OVER-EXPRESSION OF ACETOLACTATE SYNTHASE CONFERS RESISTANCE TO VALINE IN TRANSGENIC TOBACCO

Previous reports described a high level of resistance to chlorsulfuron resulting from the transfer into tobacco of the mutated acetolactate synthase (ALS) gene from the csr-1 mutant of Arahidopsis. Here we report a comparison of metabolic changes resulting from the expression of the csr-1 gene in transgenic tobacco, either under the control of its own promoter or under the control of the 35S promoter with a duplicated enhancer (p70). The Arabidopsis csr-1 gene with the native promoter confers an average 300-fold increase in resistance to chlorsulfuron in tobacco without significant modification of the amount of ALS activity in the absence of the herbicide and without cross-resistance to valine, a feed-back inhibitor of the enzyme. The expression in tobacco of the csr-1 coding sequence under the control of the p70 promoter confers an average 1500-fold increase in resistance to chlorsulfuron, with an increase by up to 12-fold in the amount of ALS activity in the absence of the herbicide and resistance to exogenous supply of valine, with an increase by up to 10-fold in the amount of valine required to inhibit bud regeneration from leaf discs. However, no correlated increase in the content of free branched chain amino acids in leaves was observed, suggesting that subsequent steps of the metabolic pathway might limit the synthesis of valine, leucine and isoleucine.