Glyphosate is the only herbicide to target the enzyme 5‐enolpyruvyl‐3‐shikimate phosphate synthase (EPSPS). It is a high use rate, non‐selective herbicide that translocates primarily to metabolic ...sinks, killing meristematic tissues away from the application site. Its phloem‐mobile properties and slow action in killing weeds allow the herbicide to move throughout the plant to kill all meristems, making it effective for perennial weed control. Since commercialization in 1974, its use has grown to dominate the herbicide market. Much of its use is on transgenic, glyphosate‐resistant crops (GRCs), which have been the dominant transgenic crops worldwide. GRCs with glyphosate provided the most effective and inexpensive weed management technology in history for a decade or more. However, as a consequence of the rapid increase in glyphosate‐resistant (GR) weeds, the effectiveness of glyphosate use in GRCs is declining. Critics have claimed that glyphosate‐treated GRCs have altered mineral nutrition and increased susceptibility to plant pathogens because of glyphosate's ability to chelate divalent metal cations, but the complete resistance of GRCs to glyphosate indicates that chelating metal cations do not contribute to the herbicidal activity or significantly affect mineral nutrition. The rates of increases in yields of maize, soybean, and cotton in the USA have been unchanged after high adoption rates of GRCs. Glyphosate is toxic to some plant pathogens, and thereby can act as a fungicide in GRCs. Ultra‐low doses of glyphosate stimulate plant growth in glyphosate‐susceptible plants by unknown mechanisms. Despite rapid and widespread increases in GR weeds, glyphosate use has not decreased. However, as GR weeds increase, adoption of alternative technologies will eventually lead to decreased use. Published 2017. This article is a U.S. Government work and is in the public domain in the USA.
Glyphosate has been the most successful herbicide in history. This perspective chronicles the reasons for its success and the impact of evolved glyphosate resistance on its current and future use.
Herbicides with new modes of action are badly needed to manage the evolution of resistance of weeds to existing herbicides. Yet no major new mode of action has been introduced to the market place for ...about 20 years. There are probably several reasons for this. New potential products may have remained dormant owing to concerns that glyphosate-resistant (GR) crops have reduced the market for a new herbicide. The capture of a large fraction of the herbicide market by glyphosate with GR crops led to significantly diminished herbicide discovery efforts. Some of the reduced herbicide discovery research was also due to company consolidations and the availability of more generic herbicides. Another problem might be that the best herbicide molecular target sites may have already been discovered. However, target sites that are not utilized, for which there are inhibitors that are highly effective at killing plants, suggests that this is not true. Results of modern methods of target site discovery (e.g. gene knockout methods) are mostly not public, but there is no evidence of good herbicides with new target sites coming from these approaches. In summary, there are several reasons for a long dry period for new herbicide target sites; however, the relative magnitude of each is unclear. The economic stimulus to the herbicide industry caused by the evolution of herbicide-resistant weeds, especially GR weeds, may result in one or more new modes of action becoming available in the not too distant future.
Herbicide-resistant crops have had profound impacts on weed management since they were introduced in 1995. Most of the impact has been by glyphosate-resistant maize, cotton, soybean, and canola. ...Significant economic savings, yield increases, and more efficacious and simplified weed management resulted in widespread adoption of the technology. Initially, glyphosate-resistant crops enabled significantly reduced tillage and reduced the environmental impact quotient of weed management. Continuous use of glyphosate with glyphosate-resistant crops over broad areas facilitated the evolution of glyphosate-resistant weeds, which have resulted in increases in the use of tillage and other herbicides with glyphosate, reducing some of the initial environmental benefits of glyphosate-resistant crops. Transgenic crops with resistance to auxinic herbicides, as well as to herbicides that inhibit acetolactate synthase, acetylCoA carboxylase, and hydroxyphenylpyruvate dioxygenase, stacked with glyphosate and/or glufosinate resistance, will become available in the next few years. These technologies will provide additional options for farmers, but will not have the positive impacts that glyphosate-resistant crops had initially. In the more distant future, other herbicide-resistant crops (including non-transgenic ones), herbicides with new modes of action, and technologies that are currently in their infancy (e.g., bioherbicides, sprayable herbicidal RNAi, and/or robotic weeding) may impact the role of transgenic, herbicide-resistant crops in weed management.
