Monday, 28 October 2019

Herbicide Resistance in Weeds

Herbicide Resistance in Weeds

The recurrent use of an herbicide or herbicide mechanism of action imparts significant selection pressure on the weed population and provides an ecological advantage to those rare individuals within the population that have the heritable mutation conferring herbicide resistance. herbicide resistance in weeds was reported in the scientific literature in 1970, but the occurrence of evolved resistance to herbicides in plants was suggested in 1956 by Harper 1956; Ryan 1970.

Adoption for Herbicide Resistance

Since the commercial introduction of genetically modified crops in 1996,  the area planted to these cultivars has increased globally at an increasing rate. In 2006, approximately 100 million hectares of genetically modified herbicide-resistant crops were planted worldwide and an estimated 80% had the genetically modified trait conferring glyphosate resistance. The use of these cultivars increased an estimated 12% in 2007 and represented 114.3 million hectares and included 23 countries. The primary countries that plant genetically modified herbicide-resistant crops are the United States (US), Argentina, Brazil, and Canada. North America represents 57% of the genetically modified herbicide-resistant crops planted globally while Central and South America contribute 33% of the total hectares. 

It is estimated that approximately 90% of the genetically modified herbicide-resistant crops grown globally are glyphosate-resistant crop cultivars and are represented primarily by soybean, cotton, and maize. Genetically modified herbicide-resistant canola dominates the cultivars grown in Canada and the US, representing >80% of the total crop is grown. Recently there has been significant adoption of glyphosate-resistant genetically modified maize in the US, in part attributable to maize-based ethanol production. Overall, the revolution of the adoption of genetically modified crops likely represents the largest man-caused biological experiment in history.

This will cause enormous selection pressure that the wide-spread application of glyphosate on millions of hectares will impose on weed communities and inevitably result in significant changes by selecting for weeds that do not respond to the prevalent control tactics. Recently genetically modified herbicide-resistant (glyphosate) sugarbeets were commercially introduced in the US, which will add more selection pressure on weed communities (Duke 2005; Gianessi 2005).

Types of Herbicide Resistance

the sensitive weed population, resulting in a resistant biotype (Gressel and Levy 2006). However, recent studies suggest that weeds can also evolve monogenic herbicide resistance by “losing” amino acid in the target protein. A partial list of target-site resistance demonstrated in weed populations includes resistant weed biotypes for acetolactate synthase (ALS) inhibiting herbicides, protoporphyrinogen oxidase (PPO) inhibiting herbicides, triazine herbicides, and glyphosate. Herbicide resistance in weeds also is the result of differential translocation of the herbicide to the target site (Feng et al. 2004). Weeds are also able to evolve herbicide resistance by rapidly and efficiently metabolizing the herbicide prior to the accumulation of a toxic concentration of the herbicide at the target site. This is also known as non-target site resistance and is typically mediated by cytochrome P450 monooxygenases, glutathione S-transferases or glycosyltransferases, depending on the herbicide.

Herbicide resistance can also be a function of ABC transporters which serve to facilitate compartmentalization of the herbicide, again protecting the target site of the herbicide (Lu et al. 1997). Finally, weeds have demonstrated other novel forms of herbicide resistance, such as morphological adaptations (i.e. leaf pubescence) and phenological changes (i.e. avoidance attributable to delayed germination) in weed populations (Owen 2001). Interestingly, weeds have demonstrated the ability to evolve multiple resistances to several herbicide modes of action (Patzoldt et al. 2005; Legleiter and Bradley 2008). Herbicide resistance in crops has been established using altered target site, the most common strategy used (i.e. glyphosate-resistant crops), enhanced metabolism (i.e. glufosinate-resistant crops) and cultivars with multiple resistances to herbicides have been developed (Green 2007; Green et al. 2008; Green et al. 2009).

