A mutant acetolactate synthase (ALS) enzyme that confers cross-resistance to all sulfonylurea, imidazolinone, pyrimidinyloxybenzoate, triazolopyrimidine and sulfonylamino-carbonyl-triazolinone herbicides is provided. The mutant enzyme contains an aspartic acid to glutamic acid substitution mutation at a newly identified conserved region of the ALS enzyme. A gene encoding the enzyme is also provided, as are transgenic plants that have been genetically engineered to contain and express the gene. The transgenic plants are cross-resistant to sulfonylurea, imidazolinone, pyrimidinyloxybenzoate, triazolopyrimidine and sulfonylamino-carbonyl-triazolinone herbicides.
