Phytoalexins are

antimicrobial An antimicrobial is an agent that kills microorganisms (microbicide) or stops their growth (bacteriostatic agent). Antimicrobial medicines can be grouped according to the microorganisms they are used to treat. For example, antibiotics are used aga ...

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

phytochemical Phytochemicals are naturally-occurring chemicals present in or extracted from plants. Some phytochemicals are nutrients for the plant, while others are metabolites produced to enhance plant survivability and reproduction. The fields of ext ...

s 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 In chemistry and biology, reactive oxygen species (ROS) are highly Reactivity (chemistry), reactive chemicals formed from diatomic oxygen (), water, and hydrogen peroxide. Some prominent ROS are hydroperoxide (H2O2), superoxide (O2−), hydroxyl ...

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 Plant disease resistance, plant immunology, the hypersensitive response (HR) is a mechanism used by plants to prevent the spread of infection by microbial pathogens. HR is characterized by the rapid apoptosis, death of cells in the local regi ...

, 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 Systemic acquired resistance (SAR) is a "whole-plant" resistance response that occurs following an earlier localized exposure to a pathogen. SAR is analogous to the innate immune system found in animals, and although there are many shared aspects b ...

(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 Jasmonic acid (JA) is an organic compound found in several plants including jasmine. The molecule is a member of the jasmonate class of plant hormones. It is biosynthesized from linolenic acid by the octadecanoid pathway. It was first isolated ...

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 Abscisic acid (ABA or abscisin II) is a plant hormone. ABA functions in many plant developmental processes, including seed and bud dormancy, the control of organ size and stomatal closure. It is especially important for plants in the response to ...

, or

salicylic acid Salicylic acid is an organic compound with the formula HOC6H4COOH. A colorless (or white), bitter-tasting solid, it is a precursor to and a active metabolite, metabolite of acetylsalicylic acid (aspirin). It is a plant hormone, and has been lis ...

. 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 Allixin is a phytoalexin found in garlic (''Allium sativum'') bulbs. It was first isolated and characterized in 1989. When garlic is stored for long periods of time, it can form visible accumulations of crystalline allixin on its surface, partic ...

(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 Garlic (''Allium sativum'') is a species of bulbous flowering plants in the genus '' Allium''. Its close relatives include the onion, shallot, leek, chives, Welsh onion, and Chinese onion. Garlic is native to central and south Asia, str ...

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 Aflatoxins are various toxicity, poisonous carcinogens and mutagens that are produced by certain Mold (fungus), molds, especially ''Aspergillus'' species such as ''Aspergillus flavus'' and ''Aspergillus parasiticus''. According to the USDA, "The ...

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

isoflavonoid Isoflavonoids are a class of flavonoid phenolic compounds, many of which are biologically active. Isoflavonoids and their derivatives are sometimes referred to as phytoestrogens, as many isoflavonoid compounds have biological effects via the estr ...

s 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 Fungal infection, also known as mycosis, is a disease caused by fungi. Different types are traditionally divided according to the part of the body affected: superficial, subcutaneous, and systemic. Superficial fungal infections include common ...

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

. Sakuranetin is a

flavanone The flavanones, a type of flavonoids, are various aromatic, colorless ketones derived from flavone that often occur in plants as glycosides. List of flavanones * Blumeatin * Butin * Dichamanetin * Eriodictyol * Hesperetin * Hesperidin * Hom ...

, 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 ''Colletotrichum'' (sexual stage: ''Glomerella'') is a genus of fungi that are symbionts to plants as endophytes (living within the plant) or phytopathogens. Many of the species in this genus are plant pathogens, but some species may have a mu ...

'' interactions. 6-Methoxymellein is a dihydroisocoumarin and a phytoalexin induced in carrot slices by

UV-C Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of the ...

, 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 The papaya (, ), papaw, () or pawpaw () is the plant species ''Carica papaya'', one of the 21 accepted species in the genus '' Carica'' of the family Caricaceae, and also the name of its fruit. It was first domesticated in Mesoamerica, within ...

fruit. This compound showed high antifungal activity against '' Colletotrichum gloesporioides'', a pathogenic fungus of papaya.

Stilbene Stilbene may refer to one of the two stereoisomers of 1,2-diphenylethene: * (''E'')-Stilbene (''trans'' isomer) * (''Z'')-Stilbene (''cis'' isomer) See also * Stilbenoid Stilbenoids are hydroxylated derivatives of stilbene. They have a C6–C ...

s are produced in ''

Eucalyptus sideroxylon ''Eucalyptus sideroxylon'', commonly known as mugga ironbark, or red ironbark is a small to medium-sized tree that is endemic to eastern Australia. It has dark, deeply furrowed ironbark, lance-shaped adult leaves, flower buds in groups of s ...

'' in case of pathogen attacks. Such compounds can be implied in the

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.

Avenanthramide Avenanthramides (anthranilic acid amides, formerly called "avenalumins") are a group of phenolic alkaloids found mainly in oats (''Avena sativa''), but also present in white cabbage butterfly eggs (''Pieris brassicae'' and '' P. rapae''), and in ...

s are phytoalexins produced by ''

Avena sativa The oat (''Avena sativa''), sometimes called the common oat, is a species of cereal grain grown for its seed, which is known by the same name (usually in the plural). Oats appear to have been domesticated as a secondary crop, as their seeds ...

'' 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