A NATURALLY-OCCURRING POINT MUTATION CONFERS BROAD RANGE TOLERANCE TOHERBICIDES THAT TARGET ACETOLACTATE SYNTHASE

Acetolactate synthase (ALS) inhibitors are among the most commonly use d herbicides. They fall into four distinct families of compounds: sulf onylureas, imidazolinones, triazolopyrimidine sulfonanilides, and pyri midinyl oxybenzoates. We have investigated the molecular basis of imid azolinone tolerance of two field isolates of cocklebur (Xanthium sp.) from Mississippi and Missouri. In both cases, tolerance was conferred by a form of ALS that was less sensitive to inhibitors than the wild t ype. The insensitivity pattern of the Mississippi isolate was similar to that of a commercial mutant of corn generated in the laboratory: IC I 8532 IT. Sequencing revealed that the same residue (Ala(57) --> Thr) was mutated in both Mississippi cocklebur and ICI 8532 IT corn. ALS f rom the Missouri isolate was highly insensitive to all the ALS herbici de families, similar in this respect to another commercial corn mutant : Pioneer 3180 IR corn, Sequencing of ALS from both plants revealed a common mutation that changed Trp(552) to Leu. The sensitive cocklebur ALS cDNA, fused with a glutathione S-transferase, was functionally exp ressed in Escherichia coli. The recombinant protein had enzymatic prop erties similar to those of the plant enzyme. All the possible point mu tations affecting Trp(552) were investigated by site-directed mutagene sis. Only the Trp --> Leu mutation yielded an active enzyme. This muta tion conferred a dramatically reduced sensitivity toward representativ es of all four chemical families, demonstrating its role in herbicide tolerance. This study indicates that mutations conferring herbicide to lerance, obtained in an artificial environment, also occur in nature, where the selection pressure is much lower. Thus, this study validates the use of laboratory models to predict mutations that may develop in natural populations.