Effects of Glyphosate on Metabolism of Phenolic Compounds

Abstract

Amounts of extractable phenylalanine ammonia-lyase (PAL; E.C. 4.3.1.5) activity increased in the axes of 3-day-old, dark-grown soybean seedlings IGlyeine max (L.) Merr.] shortly after the seedlings were transferred to glyphosate IAf-(phosphonomethyl)-glychie] solutions. The stimulation of PAL activity in herbicide-treated tissue (as compared to control tissue) was detectable as early as 12 h after treatment, whereas growth inhibition (length, fresh weight and dry weight) was not significantly affected until 24 h on a fresh-weight basis and at 48 h on a dry-weight basis. PAL activity increased with time (12-72 h) in herbicide-treated axes when expressed as activity per gram fresh weight, specific activity, and on a per axis basis. PAL activity stimulation correlated positively with glyphosate concentration from 10"* to 10~' M. PAL activity in control tissues remained nearly constant over the sampling period (12-72 h). Total alcohol-soluble hydroxyphenolic compound levels in treated axes were not significantly different from the control at any sampling period. The total soluble amino acid pool showed a general decrease with time in glyphosate-treated tissues. The phenylalanine pool was lowered with treatment time and the ammonia concentration (per g fr. wt, basis) was increased after treatment. No significant differences were noted in the concentration of soluble protein of glyphosate-treated tissue when compared to controls. Visual effects {stunting, lack of secondary root formation, and necrotic areas) of glyphosate were more obvious in the root than in the hypocotyl. Analysis of various chemical constituents substantiated that other glyphosate effects were more clearly demonstrable in the root than in the hypocotyl or in the intact axis. On a per root basis glyphosate markedly increased PAL activity while reducing free phenylalanine, free tyrosine, soluble hydroxyphenolics, total free amino acids and ammonia content. The effect of glyphosate in the root was greatest on phenyl-' Mississippi Agricultural and Forestry Experiment Station »operating. alanine content, reducing it five-fold. The results indicate that glyphosate could exert its effect through either induction of PAL activity and/or inhibition of aromatic amino acid synthesis.Recently, we postulated that glyphosate might reduce growth in maize {Zea mays L.) by inducing phenylalanine 0031-9317/79/080357-10803,00/0 © 1979 Physiologia Plantarum

Herbicides Effect on Nitrogen Cycling in Agroecossystems

“…However, as a secondary effect, there is an inhibition in the synthesis of phenolic compounds deriving from those amino acids, which increases the activity of the PAL (phenylalamine ammonia lyase) enzyme due to a response effect (Duke & Hoagland, 1985). The PAL acts in the lyase of phenylalanine and tyrosine amino acids, resulting in the formation of phenolic acids and NH 4 + (Hoagland et al, 1979). The glufosinate ammonium herbicide inhibits the action of the glutamine sinthetase, which is responsible for the glutamate synthesis from ammonium and glutamine.…”

Enzymological Basis for Herbicidal Action of Glyphosate

“…To date, the only nonmicrobial studies relating enzyme data to glyphosate effects were those (9)(10)(11)17), testing a hypothesis that glyphosate-mediated induction of PAL in higher plants might account for growth inhibition. However, it now seems likely that PAL induction is an effect, rather than the cause, of metabolic events initiated by glyphosate (1).…”

Reversal of Glyphosate Inhibition of Carrot Cell Culture Growth by Glycolytic Intermediates and Organic and Amino Acids

“…Gresshoff (12) reported reversal of glyphosate inhibition of Arabidopsis thaliana seedlings with a combination of phenylalanine and tyrosine. In contrast, Duke and Hoagland (6) observed increased inhibition by glyphosate of corn seedling growth when phenylanine and tyrosine were added.Several studies (7,8,16) show that glyphosate treatment can decrease free phenylalanine and tyrosine levels. Recent studies have indicated that glyphosate inhibits the synthesis of flavonoids and chlorogenic acid in buckwheat hypocotyls (17) and causes an accumulation of shikimate that is significantly correlated with a…”