Phytotoxins are substances that are poisonous ortoxic to the growth of plants. Phytotoxic substances may result from human activity, as withherbicides, or they may be produced by plants, by microorganisms, or by naturally occurring chemical reactions.
The term is also used to describe toxic chemicals produced by plants themselves, which function as defensive agents against their predators. Most examples pertaining to this definition of phytotoxin are members of various classes ofspecialised or secondary metabolites, includingalkaloids,terpenes, and especiallyphenolics, though not all such compounds are toxic or serve defensive purposes.[1] Phytotoxins may also be toxic to humans.[2][3]
Terpenes are made of water-insolublelipids, and synthesized fromacetyl-CoA or basic intermediates ofglycolysis[5] They often end in -ol (menthol) and comprise the majority of plant essential oils.
Monoterpenes are found ingymnosperms and collect in theresin ducts and may be released after an insect begins to feed to attract the insect's natural enemies.
Diterpenes are contained in resin and block and deter insect feeding.Taxol, an important anticancer drug is found in this group.
Triterpenes mimic the insect molting hormoneecdysone, disrupting molting and development and is often lethal. They are usually found incitrus fruit, and produce a bitter substance calledlimonoid that deters insect feeding.
Glycosides are made of one or more sugars combined with a non-sugar likeaglycone, which usually determines the level oftoxicity.Cyanogenic glycosides are found in many plant seeds likecherries,apples, andplums. Cyanogenic glycosides producecyanide and are extremely poisonous.Cardenolides have a bitter taste and influence NA+/K+ activatedATPases in human heart, they may slow or strengthen the heart rate.Saponins have lipid- and water-soluble components with detergent properties. Saponins form complexes withsterols and interfere with their uptake.
Phenolics are made of a hydroxyl group bonded to an aromatichydrocarbon.Furanocoumarin is aphototoxic phenolic, and is non-toxic until activated by light. Furanocoumarin blocks the transcription and repair ofDNA.Tannins are another group of phenolics important intanning leather.Lignins, also a group of phenolics, are the most common compounds on Earth, and help conduct water in plant stems and fill spaces in the cell.
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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 membrane production in themeristems. They do not affect dicots plants.[6]
ALS Inhibitors affect grasses and dicots by inhibiting the first step in some amino acid synthesis,acetolactate synthesis. The plants are slowly starved of these amino acids and eventually DNA synthesis stops.
ESPS Inhibitors affect grasses and dicots by inhibiting the first step in the synthesis oftryptophan,phenylalanine andtyrosine, enolpyruvylshikimate 3-phosphate synthase enzyme.
Photosystem II Inhibitors reduce theelectron flow from water to NADPH2+ causing electrons to accumulate onchlorophyll molecules and excessoxidation to occur. The plant will eventually die.
SyntheticAuxin mimics plant hormones and can affect the plant cell membrane.
Glycopeptides are produced by a number of bacteria and have been indicated in disease development.[7] Aglycopeptide fromCorynebacterium sepedonicum causes rapid wilt and marginalnecrosis. A toxin fromCorynebacterium insidiosum causes plugging of the plant stem interfering with water movement between cells.[7] Amylovorin is apolysaccharide fromErwinia amylovora and causes wilting in rosaceous plants. A polysaccharide fromXanthomonas campestris obstructs water flow throughphloem causing black rot in cabbage.
Phaseolotoxin is a modified tripeptide [Nδ-(N′-sulfodiaminophosphinyl)-ornithyl-alanyl-homoarginine] produced by certains strains ofPseudomonas syringae pv.phaseolicola,Pseudomonas syringae pv.actinidiae and strainPseudomonas syringae pv.syringae CFBP 3388.[8][9][10] Phaseolotoxin is a reversible inhibitor of the enzyme ornithine carbamoyltransferase (OCTase; EC 2.1.3.3), which catalyzes the formation of citrulline from ornithine and carbamoylphosphate in the arginine biosynthetic pathway. Phaseolotoxin is an effective inhibitor of OCTase activity from plant, mammalian, and bacterial sources and causes a phenotypic requirement for arginine. Additionally, phaseolotoxin inhibits the enzyme ornithine decarboxylase (EC 4.1.1.17), which is involved in the biosynthesis ofpolyamines.[11]
Rhizobiotoxine, produced byRhizobium japonicum, causes the root nodules of some soy bean plants to become chlorotic.
^abcStrobel, Gary A. 1977. Annual Review Microbiology "Bacterial Phytotoxins. 31:205-224
^Bender CL, Alarcón-Chaidez F, Gross DC, 1999. Pseudomonas syringae phytotoxins: mode of action, regulation, and biosynthesis by peptide and polyketide synthetases. Microbiology and Molecular Biology Reviews 63, 266-292
^Tourte C, Manceau C, 1995. A strain of Pseudomonas syringae which does not belong to pathovar phaseolicola produces phaseolotoxin. European Journal of Plant Pathology 101, 483-490
^Murillo J, Bardaji L, Navarro de la Fuente L, Führer ME, Aguilera S, Alvarez-Morales A, 2011. Variation in conservation of the cluster for biosynthesis of the phytotoxin phaseolotoxin in Pseudomonas syringae suggests at least two events of horizontal acquisition. Research in Microbiology 162, 253-261
^Bachmann AS, Matile P, Slusarenko AJ, 1998. Inhibition of ornithine decarboxylase activity by phaseolotoxin: Implications for symptom production in halo blight of French bean. Physiological and Molecular Plant Pathology 53, 287-299.