Modes of Herbicide Actions

Modes of Herbicide Actions

Most of the current herbicide-resistant crop cultivars are represented by cultivars created by transgenic modifications. (Duke 2005) These herbicide modes of action include inhibition of photosystem II (bromoxynil), inhibition of glutamine synthetase (glufosinate) and inhibition of EPSPS (glyphosate). They are facilitated by the insertion of five transgenes to confer resistance to the respective herbicides: CP4, GOX or a mutated EPSPS for glyphosate resistance, a nitrilase gene for the bar gene and bromoxynil resistance for glufosinate resistance. Historically, there are non-transgenic herbicide resistance traits for cyclohexanedione herbicides, imidazolinone herbicide, sulfonylurea herbicides, and triazine herbicides; however, the dominant herbicide-resistant trait on the market is for transgenic glyphosate resistance (Duke 2005; Duke and Powles 2008). Recently, two novel transgenes, gat4621, and hra were introduced that confer high levels of resistance to glyphosate- and ALS-inhibiting herbicides, respectively (Castle et al. 2004; Green et al. 2008; Green et al. 2009).

A gene that codes for dicamba monooxygenase (DMO), a Rieske non-heme monooxygenase that metabolizes dicamba, has been discovered in the soil bacteria Pseudomonas maltophilia and can be biotechnologically inserted into the nuclear and chloroplast genome of soybean, thus conferring these transgenic plants resistance to dicamba (Behrens et al. 2007). These cultivars are anticipated to be commercially released in several years. Furthermore, transgenes that code for resistance to 2,4-D and ACCase inhibitor herbicides are also anticipated to be inserted into the various crops in the near future. Thus, the number of herbicide modes of action with transgenic resistant crop cultivars appears to be increasing and it is anticipated that these new transgenes will improve weed management options for growers and help resolve current and future problems with the evolution of herbicide-resistant weed biotypes. However, whether or not the mitigation of current and future herbicide-resistant weed problems actually occurs depends entirely on how growers utilize the technologies and whether or not they establish appropriate integrated weed management strategies.

Implications of Genetically Modified Herbicide-Resistant Crops

The widespread adoption of genetically modified herbicide-resistant crops has made a number of significant impacts on agricultural systems. Notably, the level of weed control and consistency of efficacy has increased compared to “traditional” soil-applied herbicides (Duke 2005). Furthermore, given that genetically modified herbicide-resistant crops are represented largely by resistance to glyphosate and to a lesser amount glufosinate, and given that these herbicides are used post-emergence to the weeds and have generally favorable edaphic and toxicological characteristics, there are likely significant positive environmental benefits. 

Another important environmental benefit attributable to these crops is the adoption of conservation tillage practices including no-tillage production systems which result in important reductions of soil erosion, thus improving water quality and lessening the degradation of soil (Young 2006). The benefits that growers attribute to genetically modified herbicide-resistant crops reflect the perceived simplicity and convenience of weed control (Owen 2008a, b). However, an objective review of the implications of genetically modified herbicide-resistant would suggest that there are important risks that must also be considered.

Selection Pressure Indirectly Attributable to Genetically Modified Herbicide-Resistant Crops

The consistent and widespread use of one herbicide has considerable implications on the weed community (Owen 2008a, b). Differential response of weed species to the herbicide results in some weeds that are ecologically favored in the system. The recurrent use of a specific herbicide with a high level of efficacy on the sensitive weeds results in weeds that are favored by the system and thus become the dominant members of the weed community (Scursoni et al. 2006; Scursoni et al. 2007). For example, Asiatic dayflower (Commelina Cummins) is known to be tolerant to glyphosate and has become an increasing problem in genetically modified glyphosate-resistant crops (Ulloa and Owen 2009). 

The other aspect of selection pressure is the shift in a weed species that is predominantly sensitive to the herbicide to a biotype that has a mutation conferring resistance to the herbicide (Owen 2008a, b). Regardless of the ultimate type of weed shift, the greater the selection pressure that the herbicide imparts upon the agroecosystem, the more pervasive the change in the weed community; it should be recognized that it is not a matter of “if” the change in the weed community occurs but rather “when” the change is identified. Selection pressure from herbicides used in agriculture will inevitably result in changes in weed communities (Owen and Zelaya 2005).

Evolved Herbicide Resistance

The evolution of herbicide resistance predates the adoption of genetically modified herbicide-resistant crops by almost four decades (Ryan 1970; Duke 2005). Resistance to 19 herbicide mechanisms of action has been documented globally, with evolved resistancetoALSinhibitors, triazines, ACCase inhibitors, synthetic auxins, by pyridinium, ureas and amides, glycines and dinitroaniline herbicides being the most prevalent. Interestingly, some weeds demonstrate the ability to evolve resistance to multiple mechanisms of herbicide action (Preston et al. 1996; Patzoldt et al. 2005). 

