Phytotoxins are substances that are poisonous or toxic to the growth of plants. Phytotoxic substances may result from human activity, as with herbicides , or they may be produced by plants, by microorganisms, or by naturally occurring chemical reactions.
7-489: Tagetitoxin (TGT) is a bacterial phytotoxin produced by Pseudomonas syringae pv. tagetis . When TGT was first isolated, it was only partially characterized. The first proposed chemical structure of TGT involved an eight-membered ring, but this was revised shortly afterward to a bicyclic structure (shown at right) based on NMR and mass spectrometry . This structure, however, has been questioned. The absolute configuration remains undetermined, and attempts at confirming
14-512: Is most closely related to bacterial enzymes. In vitro , TGT inhibits bacterial RNAPs from Escherichia coli and Thermus thermophilus , and eukaryotic RNA polymerase III. In contrast, eukaryotic RNA polymerase I and II as well as single-subunit RNA polymerases of bacteriophage T7 and SP6 are relatively insensitive to the compound. TGT binds in the RNAP active site and inhibits initiation and elongation phases of transcription as well as pyrophosphorolysis of
21-399: 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 They often end in -ol ( menthol ) and comprise
28-738: The DNA by mimicking the transcription byproduct pyrophosphate. Phytotoxin#Bacterial Phytotoxins 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 of specialised or secondary metabolites , including alkaloids , terpenes , and especially phenolics , though not all such compounds are toxic or serve defensive purposes. Phytotoxins may also be toxic to humans. Alkaloids are derived from amino acids , and contain nitrogen . They are medically important by interfering with components of
35-474: The majority of plant essential oils. Phenolics are made of a hydroxyl group bonded to an aromatic hydrocarbon . Furanocoumarin is a phototoxic phenolic, and is non-toxic until activated by light. Furanocoumarin blocks the transcription and repair of DNA . Tannins are another group of phenolics important in tanning 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
42-450: The nascent RNA. However, the detailed mechanism of inhibition remains a subject of heated debate. It has been suggested that TGT forms a ternary RNAP-NTP-TGT complex and inhibits phosphodiester bond synthesis either by binding an inhibitory magnesium ion or by trapping a flexible active site domain in an inactive conformation. The third theory suggests that TGT forms predominantly a binary RNAP-TGT complex and inhibits RNAP translocation along
49-405: The structure by organic synthesis are underway. Recently Porter et al. published a revised structure of TGT based on extensive 2D NMR data. TGT interferes with development of chloroplasts in young plant leaves thereby causing chlorosis . The natural target of the toxin is chloroplast RNA polymerase. Chloroplast RNA polymerase belongs to ubiquitous family of multisubunit RNA polymerases (RNAP) and
#206793