PATTERNS OF CROSS-TOLERANCE TO HERBICIDES INHIBITING ACETOHYDROXYACIDSYNTHASE IN COMMERCIAL CORN HYBRIDS DESIGNED FOR TOLERANCE TO IMIDAZOLINONES

Two commercial corn (Zea mays L.) hybrids, ICI 8532 IT and Pioneer 318 0 IR, designed for tolerance to imidazolinones, were evaluated for tol erance to various acetohydroxyacid synthase-inhibiting herbicides at t he whole plant and enzyme levels. The purpose was to establish and com pare the underlying enzymatic basis for tolerance in these plants, whi ch were produced by contrasting methods and contain different genetic complements of the altered target enzyme. ICI IT plants exhibited sign ificant tolerance (40-fold higher rates for 50% inhibition of growth t han the control hybrid) to imazethapyr, and somewhat less to imazaquin and pyrimidyloxybenzoate. ICI IT plants were no more tolerant to chlo rsulfuron or flumetsulam than the control hybrid. Pioneer IR plants we re highly tolerant to all of the AHAS inhibitors tested, requiring rat es 200- to 2000-fold higher than those of the control hybrid for 50% i nhibition of growth. The basis for tolerance was evaluated on acetohyd roxyacid 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 sensitiv e to pyrimidyloxybenzoate and two-fold less sensitive to imazaquin, bu t 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 alter ed in the modified enzymes. Substrate saturation kinetics of AHAS isol ated from ICI IT corn were identical with those of unmodified AHAS, wh ile AHAS from Pioneer IR required a two-fold higher concentration of p yruvate for half-maximal saturation. The results support the theory th at because the catalytic and herbicide binding sites of AHAS are disti nct from each other, crops can be designed for tolerance to AHAS-inhib iting herbicides with little effect on performance.