Post emergence weed control by ALS inhibitors works in corn and soybean genotypes with altered ALS genes. The altered genes code for a slightly modified ALS enzyme which is resistant to ALS inhibitors. The molecular basis of this alteration can be determined and help us understand what alleles of a gene are. Even without molecular analysis, geneticists discovered alleles of the ALS gene that would make plants resistant to the ALS herbicides. How were these alleles discovered?
Corn breeders knew that ALS resistance did not naturally occur in their breeding lines. This discovery was easy to make. They planted thousands of different lines in short rows in the field, sprayed the field with the herbicide and observed the reaction of the lines. All of the lines tested had significant stunting. Variation did not exist in their lines so they decided to try to induce variation by mutagenesis. Pollen was collected from plants that had been exposed to a chemical mutagen. The chemical induced mistakes in DNA replication during pollen formation. The pollen was viable but carried mutant alleles. This pollen was applied to the silks of other corn plants to produce thousands of seeds. This seed was planted and the field test for ALS resistance was repeated. This time some plants resistant to stunting from ALS were observed, tagged and self pollinated. From this experiment, the IT (immi tolerant) allele was discovered. The IT allele appeared to be dominant over the normal ALS allele because plants with one copy of the IT allele per cell (heterozygotes) had the same level of resistance as plants that had two copies of the IT allele (homozygotes). Molecular analysis of these plants was done to reveal how the IT allele was different for the original version.