Herbicide-resistant acetolactate synthase

Acetolactate synthase inhibition by imidazolinones and triazolopyrimidines, 460 sensitivity to sulfonylurea herbicides, 460 Acetolactate synthase gene activity and inheritance of resistance in tobacco, 461... 

Haughn, G., Smith, J., Mazur, B. Sommerville, C. (1988). Transformation with a mutant Arabidopsis acetolactate synthase gene renders tobacco resistant to sulfonylurea herbicides. Molecular and General Genetics 211, 266-71. 

Maertens, K.D., C.L. Sprague, P.J. Tranel, and R.A. Hines (2004). Amaranthus hyridus populations resistant to triazine and acetolactate synthase-inhibiting herbicides. Weed Res., 44 21-26. 

Christopher, J.T., S.B. Powles, and J.A.M. Holtum (1992). Resistance to acetolactate synthase-inhibiting herbicides in annual ryegrass (Lolium rigidum) involves at least two mechanisms. Plant Physiol., 100 1909-1913. 

Saari, L.L., J.C. Cotterman, and D.C. Thill (1994). Resistance to acetolactate synthase-inhibitor herbicides. In Powles, S.B. and Holtum, J.A.M., eds., Herbicide Resistance in Plants Biology and Biochemistry. Boca Raton, FL Lewis Publishers, pp. 83-139. 

Sibony, M., A. Michel, H.U. Haas, B. Rubin, and K. Hurle (2001). Sulfometuron-resistant Amaranthus retroflexus Cross-resistance and molecular basis for resistance to acetolactate synthase-inhibiting herbicides. Weed Res., 41 509-522. 

Sprague, C.L., E.W. Stoller, and L.M. Wax (1997c). Response of an acetolactate synthase (ALS)-resistant biotype of Amaranthus rudis to selected ALS-inhibiting and alternative herbicides. Weed Res., 37 93-101. 

White, A.D., M.D.K. Owen, R.G. Hartzler, and J. Cardina (2001). Common sunflower (Helianthus annuus) resistance to acetolactate synthase inhibiting herbicides. Resistant Pest Management. East Lansing, MI Michigan State University, 11 3-5. 

Zelaya, I.A. and M.D.A. Owen (2004). Evolved resistance to acetolactate synthase-inhibiting herbicides in common sunflower (Helianthus annuus), giant ragweed (Ambrosia trifida), and shattercane (Sorghum bicolor) in Iowa. Weed Sci., 52 538-548. 

The sulfonylureas, an extremely potent class of herbicides, act by inhibiting acetolactate synthase (ALS), which is the first common enzyme in the biosynthetic pathways leading to the branched chain amino acids. Two other unrelated classes of herbicides also act by interfering with this enzyme. We have cloned and characterized the genes encoding ALS from several higher plants. The ALS genes isolated from herbicide sensitive and herbicide resistant plants have been compared, and several mutations which confer the herbicide resistant phenotype have been identified. 

This has changed dramatically in recent years, as is apparent throughout the herbicide chapters of this book. Of particular note are the surprising speed of resistance development to acetolactate synthase inhibitors (Mazur al., Gressel, this volume), and the emergence of multiply resistant ryegrass and blackgrass biotypes noted earlier. 

Lolium biotypes exist which have resistance to the sulfonylurea herbicides chlorsulfuron and metsulfuron methyl (4). The biotype used in the studies presented here is resistant to both these sulfonylurea herbicides. Sulfonylurea herbicides inhibit the chloroplastic enzyme acetolactate synthase (ALS), also known as acetohydroxyacid synthase (AHAS) (16). Inhibition of this enzyme results in disruption of the synthesis of the branched-chain amino acids valine and isoleucine (161. The imidazolinone herbicides also inhibit ALS Q2). In some species auxins can protect against chlorsulfuron inhibition (S. Frear, USDA North Dakota, personal communication) the mechanistic basis for this protection is not known. We have measured the ALS activity in the resistant and susceptible Lolium and have also checked for any induction of ALS activity following treatment with the sulfonylurea herbicide chlorsulfuron. 

It is not yet clear whether such factors will delay the rate of appearance of other types of resistance e.g. the multiple resistances to grass-killing herbicides in wheat, or to inhibitors of acetolactate synthase, where the fitness of resistant biotypes may be higher. 

Herbicide-Resistant Plants Carrying Mutated Acetolactate Synthase Genes... 

Acetolactate synthase (ALS) is the target enzyme for three unrelated classes of herbicides, the sulfonylureas, the imidazolinones, and the triazolopyrimidines. We have cloned the genes which specify acetolactate synthase from a variety of wild type plants, as well as from plants which are resistant to these herbicides. The molecular basis of herbicide resistance in these plants has been deduced by comparing the nucleotide sequences of the cloned sensitive and resistant ALS genes. By further comparing these sequences to ALS sequences obtained from herbicide-resistant yeast mutants, two patterns have become clear. First, the ALS sequences that can be mutated to cause resistance are in domains that are conserved between plants, yeast and bacteria. Second, identical molecular substitutions in ALS can confer herbicide resistance in both yeast and plants. 

D. C. Resistance to Acetolactate Synthase Inhibiting Herbicides, in Herbicide Resistance in Plants, Powles, S. B., Holtum, J. A. M. (Eds.), CRC Press, Boca Raton, FL, 1994, 83-139. 

Both compounds are inhibitors of the acetolactate synthase enzyme, also known as aceto hydroxy acid synthase (AHAS) and are classified in group B by the Herbicide Resistance Action Committee HRAC. Table 2.6.1 gives the physicochemical properties of 1 and 2. 

This strategy was used to engineer resistance against glyphosate, and imidazoli-nones and sulfonylureas that inhibit acetolactate synthase (ALS), a key enzyme in the biosynthesis of branched chain amino acids. ALS resistant crops have primarily been generated through selection for an herbicide insensitive ALS allele from natural or mutagenized cell or plant populations [3]. 

Compounds 296-299 inhibit acetohydroxy acid synthase (AHAS), formerly known as acetolactate synthase. Its activity is not present in animals, but it has been found in all plants where measurements have been attempted. Acetohydroxy acid synthase catalyses the first step in production of branched amino acids (leucine, valine and isoleucine) (Scheme 73), which are obviously needed for the protein synthesis and cell growth. The compounds 296-299 seem to bind within the substrate-access channel of the enzyme, thus blocking a-ketocarboxylate access to the active site. While these herbicides are undoubtedly highly successful, resistance developed due to mutations within AHAS is becoming a serious problem [274, 275]. 

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