ALS and Weed Control

The function of the ALS enzyme in plants is also an interesting issue in the area of weed control. Herbicides have been discovered with sulfonyl urea or imidazilinone chemistries that will bind to ALS enzymes in plants and disrupt their catalytic functions (Fig. 2).

Because the herbicides bind tightly to ALS and ALS is not found in high copies in the cell, the herbicides will effectively halt the synthesis of the Leucine, Isoleucine, and Valine amino acids (Leu, Ile, Val). How will plants respond to the shortage of these three amino acids? They quit growing because they cannot make three of the twenty amino acids.

Fig. 2: The ALS enzyme functions as a part of a metabolic pathway in the chloroplast that takes simple molecules (parts of the star) and synthesizes amino acids (stars). Herbicides can bind to the enzyme and block the amino acid synthesis. (Image by D. Lee)

The proteins that need these amino acids will be only partially synthesized. The partial proteins will not work because they lack the structure needed for their function. The plant cells cannot grow and divide and the plant’s growth and development is halted. They will not necessarily die but they will no longer be able to compete for light and other resources if nearby plants are not affected by the ALS inhibitors. If the nearby plant that is not effected by the ALS inhibitors is a crop species such as corn or soybean, a valuable post emergence weed control strategy can be implemented.