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United States Department of Agriculture

Agricultural Research Service

Research Project: DISCOVERY AND DEVELOPMENT OF NATURAL PRODUCT-BASED WEED MANAGEMENT METHODS Title: Glyphosate metabolism in plants

Author
item Duke, Stephen

Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: December 20, 2011
Publication Date: March 15, 2012
Citation: Duke, S.O. 2012. Glyphosate metabolism in plants. Book Chapter. pp.17-38.

Interpretive Summary: Many soil microbes and plant species metabolically degrade the herbicide glyphosate. The primary degradation routes are by a glyphosate oxidoreductase (GOX) to form aminomethylphosphonic acid (AMPA) as the distinctive metabolite and by a C-P lyase that forms sarcosine as a main metabolite. AMPA appears to be the main glyphosate metabolite of most microbes and plants. High levels of AMPA have been found in glyphosate-treated, glyphosate-resistant (GR) soybean, apparently due to plant GOX-like activity. AMPA is mildly phytotoxic, and under some conditions the AMPA accumulating in GR soybean correlates with glyphosate-caused phytotoxicity. A bacterial GOX is used in GR canola to help provide glyphosate resistance, and an altered bacterial glyphosate N-acetyltransferase is planned for a new generation of GR crops. In some weed species, glyphosate degradation could contribute to natural resistance. Neither an isolated plant GOX enzyme nor a gene for it has yet been reported in plants. Gene mutation or amplification of plant genes for GOX-like enzyme activity or horizontal transfer of microbial genes from glyphosate-degrading enzymes could produce GR weeds. Yet, there is no evidence that metabolic degradation plays a significant role in evolved resistance to glyphosate. This is unexpected, considering the extreme selection pressure for evolution of glyphosate resistance in weeds and the difficulty in plants evolving glyphosate resistance via other mechanisms.

Technical Abstract: Many soil microbes and plant species metabolically degrade the herbicide glyphosate. The primary degradation routes are by a glyphosate oxidoreductase (GOX) to form aminomethylphosphonic acid (AMPA) as the distinctive metabolite and by a C-P lyase that forms sarcosine as a main metabolite. AMPA appears to be the main glyphosate metabolite of most microbes and plants. High levels of AMPA have been found in glyphosate-treated, glyphosate-resistant (GR) soybean, apparently due to plant GOX-like activity. AMPA is mildly phytotoxic, and under some conditions the AMPA accumulating in GR soybean correlates with glyphosate-caused phytotoxicity. A bacterial GOX is used in GR canola to help provide glyphosate resistance, and an altered bacterial glyphosate N-acetyltransferase is planned for a new generation of GR crops. In some weed species, glyphosate degradation could contribute to natural resistance. Neither an isolated plant GOX enzyme nor a gene for it has yet been reported in plants. Gene mutation or amplification of plant genes for GOX-like enzyme activity or horizontal transfer of microbial genes from glyphosate-degrading enzymes could produce GR weeds. Yet, there is no evidence that metabolic degradation plays a significant role in evolved resistance to glyphosate. This is unexpected, considering the extreme selection pressure for evolution of glyphosate resistance in weeds and the difficulty in plants evolving glyphosate resistance via other mechanisms.

Last Modified: 10/25/2014
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