Inhibition of Plant Acetolactate Synthase by Herbicides, Pyrimidinylsalicylic Acids

Abstract

The growth inhibition of rice seedlings and chlorella by the herbicides, pyrimidinyl-salicylic acids (PS's) represented by pyrithiobac (2-chloro-6-(4, 6-dimethoxypyrimidine-2-ylthio)benzoic acid) was alleviated almost completely by simultaneous application of three branched-chain amino acids, namely leucine, valine and isoleucine. PS's inhibited potently acetolactate synthase (EC 4. 1.3. 18) in various species of plants including crops and weeds, which catalyzes the condensation step of two molecules of pyruvate to form acetolactate in the biosynthetic pathway of branched-chain amino acids. PS's, however, affected neither ketol-acid reductoisomerase which catalyzes the next reaction step from acetolactate synthase in the pathway nor direct acetoin forming enzyme deduced as pyruvate decarboxylase. PS's showed no inhibitory effect on photosynthetic electron transport system of pea, while they inhibited slightly chlorophyll biosynthesis of cotton cotyledons. In spite of high selectivity of pyrithiobac for cotton plants, there was no difference in sensitivity of ALS to pyrithiobac between cotton and other plants. © 1994, Pesticide Science Society of Japan. All rights reserved.