Glyphosate shikimic acid accumulation

The following experiments enabled us to reduce considerably the number of putative enzyme target sites of the herbicide glyphosate inhibited the incorporation of radioactively labelled shikimate into all three aromatic amino acids, and the labelled precursor was metabolized to only a minor extent. Following this lead, we found that buckwheat hypocotyls, as well as intact plants and cultured plant cells, accumulate large amounts of shikimic acid in the presence of the herbicide. In buckwheat hypocotyls, there was an excellent correlation between the accumulation of shikimic acid and the depletion of anthocyanin, and a correlation between the accumulation of shikimic acid and the reduced growth of cultured plant cells (in the absence of amino acids in the nutrient mediijm)... 

The pattern for glyphosate looks different again. One metabolite increases dramatically, namely shikimic acid. It is known that, after glyphosate treatment, plants accumulate shikimic acid as a consequence of inhibiting its further transformation (11). If we had not known of this mode of action, the response pattern would have given a clue to it. 

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). 

Jaworski (4) reported that growth inhibition of both plant and microbes by glyphosate could be reversed by aromatic amino acids. Further work of Amrhein and his coworkers revealed that glyphosate inhibits the shikimate pathway enzyme, 5-enolpyruvylshikimate 3-phosphate (EPSP) synthase (5). This enzyme catalyzes the reaction shown in Figure 1. Glyphosate-treated plant and bacterial cultures accumulate shikimate and/or shikimate 3-phosphate (S3P), confirming that inhibition of EPSPS is at least a part of the in vivo mechanism of action of this herbicide (6, 7). 

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