Patterns of Cross-Tolerance to Herbicides Inhibiting Acetohydroxyacid Synthase in Commercial Corn Hybrids Designed for Tolerance to Imidazolinones

Two commercial corn (Zea mays L.) hybrids, ICI 8532 IT and Pioneer 3180 IR, designed for tolerance to imidazolinones, were evaluated for tolerance to various acetohydroxyacid synthase-inhibiting herbicides at the whole plant and enzyme levels. The purpose was to establish and compare the underlying enzymatic basis for tolerance in these plants, which were produced by contrasting methods and contain different genetic complements of the altered target enzyme. ICI IT plants exhibited significant tolerance (40-fold higher rates for 50% inhibition of growth than the control hybrid) to imazethapyr, and somewhat less to imazaquin and pyrimidyloxybenzoate. ICI IT plants were no more tolerant to chlorsulfuron or flumetsulam than the control hybrid. Pioneer IR plants were highly tolerant to all of the AHAS inhibitors tested, requiring rates 200- to 2000-fold higher than those of the control hybrid for 50% inhibition of growth. The basis for tolerance was evaluated on acetohydroxyacid synthase (AHAS, EC 4.1.3.18) in vitro. Relative to unmodified AHAS isolated from the control hybrid, AHAS isolated from ICI IT corn was seven-fold less sensitive to imazethapyr, five-fold less sensitive to pyrimidyloxybenzoate and two-fold less sensitive to imazaquin, but was as sensitive to chlorsulfuron and flumetsulam. AHAS from Pioneer IR corn was highly insensitive to all of the AHAS inhibitors tested. Sensitivity to the feedback inhibitor leucine was not detectably altered in the modified enzymes. Substrate saturation kinetics of AHAS isolated from ICI IT corn were identical with those of unmodified AHAS, while AHAS from Pioneer IR required a two-fold higher concentration of pyruvate for half-maximal saturation. The results support the theory that because the catalytic and herbicide binding sites of AHAS are distinct from each other, crops can be designed for tolerance to AHAS-inhibiting herbicides with little effect on performance.