Phytoalexins are

antimicrobial An antimicrobial is an agent that kills microorganisms or stops their growth. Antimicrobial medicines can be grouped according to the microorganisms they act primarily against. For example, antibiotics are used against bacteria, and antifungals ar ...

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 predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclu ...

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''; plural 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 nam ...

. Phytoalexins tend to fall into several chemical classes, including

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

s, glyco steroids and

alkaloid Alkaloids are a class of basic, naturally occurring organic compounds that contain at least one nitrogen atom. This group also includes some related compounds with neutral and even weakly acidic properties. Some synthetic compounds of similar ...

s, however the term applies to any

phytochemicals Phytochemicals are chemical compounds produced by plants, generally to help them resist fungi, bacteria and plant virus infections, and also consumption by insects and other animals. The name comes . Some phytochemicals have been used as poison ...

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, 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

cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery ...

s 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, reactive oxygen species (ROS) are highly reactive chemicals formed from diatomic oxygen (). Examples of ROS include peroxides, superoxide, hydroxyl radical, singlet oxygen, and alpha-oxygen. The reduction of molecular oxygen () p ...

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 viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3%� ...

to kill invading cells. In pathogen interactions, the common short-term response is the

hypersensitive response Hypersensitive response (HR) is a mechanism used by plants to prevent the spread of infection by microbial pathogens. HR is characterized by the rapid death of cells in the local region surrounding an infection and it serves to restrict the growt ...

, 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 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 ...

(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 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

vector Vector most often refers to: *Euclidean vector, a quantity with a magnitude and a direction *Vector (epidemiology), an agent that carries and transmits an infectious pathogen into another living organism Vector may also refer to: Mathematic ...

s 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 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, parti ...

(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 Stress may refer to: Science and medicine * Stress (biology), an organism's response to a stressor such as an environmental condition * Stress (linguistics), relative emphasis or prominence given to a syllable in a word, or to a word in a phrase ...

. 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 poisonous carcinogens and mutagens that are produced by certain molds, particularly ''Aspergillus'' species. The fungi grow in soil, decaying vegetation and various staple foodstuffs and commodities such as hay, sweetcorn ...

B2 DNA 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 organic compounds characterized by multiples of phenol units. They are abundant in plants and structurally diverse. Polyphenols include flavonoids, tannic acid, and ellagitannin, some o ...

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 region, Central Europe, and southwestern Asia, from Morocco and Portugal north to southern Germany and east to northern Iran. There are curr ...

'' grape, ''trans''- resveratrol 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" o ...

'' and

delta-viniferin δ-Viniferin is a resveratrol dehydrodimer. It is an isomer of epsilon-viniferin. It can be isolated from stressed grapevine (''Vitis vinifera'') leaves. It is also found in plant cell cultures and wine. It can also be found in ''Rheum maximowicz ...

is another grapevine phytoalexin produced following fungal infection by ''

Plasmopara viticola ''Plasmopara viticola'', the causal agent of grapevine downy mildew, is a heterothallic oomycete that overwinters as oospores in leaf litter and soil. In the spring, oospores germinate to produce macrosporangia, which under wet condition releas ...

''.

Pinosylvin Pinosylvin is an organic compound with the formula C6H5CH=CHC6H3(OH)2. A white solid, it is related to trans-stilbene, but with two hydroxy groups on one of the phenyl substituents. It is very soluble in many organic solvents, such as acetone. O ...

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 chalcones. Most stilbenoids are prod ...

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

heartwood Wood is a porous and fibrous structural tissue found in the stems and roots of trees and other woody plants. It is an organic materiala natural composite of cellulose 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 cedars, firs, hemlocks, larches, pines and spruces. The family is included in the order Pinales, formerly kn ...

''. It is a fungitoxin protecting the wood from fungal infection.

Sakuranetin Sakuranetin is a flavan-on, the 7-methoxy derivative of naringenin, found in Polymnia fruticosa and rice, where it acts as a phytoalexin against spore germination of '' Pyricularia oryzae''. Glycosides Sakuranin is the 5-O- glucoside of sakura ...

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 * Eriodictyol * Hesperetin * Hesperidin * Homoeriodictyol * ...

, a type of flavonoid. It can be found in '' Polymnia fruticosa'' and

rice Rice is the seed of the grass species '' Oryza sativa'' (Asian rice) or less commonly ''Oryza glaberrima'' (African rice). The name wild rice is usually used for species of the genera '' Zizania'' and '' Porteresia'', both wild and domesticat ...

, where it acts as a phytoalexin against spore germination of ''

Pyricularia oryzae ''Pyricularia'' is a genus of fungi which was named by Saccardo in 1880.Saccardo, P.A. Michelia 2(no. 6): 20. 1880 The polyphyletic nature of ''Pyricularia'' has been resolved and species of ''Pyricularia s. str.'' were shown to belong to a mon ...

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

pathogen In biology, a pathogen ( el, πάθος, "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 ...

-specific 3-deoxyanthocyanidin phytoalexins 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 mutu ...

'' interactions. 6-Methoxymellein is a

dihydroisocoumarin Dihydroisocoumarins are phenolic compounds related to isocoumarin. Dihydroisocoumarin glucosides can be found in ''Caryocar glabrum ''Caryocar glabrum'' is a species of tree in the family Caryocaraceae. It is native to South America ...

and a phytoalexin induced in carrot slices by

UV-C Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...

, that allows resistance to ''

'' and other

microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...

s. Danielone is a phytoalexin found in the papaya 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– ...

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 se ...

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

of plants. High levels of

polyphenol Polyphenols () are a large family of naturally occurring organic compounds characterized by multiples of phenol units. They are abundant in plants and structurally diverse. Polyphenols include flavonoids, tannic acid, and ellagitannin, some o ...

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, unlike other cereals and pseudocereals). While oats are suitable for human co ...

'' in its response to ''

Puccinia coronata ''Puccinia coronata'' is a plant pathogen and causal agent of oat and barley crown rust. The pathogen occurs worldwide, infecting both wild and cultivated oats. Crown rust poses a threat to barley production, because the first infections in barl ...

'' 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( ...

. (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