ALKYLATION: A POSSIBLE MODE OF ACTION OF THE CHLOROACETANILIDE HERBICIDES (PLANT PROTEINS, GROWTH INHIBITORS, METOLACHLOR, ALACHLOR)

JANIS EILEEN MCFARLAND, Purdue University

Abstract

Chloroacetanilide herbicides are used extensively in corn and soybeans for preemergence control of annual grasses and certain broadleaves. These herbicides are classified as general growth inhibitors, but their mechanism of action is unknown. I investigated the ability of three chloroacetanilide herbicides (alachlor, propachlor, and metolachlor) to alkylate (covalently bind to) plant proteins in vitro and in vivo. In bioassays of oat root growth, the herbicides and iodoacetamide, a known alkylating agent, inhibited growth at 10 uM. Non-halogentated analogs of the herbicides, including the fungicide metalaxyl, were between 1 and 2 orders of magnitude less active in the bioassay of root growth than their chlorinated derivatives. An analog of metolachlor in which bromine is substituted for the chlorine also inhibited at root growth at 10 (mu)M, while an analog with a fluorine substituted was not inhibitory at 100 uM. Alkylation reactivities of the herbicides with crysteine in vitro were propachlor > alachlor > metolachlor. Halogenated analogs of metolachlor also had different alkylation reactivities, with the bromine analog > metolachlor > the fluorine analog. Neither non-halogentated nor fluorinated analogs of metolachlor alkylated cysteine in vitro. The chloroacetanilides also alkylated oat root proteins in vitro and proteins in an in vitro system for protein synthesis. Metolachlor covalently bound to several proteins in oat seedlings grown in sand culture. Although some large (>20 kd) proteins were alkylated by metolachlor, the majority of the alkylation products of metolachlor were smaller than 6 kd. The amounts of alkylated proteins increased with herbicide concentration and treatment duration in sand culture. Herbicide analogs that did not alkylate cysteine or proteins in vitro were relatively inactive in the bioassay of oat root growth. All acetanilides that inhibited oat root growth also alkylated cysteine but their alkylation reactivities varied greatly even though growth inhibition occurred at similar concentrations and treatment durations. Therefore, general alkylation may not be important for the inhibition of oat root growth, but herbicide alkylation to specific nucleophiles might be involved in the mode of action of the chloroacetanilides. This is the first report that chloroacetanilide herbicides covalently bind to plant proteins in vitro and in vivo.

Degree

Ph.D.

Subject Area

Botany

COinS