Herbicide resistance has evolved in weeds in several general forms. The most common type of herbicide resistance in weeds is the modification of the herbicide target site (Zelaya and Owen 2004). Target site resistance can be either monogenic or polygenic; the latter is often referred to as "creeping resistance" and may result from recurrent applications of low herbicide rates (Gressel 1995). In the case of monogenic herbicide resistance, typically resistance is accrued when there is a single-nucleotide point mutation of one amino acid, representing a substitution in 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" an amino acid in the target protein (Patzoldt et al. 2006). 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 (Ryan 1970; Baerson et al. 2002; Ng et al. 2003; Zelaya and Owen 2004; Patzoldt et al. 2006). 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 (Yuan et al. 2006). 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. glypho-sate-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).
9.1.3 Modes of Herbicide Action in Herbicide-Resistant Crops
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 bromoxynil resistance and the bar gene for glufosinate resistance. Historically, there are non-transgenic herbicide resistance traits for cyclohexanedione herbicides, imidazoli-none 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.
9.1.4 Implications of Genetically Modified Herbicide-Resistant Crops
The wide-spread 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.
188.8.131.52 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 cumminus) 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).
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 resistance to ALS inhibitors, triazines, ACCase inhibitors, synthetic auxins, bypyridiliums, 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).
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).
The primary benefits of the genetically modified herbicide-resistant crops, as stated by growers, is the convenience and simplicity of weed control (Bonny 2007; Owen 2008a, b). 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 sus-tainability of cropping systems based on genetically modified herbicide-resistant crops (Owen and Boerboom 2004; Sammons et al. 2007; Owen 2008a, b).
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