Action mechanisms of acetolactate synthase-inhibiting herbicides

Herbicides that target the acetolactate synthase (ALS) are among the most widely used weed control chemicals since their introduction into the marketplace in the early 1980s, including five classes (sulfonylureas, imidazolinones, triazolopyrimidines, pyrimidinylthio (or oxy)-benzoates and sulfonylamino-carbonyltriazolinones). The mechanism researches have progressed unprecedentedly in the last two decades. Primary mode of action of the ALS-inhibiting herbicides that interfere with the activity of ALS enzyme seems no longer in doubt. Three lines of investigation from physiology, genetics, molecular and chemical structure aspects came together to prove that ALS is the site of action. Researches on the effects of branched chain amino acids (BCAAs) synthesis or protein metabolism caused by ALS-inhibiting herbicide elicit lots of disputations. Besides these two main works, other secondary effects of ALS inhibition, such as buildup of 2-ketobutyrate (α-ketobutyrate or 2-KB) or 2-aminobutyrate (2-AB, the transamination product of 2-KB), depletion of intermediates of the pathway for some critical processes, disruption of photosynthesis transport and respiration system etc., have also been implicated in the mechanism of plant death. However, there are still some disputations and doubts on the precise mechanisms that need further probing into. Further more, as many ALS-inhibiting herbicides and their derivatives are chiral with one or even more enantiomers, which may behave quite differently in biochemical processes, the effects and the environmental fate of chiral herbicides need to be investigated stereospecifically. By this, we can have a better understanding about the herbicides and avoid unnecessary pollution load.