Phytoalexins
Phytoalexins are defined as low molecular weight anti-microbial compounds that are synthesized and accumulated in plants after exposure to microorganisms or abiotic agents. The term 'phytoalexin' originates from Greek: phyto – plant, alexin – warding off.
Chemical Nature
Broad spectrum inhibitors.
Phytoalexins can be put into several classes including terpenoids, glycosteroids and alkaloids.
Caused by 4 simple plant propenoid pathways:
* Shikimic acid pathway
* Tryptophan pathway
* Terpenoid pathway
* Acetate-malonate pathway
Capsoid – MVA (mevalonic acid pathway)
Pisatin – Shikimic acid and acetate malonate pathway
Phytoalexins are considered as a molecular marker of disease resistance.
Types of Phytoalexins (Some Important Phytoalexins)
1. Ipomeamarone: This is a sesquiterpenoid induced in sweet potato tissue infected with Ceratocystis fimbriata, has striking inhibitory effect on fungus.
2. Pisatin (Isoflavonoid): The chromone-coumarin ring system and is a phenolic alcohol produced in infection with Monilia fructicola. It is a broad spectrum antibiotic.
3. Phaseollin/Phaseollidin (Isoflavonoid): Found in Phaseolus vulgaris (French bean), similar to pisatin. High concentration is fungicidal; low concentration is fungistatic. Produced in infection with S. fructigena.
4. Glyceollin: Produced in soybean infected with fungus Phytophthora megasperma f.sp. glycinea.
5. Iso-coumarin: Carrot root inoculated with fungus non-pathogenic Ceratocystis fimbriata and other non-pathogenic C. ulmi, Helminthosporium carbonum, Fusarium oxysporum f.sp. lycopersici and Thielaviopsis basicola. Chemically related to phaseollin.
6. Trifolirhizin (Isoflavonoid): Trifolium pratense (Red clover), chemically related to pisatin.
7. Rishitin: Kuc and Bonquet 1940 – R1 gene potato P. infestans produces phytoalexin. It is a non-sesquiterpenic alcohol.
8. Vestiyal: Ether soluble phenol – spot of rose (Diplocarpon rosae), leaf spot of walnut (Coryneum trifolii).
9. Xanthoalexin: Parsnip root inoculated with C. fimbriata.
10. Capsidiol: Pepper fruit inoculated with non-pathogenic fungi, sesquiterpene.