Rigid ryegrass (Lolium rigidum) biotypes have been documented to resist as many as seven mechanisms of herbicide action(Heap 2009). Furthermore, a number of weed species have demonstrated the ability to evolve cross-resistance to different herbicide families with similar mechanisms of action (Hinz and Owen 1997). Despite the fact that the mutations that confer resistance to herbicides typically occur at extremely low frequencies within non-selected weed populations, resistance to any and all herbicides can evolve given the current management of weeds in most crop production systems and the strategies of resistance that weeds have demonstrated (Gressel 1996; Gressel and Levy 2006).

Changes in Herbicide Use

One of the pervasive questions surrounding the adoption of genetically modified herbicide-resistant crops is the impact on herbicide use. It is well documented that, initially, the number of active herbicide ingredients used in genetically modified herbicide-resistant crops declined dramatically (Young 2006; Bonny 2007). However, whether or not the herbicide load on the environment was lessened in genetically modified herbicide-resistant crops depends on the measurement metric. 

It is argued that, with the genetically modified herbicide-resistant crops, fewer applications of herbicides are required and thus less herbicide is used. However, given that the herbicides used on genetically modified herbicide-resistant crops are used at amounts that are many-folds higher than the herbicides that were replaced, it is argued that more herbicide is used compared to conventional crops (Benbrook 2001). Furthermore, the number of herbicide applications in genetically modified herbicide-resistant crops has increased steadily since the introduction of these crops (Young 2006).

Lack of Integrated Weed Management

The primary benefits of the genetically modified herbicide-resistant crops, as stated by growers, is the convenience and simplicity of weed control. This has contributed to the dramatic decline in alternative tactics used to manage weeds and thus a loss of integrated weed management in genetically modified herbicide-resistant crops. The loss of integrated weed management then results in weed shifts in the genetically modified crops which negatively impacts crop production economics and has important long-term implications on the sustainability of cropping systems based on genetically modified herbicide-resistant crops.

Some Herbicide Resistance Crops


Corn cultivars with resistance to herbicides include genetically modified transgenic (glyphosate and glufosinate) and non-transgenic (sethoxydim and imidazolinone) hybrids. Imidazolinone-resistant hybrids were introduced in 1993, sethoxydim- resistant hybrids in 1996, transgenic glyphosate-resistant hybrids in 1997 and transgenic glufosinate-resistant hybrids in 1998 (Dill 2005). Genetically modified glyphosate resistance in maize is the result of either the cp4 transgene that codes for an altered EPSPS that does not allow binding of glyphosate or N-acetylation of 164 M.D.K. Owen glyphosate resulting in the non-herbicidal metabolite N-acetyl glyphosate (Padgette et al. 1995; Castle et al. 2004). Recently, maize cultivars with an hra transgene that confers 1000-fold cross-resistance to ALS-inhibiting herbicides were introduced (Green et al. 2009). The adoption of transgenic herbicide-resistant corn hybrids appears to be ever-increasing (Owen and Zelaya 2005; Dill et al. 2008).


Genetically modified herbicide-resistant soybean became commercially available in the US in 1996. The cultivars utilize the cp4 transgene from Agrobacterium sp. that codes for a glyphosate-resistant form of EPSPS. Soybean cultivars with glyphosate resistance represent more than 90% of soybean planted in the US (Duke and Powles 2008). Soybean cultivars possessing the bar transgene from Streptomyces hygroscopicus thus conferring resistance to glufosinate have been developed and are now commercially available (Green 2009). A newly-reported mechanism, N-acetylation of glyphosate, provides considerable resistance to glyphosate and is currently under development in soybean (Siehl et al. 2005).


Cotton resistance to glyphosate was originally due to the cp4 epsps transgene and grower adoption of the genetically modified glyphosate-resistant cultivars has been rapid since their introduction in 1997 (Cerdeira and Duke 2006). However, there were problems with the transgene expression in reproductive structures which resulted in the development of cultivars with two cp4 epsps transgenes and various promoters to provide a better expression of resistance later in the development of the lants (CaJacob et al. 2007; Dill et al. 2008). Cotton with transgenic resistance to bromoxynil was introduced in the US in 1994, and glufosinate-resistant cultivars were introduced in 2003 (Duke 2005).

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