Glyphosate is the most used herbicide worldwide, which has contributed to concerns about its environmental impact. Compared with most other herbicides, glyphosate has a half-life in soil and water ...that is relatively short (averaging about 30 d in temperate climates), mostly due to microbial degradation. Its primary microbial product, aminomethylphosphonic acid, is slightly more persistent than glyphosate. In soil, glyphosate is virtually biologically inactive due to its strong binding to soil components. Glyphosate does not bioaccumulate in organisms, largely due to its high water solubility. Glyphosate-resistant crops have greatly facilitated reduced-tillage agriculture, thereby reducing soil loss, soil compaction, carbon dioxide emissions, and fossil fuel use. Agricultural economists have projected that loss of glyphosate would result in increased cropping area, some gained by deforestation, and an increase in environmental impact quotient of weed management. Some drift doses of glyphosate to non-target plants can cause increased plant growth (hormesis) and/or increased susceptibility to plant pathogens, although these non-target effects are not well documented. The preponderance of evidence confirms that glyphosate does not harm plants by interfering with mineral nutrition and that it has no agriculturally significant effects on soil microbiota. Glyphosate has a lower environmental impact quotient than most synthetic herbicide alternatives.
High levels of aminomethylphosphonic acid (AMPA), the main glyphosate metabolite, have been found in glyphosate-treated, glyphosate-resistant (GR) soybean, apparently due to plant glyphosate ...oxidoreductase (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, 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 of evolving glyphosate resistance via other mechanisms.
Glyphosate is the most used herbicide globally. It is a unique non-selective herbicide with a mode of action that is ideal for vegetation management in both agricultural and non-agricultural ...settings. Its use was more than doubled by the introduction of transgenic, glyphosate-resistant (GR) crops. All of its phytotoxic effects are the result of inhibition of only 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), but inhibition of this single enzyme of the shikimate pathway results in multiple phytotoxicity effects, both upstream and downstream from EPSPS, including loss of plant defenses against pathogens. Degradation of glyphosate in plants and microbes is predominantly by a glyphosate oxidoreductase to produce aminomethylphosphonic acid and glyoxylate and to a lesser extent by a C-P lyase to produce sarcosine and phosphate. Its effects on non-target plant species are generally less than that of many other herbicides, as it is not volatile and is generally sprayed in larger droplet sizes with a relatively low propensity to drift and is inactivated by tight binding to most soils. Some microbes, including fungal plant pathogens, have glyphosate-sensitive EPSPS. Thus, glyphosate can benefit GR crops by its activity on some plant pathogens. On the other hand, glyphosate can adversely affect some microbes that are beneficial to agriculture, such as Bradyrhizobium species, although GR crop yield data indicate that such an effect has been minor. Effects of glyphosate on microbes of agricultural soils are generally minor and transient, with other agricultural practices having much stronger effects.
Chemical pesticides and their formulation ingredients can have unintended effects on microbes associated with plants and plant pests. These effects can be due to direct effects on the microbes or to ...effects on crops or weeds that subsequently affect the microbes. In addition to fungicides, some insecticides, herbicides, and formulation compounds are toxic to plant pathogenic microbes, as well as to potentially beneficial microbes, such as those that infect insect pests. These chemicals, especially herbicides, can also indirectly affect microbes through their effects on crops and weeds. For example, glyphosate strongly impairs shikimic acid pathway-based plant defenses to microbial diseases in glyphosate-susceptible plants, significantly increasing its efficacy as an herbicide. Some herbicides induce plant defenses against plant pathogens. For a complete understanding of integrated pest management and overall cost/benefit of pesticide use, much more information is needed on microbial/pesticide interactions.
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