The IR Allele, Tissue Culture Selection for Mutations

ALS alleles that confer resistance to immidazilanone herbicides have also been selected from mutations that have occurred in tissue culture cells. DNA replication does not occur 100 percent without errors so it is not surprising that natural mutations occur in cells. A mutation, though, that gives a single cell in a corn plant resistance to ALS herbicides will not render the whole plant resistant. Geneticists have used the power of tissue culture to select for these mutations at the single cell level. Hundreds of thousands of corn cells can be grown in tissue culture plates that contain the herbicide. Most of the cells will quit growing and dividing when they take up the herbicide but some cells continue to grow and form clumps of undifferentiated cells called callus. Plants regenerated from these callus cells will often express the herbicide resistance as well. The IR allele of the ALS gene was selected in this way. 

This allele also has an altered coding region sequence that blocks the ALS herbicides from binding. The ALS enzyme encoded by the IR allele, however, does not provide desired levels of herbicide resistance in a heterozygous plant. The molecular mechanisms causing this difference in the IR compared to IT allele are not clear. The implication of this difference in plant breeding are significant. The IT allele can be inherited from just one parent to give the 'Clearfield' or “Immi” hybrid herbicide resistance. Hybrids must be homozygous for the IR allele requiring both parents be homozygous IR inbreds (Fig. 3).  

Fig. 3: Corn plants sprayed with an ALS inhibitor herbicide vary in their growth. The plant can have the IR, IT or IN (normal) alleles and the genotype of the plant for the ALS genes determines it’s phenotype in response to these herbicides. (Image by D. Lee)