The mode of action of glyphosate has been studied with a variety of plant material. Chlorophyll formation was particularly sensitive, but was unlikely to be due to an interference with chloroplast macromolecule synthesis. Protein and RNA synthesis by isolated chloroplasts were not inhibited in vitro although chlorotic growth contained reduced levels of rRNA. The induction of nitrite reductase, a chloroplast protein was repressed in peas but not to an extent which would suggest inhibition of de novo synthesis. The induction of nitrate reductase was stimulated, but this was only transitory. Effects of glyphosate could not be negated by metal ions or glycine. Glyphosate inhibited the growth of both single node buds of Agropyron rhizome and wheat roots. 14C-glyphosate administered to single nodes was recovered from the buds principally unchanged and did not bind to cell walls or a protein/nucleic acid fraction. A metabolite was present, representing 10% of total 14C-activity. The syntheses of DNA, RNA and cellulose were inhibited to similar extents, due partly to inhibition of 14C-precursor uptake, notably in the case of DNA synthesis. The incorporation of 14C-leucine was inhibited markedly but that of 14C-phenylalanine was not greatly affected whereas the incorporation of 14C-protein hydrolysate was inhibited to an intermediate extent. These results indicated an inhibition of protein synthesis brought about by a diminution of the phenylalanine protein precursor pool. The induction of phenylalanine ammonia-lyase (PAL) was enhanced markedly in both single node buds and wheat root tips. In the latter case this was accompanied by declines in soluble protein and the rate of formazan reduction. Growth inhibition could not be alleviated by exogenous L-phenylalanine, mixed aromatic amino acids or PAL inhibitors. The specific activities of shikimic acid pathway enzymes, polyphenol oxidase and some hydrolytic enzymes characteristic of senescence were also enhanced. A 'classic wound response' was not elicited as the evolution of ethylene was not activated. The generation of ethane, however, an indicator of oxidative membrane lipid breakdown was stimulated, but the specific activities of microsomal marker enzymes were unaffected. Levels of these enzymes declined due to a reduction in the amount of microsomal protein present. Microsomal ATPases were not inhibited in vitro by glyphosate. The toxicity of glyphosate would appear to be mediated at least partly by a decrease in free phenylalanine resulting in inhibition of protein synthesis and de-repression of PAL. The involvement of other factors is not excluded.
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