytoalexin produced against the growth of fungal pathogens such as ''

Botrytis cinerea ''Botrytis cinerea'' is a necrotrophic fungus that affects many plant species, although its most notable hosts may be wine grapes. In viticulture, it is commonly known as "botrytis bunch rot"; in horticulture, it is usually called "grey mould" ...

'' and delta-viniferin is another grapevine phytoalexin produced following fungal infection by '' Plasmopara viticola''. Pinosylvin is a pre-infectious

stilbenoid Stilbenoids are hydroxylated derivatives of stilbene. They have a C6–C2–C6 structure. In biochemical terms, they belong to the family of phenylpropanoids and share most of their biosynthesis pathway with Chalconoid, chalcones. Most stilbenoids ...

toxin (i.e. synthesized prior to infection), contrary to phytoalexins which are synthesized during infection. It is present in the

heartwood Wood is a structural tissue/material found as xylem in the stems and roots of trees and other woody plants. It is an organic materiala natural composite of cellulosic fibers that are strong in tension and embedded in a matrix of lignin t ...

of ''

Pinaceae The Pinaceae (), or pine family, are conifer trees or shrubs, including many of the well-known conifers of commercial importance such as Cedrus, cedars, firs, Tsuga, hemlocks, Pinyon_pine, piñons, larches, pines and spruces. The family is incl ...

''. 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 Rice is a cereal grain and in its Domestication, domesticated form is the staple food of over half of the world's population, particularly in Asia and Africa. Rice is the seed of the grass species ''Oryza sativa'' (Asian rice)—or, much l ...

, where it acts as a phytoalexin against spore germination of '' Pyricularia oryzae''. In ''

Sorghum ''Sorghum bicolor'', commonly called sorghum () and also known as great millet, broomcorn, guinea corn, durra, imphee, jowar, or milo, is a species in the Poaceae, grass genus ''Sorghum (genus), Sorghum'' cultivated for its grain. The grain i ...

'', the ''SbF3'H2'' gene, encoding a flavonoid 3'-hydroxylase, seems to be expressed in

pathogen In biology, a pathogen (, "suffering", "passion" and , "producer of"), in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a Germ theory of d ...

-specific

3-deoxyanthocyanidin The 3-Deoxyanthocyanidins and their glycosides (3-deoxyanthocyanins or 3-DA) are molecules with an anthocyanidins backbone lacking an hydroxyl group at position 3 on the C-ring. This nomenclature is the inverse of that which is commonly used in ...

phytoalexin synthesis, for example in ''Sorghum- Colletotrichum'' interactions. 6-Methoxymellein is a dihydroisocoumarin and a phytoalexin induced in carrot slices by UV-C, that allows resistance to ''

'' and other

microorganism A microorganism, or microbe, is an organism of microscopic scale, microscopic size, which may exist in its unicellular organism, single-celled form or as a Colony (biology)#Microbial colonies, colony of cells. The possible existence of unseen ...

s. Danielone is a phytoalexin found in the papaya fruit. This compound showed high antifungal activity against '' Colletotrichum gloesporioides'', a pathogenic fungus of papaya. Stilbenes are produced in '' Eucalyptus sideroxylon'' in case of pathogen attacks. Such compounds can be implied in the hypersensitive response of plants. High levels of

polyphenol Polyphenols () are a large family of naturally occurring phenols. They are abundant in plants and structurally diverse. Polyphenols include phenolic acids, flavonoids, tannic acid, and ellagitannin, some of which have been used historically as ...

s in some woods can explain their natural

preservation Preservation may refer to: Heritage and conservation * Preservation (library and archival science), activities aimed at prolonging the life of a record while making as few changes as possible * ''Preservation'' (magazine), published by the Nat ...

against rot. Avenanthramides are phytoalexins produced by '' Avena sativa'' in its response to ''

Puccinia coronata ''Puccinia coronata'' is a plant pathology, plant pathogen and causal agent of oat and barley crown rust. The pathogen occurs worldwide, infecting both wild and cultivated oats. Crown rust (fungus), rust poses a threat to List of countries by bar ...

'' var. ''avenae'' f. sp. ''avenae'', the oat crown

rust Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO(OH) ...

. (Avenanthramides were formerly called avenalumins.)

References

External links

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

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

Loretta L. Mannix

Function

Recent research

Role of natural phenols in the plant defense against fungal pathogens

See also

References

Further reading

External links