A nucleic acid construct comprising an isolated nucleic acid fragment comprising a nucleotide sequence, wherein said nucleotide reference encodes a plant acetolactate synthase protein which is resistant to a compound selected from the group consisting of sulfonylurea, triazolopyrimidine sulfonamide and imidazolinone herbicides, said nucleotide sequence comprises at least one sub-sequence which encodes one of the substantially conserved amino acid sub-sequences designated A, B, C, D, E, F, and G in FIG. 6, the nucleic acid fragment is further characterized in that at least one of the following conditions is met,
- a) the nucleic acid fragment has a sequence which encodes an amino acid sub-sequence A wherein .epsilon..sub.1 is an amino acid other than alanine, or .epsilon..sub.2 is an amino acid other than glycine.
- b) the nucleic acid fragment has a sequence which encodes an amino acid sub-sequence B wherein .alpha..sub.1 is an amino acid other than proline,
- c) the nucleic acid fragment has a sequence which encodes an amino acid sub-sequence C wherein .delta..sub.2 is an amino acid other than alanine,
- d) the nucleic acid fragment has a sequence which encodes an amino acid sub-sequence D wherein .lambda..sub.1 is an amino acid other than lysine,
- e) the nucleic acid fragment has a sequence which encodes an amino acid sub-sequence E wherein .gamma..sub.1 is an amino acid other than aspartic acid,
- f) the nucleic acid fragment has a sequence which encodes an amino acid sub-sequence F wherein .beta..sub.3 is an amino acid other than tryptophan, or .beta..sub.8 is an amino acid other than valine, or .beta..sub.7 is an amino acid other than phenylalanine, and
- g) the nucleic acid fragment has a sequence which encodes an amino acid sub-sequence G wherein .sigma..sub.1 is an amino acid other than methionine.