Glyphosate metabolic effects

It is now well established that the primary metabolic target of glyphosate is an enzyme of the shikimic acid metabolic pathway, enolpyruvyl shikimate-3-phosphate synthase (2.f ). Via this action, glyphosate blocks the synthesis of the end products of this pathway, notably phenylalanine and tryptophan, but also various subsequent products (Figure 1) ( ,i). It has seemed logical to conclude that the herbicidal effect of glyphosate is a direct result of its effect on the shikimic acid pathway. 

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. 

Table II. Effects of glyphosate on enz3mies of phenolic metabolism (see Figure 1 for pathway).

Table III. Effect of 0.5 mM glyphosate on extractable activity of four enzymes of phenolic metabolism from wheat root tips. Adapted from Cole et al. (72).

Table IV. Summary of effects of glyphosate (0.5 mM) and two PAL inhibitors, AOPP (0.01 mM) and AOA (0.01 mM), on growth and metabolism of soybean seedlings.

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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