Glyphosate-resistant and conventional canola (Brassica napus L.) responses to glyphosate and Aminomethylphosphonic Acid (AMPA) treatment

Authors: CORREA, ELZA - Universidade Estadual Paulista (UNESP); Dayan, Franck; Owens, Daniel; Rimando, Agnes; Duke, Stephen

Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/19/2016
Publication Date: 4/19/2016
Citation: Correa, E.A., Dayan, F.E., Owens, D.K., Rimando, A.M., Duke, S.O. 2016. Glyphosate-resistant and conventional canola (Brassica napus L.) responses to glyphosate and Aminomethylphosphonic Acid (AMPA) treatment. Journal of Agricultural and Food Chemistry. 64:3508-3513.

Interpretive Summary: Over the past 20 years, the herbicide glyphosate is the most important herbicide and glyphosate-resistant (GR) crops have been the most important transgenic crops. Despite the importance of these products, there are voids in our knowledge of some aspects of these essential components of modern agricultural production. Almost nothing is known of the metabolic behavior of glyphosate in GR canola, the only GR crop to use the glyphosate oxidase (GOX) transgene along with a gene for a GR EPSPS. Significant new information on this topic is presented. Results indicate that the GOX enzyme is highly active, especially in young tissues and that AMPA is very weakly toxic to canola, not accumulating to phytotoxic levels in GR canola treated with up to three times the recommended glyphosate dose.

Technical Abstract: Glyphosate-resistant (GR) canola expresses two transgenes: 1) the microbial glyphosate oxidase gene (gox) encoding the glyphosate oxidase enzyme (GOX) that metabolizes glyphosate to aminomethylphosphonic acid (AMPA) and 2) cp4 that encodes a GR form of the glyphosate target enzyme 5-enolpyruvylshikimic acid-3-phosphate synthase. The objectives of this research were to determine the phytotoxicity of AMPA to canola, the relative metabolism of glyphosate to AMPA in GR and conventional non-GR (NGR) canola, and AMPA pool sizes in glyphosate-treated GR canola AMPA applied at 1.0 kg ha-1 was not substantially phytotoxic to GR canola, but was moderately phytotoxic to NGR canola. At this AMPA application rate, NGR canola accumulated a higher concentration of AMPA in its tissues than GR canola. At rates of 1 and 3.33 kg ae ha-1 of glyphosate, GR canola growth was stimulated. This hormetic effect is similar to that of much lower doses of glyphosate on NGR canola. Both shikimate and AMPA accumulated in tissues of these treated plants. In a separate experiment in which young GR and NGR canola plants were treated with non-phytotoxic levels of 14C-glyphosate, very little glyphosate was metabolized in NGR plants, whereas most of the glyphosate was metabolized to AMPA in GR plants at 7 days after application. Untreated leaves of GR plants accumulated only metabolites (mostly AMPA) of glyphosate, indicating that GOX activity is very high in the youngest leaves. These data indicate that more glyphosate is transformed to AMPA rapidly in GR canola, and that the accumulated AMPA is not toxic to the canola plant.