Herbicide Resistant Weeds

An in vivo acetolactate synthase assay.

Simpson, D. M., E. W. Stoller, and L. M. Wax. 1995. An in vivo acetolactate synthase assay. Weed Technology. 9: 17-22.

A method was developed and tested for the in vivo assay of acetolactate synthase (ALS). The method used foliar application of 1,1-cyclopropanedicarboxylic acid (CPCA) to inhibit ketol-acid reductoisomerase, the enzyme immediately following ALS in biosynthesis of branched-chain amino acids, thereby causing accumulation of acetolactate. Since the amount of acetolactate accumulation is a function of carbon flux through ALS, quantification of acetolactate accumulation determined ALS activity. Accumulation of acetolactate in soyabean leaves resulted from CPCA rates as low as 15 g/ha and occurred within 1.5 h. Accumulation rates in soyabean leaflets declined with leaf age from 84 æg h-1 g-1 tissue at 3 d to 17 æg h-1 g-1 tissue at 7 d. Foliar application of CPCA also caused acetolactate accumulation in corn [maize], grain sorghum, velvetleaf (Abutilon theophrasti), common cocklebur (Xanthium strumarium), and smooth pigweed (Amaranthus hybridus). The ability of the in vivo assay to quantify the reduction in ALS activity following applications of ALS-inhibiting herbicides was validated by comparing ALS activity following thifensulfuron application to soyabeans cv. Williams 82, which has a sulfonylurea-sensitive ALS, and soyabeans cv. Asgrow 3200, which has a sulfonylurea-insensitive ALS. Thifensulfuron reduced ALS activity in soyabeans cv. Williams 82 to 0, 0.8, 3.3 and 15.6% of the CPCA control at 6, 12, 24 and 48 h after treatment, but ALS activity in Asgrow 3200 soyabeans was reduced only to 34, 40, 57 and 88% of the CPCA control.