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 Antioxidants are compounds that inhibit oxidation, a chemical reaction that can produce free radicals. This can lead to polymerization and other chain reactions. They are frequently added to industrial products, such as fuels and lubricants, ...

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 excl ...

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 terpenoids, glyco

steroid A steroid is a biologically active organic compound with four rings arranged in a specific molecular configuration. Steroids have two principal biological functions: as important components of cell membranes that alter membrane fluidity; and ...

s 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 simila ...

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 pois ...

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 surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mec ...

, 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 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 g ...

, in which cells surrounding the site of infection are signaled to undergo

apoptosis Apoptosis (from grc, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes ( morphology) and death. These changes in ...

, 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 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 isolate ...

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 double bonds). Ethylene ...

abscisic acid Abscisic acid (ABA) 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 environmental s ...

salicylic acid Salicylic acid is an organic compound with the formula HOC6H4CO2H. A colorless, bitter-tasting solid, it is a precursor to and a metabolite of aspirin (acetylsalicylic acid). It is a plant hormone, and has been listed by the EPA Toxic Substa ...

. 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 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-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 plant in the genus '' Allium''. Its close relatives include the onion, shallot, leek, chive, Welsh onion and Chinese onion. It is native to South Asia, Central Asia and northeas ...

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 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 ...

s, especially

isoflavonoids 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 estro ...

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 cur ...

'' grape, ''trans''-

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

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" or ...

'' and

delta-viniferin δ-Viniferin is a resveratrol dehydro dimer. 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 maximowi ...

is another grapevine phytoalexin produced following

fungal infection Fungal infection, also known as mycosis, is 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 ti ...

by '' Plasmopara viticola''.

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. ...

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 of '' Pinaceae''. It is a fungitoxin protecting the wood from

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 sakuranet ...

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 ''Sorghum'' () is a genus of about 25 species of flowering plants in the grass family (Poaceae). Some of these species are grown as cereals for human consumption and some in pastures for animals. One species is grown for grain, while many other ...

'', the ''SbF3'H2'' gene, encoding a

flavonoid 3'-hydroxylase In enzymology, a flavonoid 3'-monooxygenase (, was wrongly classified as in the past) is an enzyme that catalyzes the chemical reaction :a flavonoid + NADPH + H+ + O2 \rightleftharpoons a 3'-hydroxyflavonoid + NADP+ + H2O The 4 substrates of t ...

, 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 g ...

-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 f ...

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. Chemica ...

and a phytoalexin induced in carrot slices by UV-C, 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 old ...

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. It was first domesticated in Mesoamerica, within modern-day southern Mexico and ...

fruit. This compound showed high antifungal activity against ''

Colletotrichum gloesporioides ''Glomerella cingulata'' is a fungal plant pathogen, being the name of the sexual stage (teleomorph) while the more commonly referred to asexual stage (anamorph) is called ''Colletotrichum gloeosporioides''. For most of this article the pathogen ...

'', a pathogenic fungus of papaya. Stilbenes 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 ...

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'' in its response to '' Puccinia coronata'' 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