The Site of the Inhibition of the Shikimate Pathway by Glyphosate

Abstract

The nonselective herbicide glyphosate (N-lphosphonomethyljglycine) 3 To whom reprint requests should be addressed.were added to the growth medium, but no effect of glyphosate was found on chorismate mutase, prephenate dehydrogenase, and prephenate dehydratase. High concentrations of the herbicide inhibited the two initial enzymes of the shikimate pathway, but this inhibition was apparently too slight to account for growth inhibition caused by low glyphosate concentrations. Inhibition by glyphosate of the growth of carrot and tobacco cells was alleviated by the addition of the three aromatic amino acids, but Haderlie et al. (15) hesitated to conclude that the synthesis of the aromatic amino acids was inhibited by glyphosate because glyphosate did not appreciably reduce the cellular concentrations of these amino acids. Just recently, Greshoff (14) extended the earlier studies of Jaworski (20) and Haderlie et al. (15) with an investigation on glyphosate's growth inhibition of E. coli, Chlamydomonas reinhardtii, carrot and soybean cell cultures, and Arabidopsis thaliana seedlings. In all cases, phenylalanine and tyrosine acted synergistically in the alleviation of the inhibitory effect of glyphosate. It was suggested that some essential process or compound derived from phenylalanine and/or tyrosine was inhibited by glyphosate or one of its metabolites (14). Nilsson (23) reported an increase in the amount of free amino acids and in NH3 in wheat plants sprayed with glyphosate, but the percentage of phenylalanine and tyrosine of the total amino acids was strongly reduced. In a series of recent papers, 16,17) hypothesized that glyphosate might exert its effect at least in part through induction of PAL4 activity, resulting in one or more of the following growth-limiting conditions: (a) depletion of free phenylalanine and, possibly, tyrosine levels and coincident inhibition of protein synthesis; (b) toxic levels of ammonia provided that the rate of deamination exceeds the rate of amination; and (c) accumulation of growth-limiting phenolics derived from trans-cinnamic acid, the product of the PAL reaction. In spite of the attractiveness of this proposal, there are alternative explanations for the induction of PAL by glyphosate (see under "Discussion"), and this hypothesis would not explain the growth-inhibitory action of glyphosate in organisms, such as bacteria, which do not have PAL. Furthermore, if conditions b or c were pertinent, one would expect that phenylalanine would increase the inhibitory effect of glyphosate rather than alleviate it.PAL, however, may contribute only partially to aromatic amino acid depletion, as was recently pointed out by Duke et al. (11). It was also suggested (11) that glyphosate's effect on aromatic compounds may not be its principal mode of action. In the present HOLLANDER AND AMRHEIN study, we used an approach basically similar to that of Duke and Hoagland (9,16,17). We reasoned that, due to the possible functioning of a homeostatic control system and/or the possible involvemen...

Nitrogen Metabolism and Flower Symmetry of Petunia Corollas Treated with Glyphosate

“…Although the decrease in anthocyanin content was a result of the reduction of PAL activity caused by glyphosate (Hollander and Amrhein, 1980), the reduction of PAL activity alone could not account for the two independent processes of flower shape and pigmentation.…”

“…Glyphosate is widely used as a nonselective, broad-spectrum, postemergence herbicide with particular potential for perennial weed control due to excellent translocation (Hollander and Amrhein, 1980;Haderlie et al, 1977). The primary mode of action of glyphosate is to competitively inhibit the binding of the substrate phosphoenolpyruvate to the active site of 5-enolpyruvylshikimate-3-phosphate synthase (EPSP synthase; 3-phosphoshikimate 1-carboxyvinyltransferase; EC 2.…”

Effects of Glyphosate on Isolated Maize Mitochondria

“…Thus, the induction of phenylalanine ammonia-lyase in maize seedlings occurs at 10~4 to 10~3 mol dm -3 glyphosate (Hoagland and Duke, 1982); depression of anthocyanin synthesis in excised hypocotyls of buckwheat is detected at 10~6 mol dm -3 while inhibition of chlorophyll synthesis takes place only at concentrations above 10^5 mol dm -3 (Hollánder and Amrhein, 1980); the conversión of shikimate to anthraquinones is blocked in G. mollugo cell cultures at concentrations above 10~4 mol dm -3 (Amrhein, Brigette, Gehrke and Steinrücken, 1980). However, it should be pointed out that direct comparison of results among different reports is difficult because of two reasons: (i) different glyphosate preparations have been used (puré acid or isopropylamine salt, with or without surfactant) and (ii) there is lack of knowledge of the intracellular glyphosate levéis attained in each case.…”

Effects of Herbicide Glyphosate and Glyphosate-Based Formulations on Aquatic Ecosystems

“…Evidence for the involvement of this pathway in glyphosate toxicity has come from a variety of studies in a wide range of microorganisms, plant cell cultures and plants (e.g. Jaworsky, 1972;Haderlie et al, 1977;Gresshoff, 1979;Hollander & Amrhein, 1980; among others). Glyphosate caused a massive accumulation of shikimate in treated cells and tissues Berlin & Witte, 1981;Bode et al, 1984;Rubin et al, 1984).…”

“…(acid equivalents). Glyphosate is an unusual herbicide, in that essentially no structurally related compounds show any herbicidal activity (Hollander & Amrhein, 1980;Franz, 1985), whith the exception of glyphosine, which has reduced herbicidal effects but shows some interesting plant growth regulatory effects, such as enhancing ripening of sugar cane (Franz, 1985). The herbicidal properties of glyphosate were reported in 1971 (Baird et al, 1971).…”