Effects of glyphosate

Glyphosate also inhibits the activity of a form of 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase (13), however, this inhibition is relatively weak. Effects of glyphosate on the enzymes of aromatic amino acid synthesis have been described in more detail in recent reviews (14-17). 

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). 

Figure 3. Effect of glyphosate on fresh weight (o), chlorophyll content (x), and anthocyanin content ( ) of excised buckwheat cotyledons after 24 h of treatment in the light. Reproduced with permission from Ref. 6. Copyright 1980, Plant Physiology.

Kleter, G.A., Harris, C.A., Stephenson, G.R. and Unsworth, J. (2008) Comparison of herbicide regimes and the associated potential environmental effects of glyphosate-resistant crops vs. what they replace in Europe. Pestic Manage Sci, 64, 479-488. 

Sharma S.D. and M. Singh (2000). Effect of glyphosate and its 2,4-D formulation on some difficult-to-control weeds. Proc. Fla. State Hort. ScL, 113 63-67. 

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. 

Richard S, Moslemi S, Sipahutar H, et al. Differential effects of glyphosate and roundup on human placental cells and aromatase. Environ Health Perspect 2005 113(6) 716—20. 

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. 

The phytotoxic effect of glyphosate can be increased by additives enhancing absorption and translocation. According to the investigations of Wills (1973) the addition of various salts and surfactants to glyphosate does not increase its initial phytotoxicity, but increases the duration of effectiveness from 2 to 8 months. Of the cations K, of the anions POJ had the best effect. 

Figure 2 (A). Effects of glyphosate on growth in wild type ( O) and mutant (G363 A, G492 ) gametophytes of Ceratopteris.

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

The effects of glyphosate on several E. coll enzymes of the aromatic amino acid biosynthe tic pathway have been studied... 

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.

Effects of Glyphosate Analogs on Secondary Phenolic Compound Synthesis. To test the specificity of glyphosate s action and/or the effect of analogs and possible degradation products (Figure 2), a study was conducted that used these compounds on... 

Figure 12. Effect of glyphosate on fresh weight increase, chlorophyll content, and anthocyanin content in excised buckwheat cotyledons (69). Cotyledons of 6-day-old etiolated seedlings were incubated at the indicated glyphosate concentrations for...

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