Amino acid, aromatic, effects glyphosate

Glyphosate also inhibits the activity of a form of 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase (13), however, this inhibition is relatively weak. Effects of glyphosate on the enzymes of aromatic amino acid synthesis have been described in more detail in recent reviews (14-17). 

The effects of glyphosate on phenolic compound production are two-fold 1) accumulation of phenolic compounds that are derivatives of aromatic amino acids is reduced and 2) pools of phenolic compounds derived from constituents of the shikimate pathway prior to 5-enolpyruvylshikimate-3-phosphate become larger. Assays that do not distinguish between effects on these two groups, such as that for hydroxyphenolics of Singleton and Rossi (18), can lead to equivocal and difficult to interpret results (e.g. 3-5). 

It is clear from the above discussion ensure that glyphosate tolerance may be conferred to plants both by overproduction of wild type EPSPS as well as mutant EPSP synthases. It has been suggested that glyphosate may have multiple sites of action in plant cells (51-56). If this is true, mutant EPSPS enzymes would not confer glyphosate tolerance to plants, which is evidently not the case. It appears, therefore, that reports concerning the effect of glyphosate on other aspects of plant metabolism are due to secondary effects of the herbicide arising as a consequence of the inhibition of aromatic amino acid biosynthesis. 

The effects of glyphosate on several E. coll enzymes of the aromatic amino acid biosynthe tic pathway have been studied... 

Chorismate is an intermediate in the biosynthesis of the aromatic amino acids tryptophan, phenylalanine, and tyrosine. Mammals do not synthesize these amino acids bom chorismate. Instead, they obtain the essential aromatic amino acids tryptophan and phenylalanine from the diet, and they can synthesize tyrosine from phenylalanine. Glyphosate is an effective herbicide because it prevents synthesis of aromatic amino acids in plants. But the compound has no effect on mammals because they have no active pathway for de novo aromatic amino acid synthesis. 

The mechanism of the action of glyphosate is thought to be associated with the metabolism of aromatic amino acids. iV-(phosphonomethyl) glycine is a highly effective herbicide because of its potent and specific inhibition of 5-enolpyruvoylshikimate 3-phosphate synthase (EPSPS) [28]. This enzyme, which is found in plants and microorganisms but not in other forms of life, catalyzes the penultimate step of the shikimic acid pathway [29], that is, the interaction of shikimate-3-phosphate 8 with phosphenolpyruvate 9 to give 5-enolpyruvoylshikimate 3-phosphate 11 and inorganic phosphate (Pi)[30,31-33]. 

Glyphosate is phloem mobile and accumulates in accordance with source-sink relationships in meristematic areas of the treated plants. " The general physiological effects of glyphosate are consistent with its acting as a metabolic poison. Death is a slow process, requiring days or weeks much more rapid effects, within hours, can be demonstrated at the biochemical level. These effects can be explained if the turnover rates of specific proteins, and therefore sensitivity to amino acid synthesis inhibition, differ. Inhibition of phenolic biosynthesis and chlorophyll formation and effects on the levels of the phytohormone 3-indoleacetic acid have been considered as target sites but can be viewed as necessary consequences of the primary interference with aromatic amino acid biosynthesis. " ... 

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