Review of Glyphosate and Als-inhibiting Herbicide Crop Resistance and Resistant Weed Management

Abstract

Weed management is a perennial challenge for growers, and continual innovation is essential to maintain the effectiveness of management technologies. The first generation of herbicide-resistant crops revolutionized weed control. However, weeds are adapting to crop systems that rely on a single mode of herbicide action. Crops with resistance to multiple modes of herbicide action could help maintain weed management. GAT/HRA is a new multiple herbicide–resistance technology for corn, soybean, and other crops. GAT/HRA combines metabolic glyphosate inactivation with an acetolactate synthase (ALS) enzyme that is insensitive to ALS-inhibiting herbicides. The mechanism to inactivate glyphosate is the glyphosate Nacetyltransferase enzyme, which transforms glyphosate into a nonphytotoxic metabolite. The gat gene is derived from a naturally occurring soil bacterium and optimized by repetitive gene shuffling and screening. The resistance mechanism to ALS-inhibiting herbicides is a double-mutant, highly resistant ALS (HRA) that is insensitive to all five classes of ALS herbicides. GAT/HRA crops will maintain natural tolerance to selective herbicides and thus provide more weed management options for growers to help deter weed spectrum shifts and delay the evolution of herbicide-resistant weeds. Nomenclature: Glyphosate; corn, Zea mays L.; soybean, Glycine max (L.) Merr. Traditionally, growers have used selective herbicides to control weeds in crops. Scientists made these selective herbicides by systematically modifying chemicals to find ones that were safe to the crop while still controlling key weeds. Now, instead of modifying the chemistry, scientists can modify the biology to attain crop safety. These biological modifications can be made through gene transfer or by selection of genetic mutations using seed or tissue-culture screens. Herbicide-resistant crops (HRCs) developed with these technologies quickly had an important impact on agriculture (Duke and Cerdeira 2005; Gutterson and Zhang 2004). However, like other technologies, the first generation of products raised many questions about putative risks, including whether the weed management systems associated with HRCs would increase the evolution of herbicide-resistant weeds (Madsen and Streibig 2004; Owen and Zelaya 2005). The increasing use of HRCs reduced the opportunity for new herbicide active ingredients and made the industry reluctant to invest capital because of shrinking market opportunities (Shaner 2000). As the use of glyphosate increased, the use of other herbicides decreased and pressured industry to reduce support of product registrations. Additionally, as herbicide markets declined, the number of chemicals that must be screened to discover a new herbicide increased from less than 1,000 in 1950 to more than 500,000 today. These trends make expanding the use of currently registered herbicide ingredients increasingly attractive to industry.