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
plants 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. Phytoalexins tend to fall into several chemical classes, including
terpenoids,
glycosteroids, and
alkaloids; 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, 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.[1]
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.[citation needed]
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
apoptosis, or programmed cell death, in order to prevent the spread of the pathogen to the rest of the plant.[citation needed]
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 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 diseases, these and other wound response aromatics seem to act as a warning that the plant is no longer edible.[citation needed] 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-4H-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.[2] This compound has been shown to have unique biological properties, such as anti-oxidative effects,[2] anti-microbial effects,[2] 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 to be useful compounds for cancer prevention or chemotherapy agents for other diseases.[citation needed]
Role of natural phenols in the plant defense against fungal pathogens
Polyphenols, especially
isoflavonoids and related substances, play a role in the plant defense against fungal and other microbial pathogens.
^
abNishino H, Nishino A, Takayama J, Iwashima A, Itakura Y, Kodera Y, Matsuura H, Fuwa T (1990). "Antitumor promoting activity of allixin, a stress compound produced by garlic". Cancer J. 3: 20–21.
^
abYamasaki T.; Teel R. W.; Lau B. H. S. (1991). "Effect of allixin, a phytoalexin produced by garlic, on mutagenesis, DNA-binding and metabolism of aflatoxin B1". Cancer Lett. 59 (2): 89–94.
doi:
10.1016/0304-3835(91)90171-D.
PMID1909211.
^Timperio, Anna Maria; d’Alessandro, Angelo; Fagioni, Marco; Magro, Paolo; Zolla, Lello (2012). "Production of the phytoalexins trans-resveratrol and delta-viniferin in two economy-relevant grape cultivars upon infection with Botrytis cinerea in field conditions". Plant Physiology and Biochemistry. 50 (1): 65–71.
doi:
10.1016/j.plaphy.2011.07.008.
PMID21821423.
^Mercier, J.; Arul, J.; Ponnampalam, R.; Boulet, M. (1993). "Induction of 6-Methoxymellein and Resistance to Storage Pathogens in Carrot Slices by UV-C". Journal of Phytopathology. 137: 44–54.
doi:
10.1111/j.1439-0434.1993.tb01324.x.
^Hoffman, R.; Heale, J.B. (1987). "Cell death, 6-methoxymellein accumulation, and induced resistance to Botrytis cinerea in carrot root slices". Physiological and Molecular Plant Pathology. 30: 67–75.
doi:
10.1016/0885-5765(87)90083-X.
^Kurosaki, Fumiya; Nishi, Arasuke (1983). "Isolation and antimicrobial activity of the phytoalexin 6-methoxymellein from cultured carrot cells". Phytochemistry. 22 (3): 669.
Bibcode:
1983PChem..22..669K.
doi:
10.1016/S0031-9422(00)86959-9.
^Hart, John H.; Hillis, W. E. (1974). "Inhibition of wood-rotting fungi by stilbenes and other polyphenols in Eucalyptus sideroxylon". Phytopathology. 64 (7): 939–48.
doi:
10.1094/Phyto-64-939.
Moriguchi, Toru; Matsuura, Hiromichi; Itakura, Yoichi; Katsuki, Hiroshi; Saito, Hiroshi; Nishiyama, Nobuyoshi (1997). "Allixin, a phytoalexin produced by garlic, and its analogues as novel exogenous substances with neurotrophic activity". Life Sciences. 61 (14): 1413–1420.
doi:
10.1016/S0024-3205(97)00687-5.
PMID9335231.