Glyphosate’s accumulation in and influence on plant disease, mineral nutrition, and associated microbiota of glyphosate-resistant soybean and maize.

Authors: Duke, Stephen

Submitted to: Meeting Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 2/27/2019
Publication Date: N/A
Citation: N/A

Interpretive Summary: There has been controversy about whether the herbicide glyphosate adversely affects transgenic (GM), glyphosate-resistant crops. Numerous well replicated studies at different locations on the effects of glyphosate on the mineral nutritution, yield, disease, and associated soil microbiota of glyphosate-resistant soybean and corn have found no significant effects, even when used for many years.

Technical Abstract: Since it was introduced in 1974, glyphosate has become the most used herbicide worldwide, largely due to the widespread adoption of glyphosate-resistant (GR) crops (Duke, 2018a). Glyphosate has been used continuously in GR crops in many fields in the USA for over 20 years. Some have suggested that glyphosate increases disease and adversely affects mineral nutrition and rhizosphere microbiota associated with these crops. Plants and microbes can metabolically degrade glyphosate (Duke, 2011). Most microbial degradation of glyphosate is due to a glyphosate oxidase (GOX) that converts it to aminomethylphosphonic acid (AMPA) and glyoxylate. A microbial C-P lyase converts some glyphosate in soil to sarcosine and phosphate. A plant GOX is responsible for most degradation of glyphosate in plants. The amount of degradation in plants varies considerably between species (Reddy et al., 2008). Both glyphosate and AMPA are found in seeds of glyphosate-treated GR soybean (Bohm et al., 2014;Bøhn et al., 2014; Duke et al, 2003, 2012a, 2018), but only trace amounts of glyphosate and no AMPA are found in seeds of glyphosate-treated GR maize (Costa et al., 2018; Reddy et al., 2018; USDA, 2018). All plants and some microbes are sensitive to glyphosate due to inhibition of 5-enolpyruvylshikimate-3-phospate synthase (EPSPS), a shikimate pathway enzyme needed for aromatic amino acid synthesis. No other herbicide targets this enzyme. There is no convincing evidence of another mode of action of glyphosate as a herbicide or antibiotic at doses used to kill plants. The best proof of this is that transgenic plants with a glyphosate-insensitive bacterial EPSPS are 50-fold less sensitive to glyphosate than near isogenic lines without the transgene (Nandula et al., 2007). Some have claimed that glyphosate adversely affects mineral nutrition of plants due to its chelating properties. However, the preponderance of data in well replicated field studies at multiple sites, including sites where GR crops were grown for many years, have not found such effects on GR crops (Costa et al., 2018; Reddy et al., 2018; Duke et al., 2012a, 2018; Kandel et al., 2015; Williams et al., 2015). Furthermore, the unaltered rates of yield increases in the principle GR crops (cotton, maize, and soybean) after the introduction of these crops in the U.S. (Duke and Reddy, 2018) strongly supports the view that glyphosate has no significant effect on plant mineral metabolism or any other plant function independent of inhibition of EPSPS. Inhibition of EPSPS reduces shikimate pathway-mediated resistance of glyphosate-sensitive plants to pathogens (Schafer et al., 2012; Hammerschmidt, 2018). Thus, there is a synergy between glyphosate and some plant pathogens in glyphosate-sensitive plants (Duke, 2018b). In the field, this is part of the mode of action of glyphosate as a herbicide. In GR crops, glyphosate has either no effect on crop disease (Duke et al., 2012b; Kandel et al., 2015; Williams et al., 2015) or acts as a fungicide on some plant disease microbes, such as fungal rusts (e.g., Feng et al., 2005). Drift levels of glyphosate can reduce severity of some plant diseases in some plant species (Dos Santos et al., 2018). Some have claimed that glyphosate significantly alters soil and rhizosphere microbiota. If this were so, the effect would be most severe in agroecosystems in which glyphosate was used for many years, such a fields in which GR crops were grown continuously. Most studies have shown that, even in GR crops treated with high glyphosate doses, effects are short-lived and of less consequence than effects associated with such factors as soil type, farming systems, and seasonal progression. These were the findings in a well replicated study in at two sites in the U.S. (Mississippi and Maryland) conducted for two years, comparing rhizosphere microbiota in plots of GR