Mutations on the A and C genomes of Brassica napus confer herbicide resistance to acetohydroxyacid synthase inhibitors

The point mutations conferring resistance to the sulfonylurea herbicide, chlorsulfuron, are reported for two <i>Brassica napus</i> lines (19c and 30a) developed by seed mutagenesis using ethyl methanesulfonate. Crosses with the herbicide-susceptible wild-type line generated F2 and backcrossed progeny and confirmed that chlorsulfuron resistance was inherited as a dominant mutation at a single locus in both lines. F2 progeny from crossing 19c and 30a segregated in a 15 resistant: 1 susceptible ratio, establishing that the two herbicide-resistant loci are independently inherited and that the mutations for resistance occur at different loci. PCR amplification and DNA sequencing of regions from the acetohydroxyacid synthase gene revealed two separate point mutations at different nucleotides in the same codon. This results in the substitution of proline-197 for leucine in 19c and proline-197 for serine in 30a. The different amino acid substitutions in the acetohydroxyacid synthase enzyme explain the differing responses of 19c and 30a to sulfonylurea and imidazolinone herbicides. Based on attempts at introgression of chlorsulfuron resistance to a wide range of forage brassicas, we conclude that the 19c mutation resides in the <i>Brassica</i> C genome, whereas the 30a mutation is in the <i>Brassica</i> A genome.