Glyphosate is a slow-acting herbicide that is metabolically degraded to aminomethylphosphonic acid (AMPA) and glyoxylate in many plant species.   AMPA is much less phytotoxic than glyphosate and glyoxylate is not phytotoxic. So, even a moderate rate of metabolic degradation of this herbicide should impart tolerance or resistance.  Considering the base line metabolism in some weed species and the massive selection pressure on weeds in glyphosate-resistant (GR) crops, one would assume that that selection pressure would select for weed biotypes with greater rates of metabolic degradation.  But, there are few data to indicate that glyphosate metabolism relates to the low levels of tolerance of some species, and there are no confirmed studies that show that enhanced glyphosate metabolism has been selected for in GR weeds.   The enzyme that degrades glyphosate to AMPA (glyphosate oxidoreductase, GOX) has not been isolated from plants, but the gene that encodes GOX has been isolated from  a soil microbe (<i>Acrhromobacter </i>sp.) and, at one time, used as a transgene to help provide some of the glyphosate resistance in GR canola.  Many soil microbes can metabolize glyphosate with GOX or a C-P lyase.  The fact that selection pressure with glyphosate has apparently not led to horizontal gene transfer of genes for either enzyme to weeds argues against horizontal gene transfer being a common phenomenon.  The role of endophytes in glyphosate degradation in plants is unexplored. <br/>

Metabolic degradation of glyphosate and glyphosate tolerance and resistance

256th National Meeting (2018)

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