refers to a substance produced by a plant that is toxic or a substance that is toxic to the plant
["Science Encyclopedia:Biochemistry" http://science.jrank.org/pages/39180/phytotoxin.html 2010/4/5.]
Many substances produced by plants are secondary metabolites and are the by-products of primary physiological processes.
[Raven, Peter H,Ray F. Evert, Susan E. Eichhorn: "Biology of Plants", pages 27-33.]
Some examples of phytotoxins are alkaloids, terpenes, phenolics, herbicides and substances produced by bacteria.
Toxins Produced By Plants
Alkaloids are derived from amino acids, and contain nitrogen
[Zeiger, L. Taiz: "Plant Physiology: Plant Defenses" pages 349-376.]
They are medically important by interfering with components of the nervous system
affecting membrane transport
, protein synthesis, and enzyme
activities.They generally have a bitter taste. Alkaloids usually end in -ine (caffeine, nicotine, cocaine, morphine, ephedrine).
Terpenes are made of water insoluble lipids, and synthesized from acetyl CoA or basic intermediates of glycolysis
[ Plant Sciences "Poisonous Plants". pages 170-175. ]
They often end in -ol (menthol) and make the majority of plant essential oils.
- Monoterpenes are found in gymnosperms and collect in the resin ducts and maybe released after an insect begins to feed to attract the insect's natural enemies.
- Sesquiterpenes are bitter tasting to humans and are found on glandular hairs or subdermal pigments.
- Diterpenes are contained in resin and block and deter insect feeding. Taxol, an important anitcancer drug is found in this group.
- Triterpenes mimic the insect molting hormone ecdysone, disrupting molting and development and is often lethal. They are usually found in citrus fruit, and produce a bitter substance called limonoid that deters insect feeding.
- Glycosides are made of one or more sugars combined with a non-sugar like aglycone, which usually determines the level of toxicity. Cyanogenic glycosides are found in many plant seeds like cherries, apples,and plums. Cyanogenic glycosides produce cyanide and are extremely poisonous.Cardenolides have a bitter taste and influence NA+/K+ activated ATPases in human heart, they may slow or strengthen the heart rate. Saponins have lipid and water soluble components with detergent properties. Saponins form complexes with sterols and interfere with their uptake.
Phenolics are made of a hydroxyl group bonded to an aromatic hydrocarbon. Furanocoumarin is a phenolic and is non-toxic until activated by light. Furancoumarin blocks the transcription and repair of DNA
. Tannins are another group of phenolics and they are important in tanning leather. Lignins, also a group of phenolics, is the most common compound on earth and helps conduct water in plant stems and fill spaces in the cell.
Substances Toxic to Plants
Herbicides usually interfere with plant growth and often imitate plant hormones.
- ACCase Inhibitors kill grasses and inhibit the first step in lipid synthesis, acetyl coA carboxylase,thus affecting cell memebrane production in the meristems. They do not affect dicots plants.
[Pike, David R., Aaron Hager, "How Herbicides Work" http://wed.aces.uiuc.edu/vista/pdf_pubs/herbwork.pdf]
- ALS Inhibitors affect grasses and dicots by inhibiting the first step in some amino acid synthesis, acetolactate synthesis. The plants are slowly starved of theses amino acids and eventually DNA synthesis stops.
- ESPS Inhibitors affect grasses and dicots by inhibiting the first step in the synthesis of tryptophan, phenylalanine and tyrosine, enolpyruvylshikimate 3-phosphate synthase enzyme.
- Photosystem II Inhibitors reduce the electron flow from water to NADPH2+ causing electrons to accumulate on chlorophyll molecules and excess oxidation to occur. The plant will eventually die.
- Synthetic Auxin mimics plant hormones and can affect the plant cell membrane.
- Tabtoxin is produced by Pseudomonas tabaci that may cause toxic concentrations ofammonia to build up. This build up of ammonia causes leaf chlorosis.
[ Strobel,Gary A. 1977. Annual Review Microbiology "Bacterial Phytotoxins. 31:205-224]
- Glycopeptides are produced by a number of bacteria and have been indicated in disease development.
[glycopeptide from Corynebacterium sepedonicum causes rapid wilt and marginal necrosis. A toxin from Corynebacterium insidiosum causes plugging of the plant stem interfering with water movement between cells.] [polysaccharide from Erwinia amylovora and causes wilting in rosaceous plants. A polysaccharide from Xanthomonas campestris obstructs water flow through phloem causing black rot in cabbage.]
- Phaseotoxins prooduced by Pseudomonas phaseolicola and Pseudomonas glycinea can cause starch accumulation, decrease cell permeability in swiss chard and inhibit bean callus tissue growth.
- Rhizobiotoxine produced by Rhizobium japonicum causes the root nodules of some soy bean plants to become chlorotic.