Modern herbicides make major contributions to global food production by easily removing weeds and substituting for destructive soil cultivation. However, persistent herbicide selection of huge weed numbers across vast areas can result in the rapid evolution of herbicide resistance. Herbicides target specific enzymes, and mutations are selected that confer resistance-endowing amino acid substitutions, decreasing herbicide binding. Where herbicides bind within an enzyme catalytic site very few mutations give resistance while conserving enzyme functionality. Where herbicides bind away from a catalytic site many resistance-endowing mutations may evolve. Increasingly, resistance evolves due to mechanisms limiting herbicide reaching target sites. Especially threatening are herbicide-degrading cytochrome P450 enzymes able to detoxify existing, new, and even herbicides yet to be discovered. Global weed species are accumulating resistance mechanisms, displaying multiple resistance across many herbicides and posing a great challenge to herbicide sustainability in world agriculture. Fascinating genetic issues associated with resistance evolution remain to be investigated, especially the possibility of herbicide stress unleashing epigenetic gene expression. Understanding resistance and building sustainable solutions to herbicide resistance evolution are necessary and worthy challenges.

/pdf/evolution-in-action-plants-resistant-to-herbicides-2gtjx1z4z2.pdf

Evolution in Action: Plants Resistant to Herbicides

Since its commercial introduction in 1974, glyphosate [N-(phosphonomethyl)glycine] has become the dominant herbicide worldwide. There are several reasons for its success. Glyphosate is a highly effective broad-spectrum herbicide, yet it is very toxicologically and environmentally safe. Glyphosate translocates well, and its action is slow enough to take advantage of this. Glyphosate is the only herbicide that targets 5-enolpyruvyl-shikimate-3-phosphate synthase (EPSPS), so there are no competing herbicide analogs or classes. Since glyphosate became a generic compound, its cost has dropped dramatically. Perhaps the most important aspect of the success of glyphosate has been the introduction of transgenic, glyphosate-resistant crops in 1996. Almost 90% of all transgenic crops grown worldwide are glyphosate resistant, and the adoption of these crops is increasing at a steady pace. Glyphosate/glyphosate-resistant crop weed management offers significant environmental and other benefits over the technologies that it replaces. The use of this virtually ideal herbicide is now being threatened by the evolution of glyphosate-resistant weeds. Adoption of resistance management practices will be required to maintain the benefits of glyphosate technologies for future generations. Copyright © 2008 Society of Chemical Industry

Glyphosate: a once-in-a-century herbicide.

After 25 years of sales, glyphosate (N-(phosphonomethyl)glycine) is the world's biggest selling and fastest growing agrochemical. Glyphosate has many strengths, particularly its systemicity and ability to control perennial weeds, which have facilitated its role as an essential tool in modern agriculture. Surveys of recent scientific and patent literature point to several key areas of current and future interest in glyphosate and its use. Biotechnology topics have been the most popular, but there is still much interest in the molecule per se, particularly in manufacturing processes. The mode of action and physiology of glyphosate are common subjects. More applied aspects concern the development of formulations with new adjuvants, or mixtures with other herbicides, to enhance performance. An increasing number of papers are reporting on its environmental fate and on agronomic techniques designed to improve the efficiency of application and minimise its environmental impact. Lack of weed resistance is a key feature of glyphosate despite its widespread use. However, differential levels of tolerance between species have led to changes in weed succession. There is a need to develop integrated weed management systems in which glyphosate still plays a key role. Glyphosate is an exceedingly effective herbicide and presents a considerable challenge to the development of new herbicides for sustainable cropping systems and situations in which cost-effective and environmentally considerate weed control is required.

Why glyphosate is a global herbicide: strengths, weaknesses and prospects

Glyphosate is the world's most important herbicide, with many uses that deliver effective and sustained control of a wide spectrum of unwanted (weedy) plant species. Until recently there were relatively few reports of weedy plant species evolving resistance to glyphosate. Since 1996, the advent and subsequent high adoption of transgenic glyphosate-resistant crops in the Americas has meant unprecedented and often exclusive use of glyphosate forweedcontroloververylarge areas.Consequently, in regionsoftheUSA wheretransgenic glyphosate- resistant crops dominate, there are now evolved glyphosate-resistant populations of the economically damaging weed species Ambrosia artemissifolia L., Ambrosia trifida L., Amaranthus palmeri SW atson,Amaranthus rudis JD Sauer, Amaranthus tuberculatus (Moq) JD Sauer and various Conyza and Lolium spp. Likewise, in areas of transgenic glyphosate-resistant crops in Argentina and Brazil, there are now evolved glyphosate-resistant populations ofSorghum halepense (L.) Pers andEuphorbia heterophylla L. respectively. As transgenic glyphosate- resistant crops will remain very popular with producers, it is anticipated that glyphosate-resistant biotypes of other prominent weed species will evolve over the next few years. Therefore, evolved glyphosate-resistant weeds are a major risk for the continued success of glyphosate and transgenic glyphosate-resistant crops. However, glyphosate-resistant weeds are not yet a problem in many parts of the world, and lessons can be learnt and actions taken to achieve glyphosate sustainability. A major lesson is that maintenance of diversity in weed management systems is crucial for glyphosate to be sustainable. Glyphosate is essential for present and future world food production, and action to secure its sustainability for future gene rations is ag lobal imperative.  2008 Society of Chemical Industry

Evolved glyphosate-resistant weeds around the world: lessons to be learnt

No-tillage corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] production has been widely accepted in the mid-Atlantic region, favoring establishment of horseweed [Conyza canadensis (L.) Cronq.]. Within 3 yr of using only glyphosate for weed control in continuous glyphosate-resistant soybeans, glyphosate failed to control horseweed in some fields. Seedlings originating from seed of one population collected in Delaware were grown in the greenhouse and exhibited 8- to 13-fold glyphosate resistance compared with a susceptible population. There were no differences between the isopropylamine or diammonium salts of glyphosate. Nomenclature: Corn; Zea mays L.; horseweed; Conyza canadensis (L.) Cronq. ERICA; soybean; Glycine max (L.) Merr.; glyphosate.

Glyphosate-resistant horseweed from Delaware

Following 15 yr of successful use, glyphosate failed to control a population of the widespread grass weed rigid ryegrass in Australia. This population proved to be resistant to glyphosate in pot dose-response experiments conducted outdoors, exhibiting 7- to 11-fold resistance when compared to a susceptible population. Some cross-resistance to diclofop-methyl (about 2.5-fold) was also observed. Similar levels of control of the resistant and susceptible populations were obtained following application of amitrole, chlorsulfuron, fluazifop-P-butyl, paraquat, sethoxydim, sirnazine, or tralkoxydim. The presence of glyphosate resistance in a major weed species indicates a need for changes in glyphosate use patterns.

James J. Dellow

Papers

Evolved resistance to glyphosate in rigid ryegrass (Lolium rigidum) in Australia