Corn (Zea mays) acetolactate synthase sensitivity to four classes of ALS-inhibiting herbicides

In vitro acetolactate synthase (ALS) activity from three commercial imidazolinone-resistant corn hybrids (ICI 8692 IT, Pioneer 3751 IR, and Ciba 4393 IMR) was compared to imidazolinone-sensitive isogenic hybrid controls for sensitivity to 11 herbicides representing four classes of ALS-inhibiting herbicide chemistry. Acetolactate synthase activity from Pioneer IR and Ciba IMR was cross-resistant to all four classes of ALS inhibitors, ranging from 48- to 5,000-fold. The ICI IT hybrid displayed only four- to eightfold resistance to the six imidazolinone herbicides and the pyrimidinylthiobenzoate herbicide, pyrithiobac, but no cross-resistance to the sulfonylurea and triazolopyrimidine sulfonanilide herbicides. The four- to eightfold enzyme resistance to imidazolinone herbicides provides whole-plant resistance; however, the sevenfold enzyme resistance to pyrithiobac was insufficient to afford whole-plant protection to a field application rate of the herbicide. A second imidazolinone-specific resistance allele,XI-12, currently under commercial development, was examined for the level of dominance at the enzyme level. In the heterozygous state, imazethapyr resistance was fivefold, compared to 250-fold in the homozygous condition, indicatingXI-12is a semidominant trait. No cross-resistance to nicosulfuron or primisulfuron was observed in the heterozygousXI-12hybrid extracts nor to nicosulfuron in theXI-12homozygote; however, a fivefold resistance to primisulfuron was detected in theXI-12homozygote.