Specific Binding of a Dichloroacetamide Herbicide Safener in Maize at a Site That Also Binds Thiocarbamate and Chloroacetanilide Herbicides

Dichloroacetamide safeners such as N,N-diallyl-2,2-dichloroacetamide and (R,S)-3-dichloroacetyl-2,2,5-trimethyl-1,3-oxazolidine protect maize (Zea mays) against injury from thiocarbamate and chloroacetanilide herbicides. Binding activity of tritium-labeled (R,S)-3-dichloroacetyl-2,2,5-trimethyl-1,3-oxazolidine (15 Ci/ mmol; referred to as [3H]Saf) was characterized in extracts of etiolated maize seedlings. The binding is saturable, involves a single class of binding sites (Kd 0.12 [mu]M; maximal binding in coleoptiles 0.53 nmol/g fresh weight, equivalent to 55 pmol/mg protein), and is sensitive to boiling and protease treatment. Binding in etiolated maize seedlings is highest in the coleoptile and lowest in the leaves. Binding of [3H]Saf also occurs in etiolated sorghum (Sorghum bi-color) shoots but not several other cereals. There is a good correlation between known safener effectiveness and the concentration that inhibits [3H]Saf binding half-maximally among 21 dichloroacetamides and related compounds. N,N-Diallyl-2,2-dichloroacetamide had the lowest inhibitor concentration that reduces specific binding by 50% (IC50), 0.01 [mu]M. [3H]Saf binding is inhibited by 4 chloroacetanilide herbicides with IC50 values of 0.07 to 0.48 [mu]M and by 12 thiocarbamate herbicides and analogs with IC50 values of 0.06 to 2.3 [mu]M. The inhibition of [3H]Saf binding by alachlor and S-ethyl dipropylthiocarbamate is competitive.