Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Plant Pathology and Crop Physiology

First Advisor

Norimoto Murai


Acetolactate synthase (ALS) is a key regulatory enzyme in the biosynthetic pathway of leucine, isoleucine, and valine. ALS is inhibited by four classes of structurally unrelated herbicides viz. sulfonylureas, imidazolinones, triazolopyrimidines, and pyrimidyl-oxy-benzoate. To understand the molecular basis of imidazolinone resistance, the ALS gene was isolated from an imazapyr-resistant mutant GH90 of Arabidopsis thaliana (Haughn and Somerville, 1990. Plant Physiol. 92:1081-1085). DNA sequence analysis of the mutant gene demonstrated a single point mutation from G to A at the 1958th nucleotide of the ALS coding sequence (Sathasivan et al., 1990. Nucl. Acids Res. 18:2188). This would result in serine to asparagine substitution at the 653rd amino acid, near the carboxyl terminal of matured ALS. The mutant ALS gene was introduced into tobacco using Agrobacterium-mediated transformation. The presence of the introduced ALS gene was confirmed by a southern hybridization analysis of transgenic plant DNA. Imidazolinone resistance of transformed calli and leaves of transgenic plants was 100 fold greater than that of wild type. The level of resistance of the ALS activity in vitro correlated with the amount of resistance in the leaves of transgenic plants. These results demonstrated that the single amino acid substitution from serine to asparagine at the 653rd amino acid near the carboxyl terminus of ALS is the molecular basis of imidazolinone herbicide resistance.