Inhibition of Acetolactate Synthase in Plants by Imazosulfuron.

Abstract

The elongation of the third, fourth and fifth leaves of rice plants was inhibited when 30ppb or more of imazosulfuron was contained in nutrient solution when the plants were treated at the two-leaf stage, whereas root dry weight was reduced at concentrations of 1ppb and above. Addition of the branched-amino acids, valine, isoleucine and leucine at a concentration of 100ppm each alleviated the growth inhibition of the third leaf of rice plants induced by 30ppb of imazosulfuron to almost the same degree as the elongation of the control. Imazosulfuron inhibited acetolactate synthase (ALS) activity from pea seedlings (Pisum sativum) in the noncompetitive with respect to pyruvate and uncompetitive with respect to thiamine pyrophosphate (TPP) and showed the I50 at 24nM. Inhibition of ALS activity by the continuous presence of imazosulfuron was time dependent and biphasic. The constant of the initial inhibition by the herbicide was 20-fold larger than that of the final steady-state. The ALS activity from rice plants which were tolerant to imazosulfuron was sensitive to the herbicide irrespective of age or chloroplast development of the plants (I50=14 to 45nM). The results suggest that imazosulfuron acts on the plant by blocking the biosynthesis of valine, isoleucine and leucine, which is due to the direct inhibition of ALS. The herbicide is a slow-binding inhibitor of ALS activity. The binding site of imazosulfuron on the enzyme is judged not to overlap with that of pyruvate and TPP. Tolerance of rice plants to imazosulfuron does not depend on the sensitivity of ALS activity irrespective of the difference in plant age or growth condition.