Phytoalexin

This article's citation style may be unclear. The references used may be made clearer with a different or consistent style of citation, footnoting, or external linking. (August 2009)

Capsidiol is a phytoalexin produced by certain plants in response to pathogenic attack

Phytoalexins are antimicrobial substances synthesized de novo by plants that accumulate rapidly at areas of incompatible pathogen infection. They are broad spectrum inhibitors and are chemically diverse with different types characteristic of particular plant species. Phytoalexins tend to fall into several classes including terpenoids, glycosteroids and alkaloids; however, researchers often find it convenient to extend the definition to include all phytochemicals that are part of the plant's defensive arsenal.

Function

Phytoalexins produced in plants 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 type. As such, host-specific pathogens capable of degrading phytoalexins are more virulent than those unable to do so. ^[2]

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 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, ethylene, abscisic acid or salicylic acid. The reception of the signal leads to global changes within the plant, which induce genes that protect from further pathogen intrusion, including enzymes involved in the production of phytoalexins. Often, if jasmonates or ethylene (both gaseous hormones) is 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.

Phytoalexins in Recent Research

Allixin (3-hydroxy-5-methoxy-6-methyl-2-penthyl-4H-pyran-4-one), a non-sulfur containing compound having a g-pyrone skeleton structure, was the first compound isolated from garlic as a phytoalexin, a product induced in plants by continuous stress.^[1] This compound has been shown to have unique biological properties, such as anti-oxidative effects,^[1] anti-microbial effects,^[1] anti-tumor promoting effects,^[3] inhibition of aflatoxin B2 DNA binding,^[4] and neurotrophic effects.^[4]Allixin showed an anti-tumor promoting effect in vivo, inhibiting skin tumor formation by TPA in DMBA initiated mice.^[3] Herein, allixin and/or its analogs may be expected useful compounds for cancer prevention or chemotherapy agents for other diseases.

See also

• Glyceollin
• Phaseolin
• Resveratrol
• Allixin
• Allicin
• Garlic
• Plant defense against herbivory

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
• THE CHEMICAL DEFENSES OF HIGHER PLANTS 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
• Relationships Among Plants, Insect Herbivores, Pathogens, and Parasitoids Expressed by Secondary Metabolites Loretta L. Mannix

References

^[1] ^ Kodera Y., Matuura H., Yoshida S., Sumida T., Itakura Y., Fuwa T., Nishino H., Chem. Pharm. Bull., 37, 1656—1658 (1989).

^[2] ^ Glazebrook and Ausbel. PNAS 91, 8955-8959.

^[3] ^ Nishino H., Nishino A., Takayama J., Iwashima A., Itakura Y., Kodera Y., Matsuura H., Fuwa T., Cancer J., 3, 20—21 (1990).

^[4] ^ Yamasaki T., Teel R. W., Lau B. H. S., Cancer Lett., 59, 89—94 (1991).

^[5] ^ Moriguchi T., Matsuura H., Itakura Y., Katsuki H., Saito H., Nishiyama N., Life Sci., 61, 1413—1420 (1997).

^[6] ^ Yukihiro Kodera, Makoto Ichikawa, Jiro Yoshida, Naoki Kashimoto, Naoto Uda, Isao Sumioka, Nagatoshi Ide and Kazuhisa Ono, “Pharmacokinetic Study of Allixin, a Phytoalexin Produced by Garlic”, Chem. Pharm. Bull., Vol. 50, 354-363 (2002) doi:10.1248/cpb.50.354 [1]