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HERBICIDES THAT INHIBIT ACETOLACTATE SYNTHASE

Abstract:

Herbicides are an essential pillar in modern agricultural practice. They are used to selectively reduce the growth of weeds in crops and natural environments. Early weed mitigating chemicals date back to Roman times and include substances as simple as table salt, sulfuric acid and carbon bisulfide. However, these were non-selective, required high application rates and could be toxic not only to the target weeds but to other plants and to animals. The first major advance in the development of tailored herbicides began in the mid 1940s with the development of the first organic growth regulators such as 2,4 dichlorophenoxyacetic acid (2,4-D)[1]. The next major advance came in the 1970s with the discovery of glyphosate[2], marketed as Roundup, a herbicide that is routinely used to eradicate broad leaf weeds and grasses. Due to the low cost to produce, the small quantities that need to be applied to the field, and the development of glyphosateresistant crops, it has been the dominant herbicide for the past 30 years. As a result of its success, recent efforts in herbicide discovery have been subdued. Glyphosate functions by acting as a transition state inhibitor of 5-enolpyruvylshikimate-3-phosphate (EPSP) synthase, an enzyme in the aromatic amino acid biosynthesis pathway. Even though glyphosate remains as a world leading commercial herbicide, an increase in the number of resistant weeds is a concern[3]. In the 1980s, another revolutionary class of herbicide, the sulfonylureas (SUs), was discovered by Dr. George Levitt of DuPont. Several years later their mode of action was shown to be inhibition of acetolactate synthase (ALS), also known as acetohydroxyacid synthase (AHAS; EC 2.2.1.6)[4]. ALS is the first enzyme in the biosynthetic pathway of the branched chain amino acids (BCAAs) (i.e., leucine, isoleucine and valine), fundamental components of most proteins. Thus, there are striking similarities in the glyphosate and SU classes of herbicide (i.e., both discovered in the 1970s−1980s, both target amino acid biosynthesis pathways, though aromatic versus branched chain, both target pathways that are found in plants and bacteria but not in animals, and in both cases weed resistance is increasing in frequency and distribution). Nonetheless, these two herbicide classes continue to be at the forefront of crop and land protection strategies employed by farmers, graziers and government agencies across the world. Since there are many reviews that track the success and history of glyphosate, it will not be discussed further here. However, recent advances to our knowledge of ALS and its herbicidal inhibitors and updates to the current status of weed resistance due to application of ALS inhibiting herbicides warrants review at this time.