Physiological consequences of mutation for ALS-inhibitor resistance

Biochemical and physiological effects of target site resistance to herbicid es inhibiting acetolactate synthase (ALS) were evaluated using sulfonylurea -resistant (R) and -susceptible (S) near isonuclear Lactuca sativa 'Bibb' l ines derived by backcrossing the resistance allele from Lactuca serriola L. into L. sativa. Sequence data suggest that resistance in L. sativa is conf erred by a single-point mutation that encodes a pro-line(197) to histidine substitution in Domain A of the ALS protein; this is the same substitution observed in R L. serriola. K-mapp (pyruvate) values for ALS isolated from R and S L. sativa were 7.3 and 11.1 mM, respectively, suggesting that the re sistance allele did not alter the pyruvate binding domain on the ALS enzyme . Both R and S ALS had greater affinity for 2-oxobutyrate than for pyruvate at the second substrate site. Ratios of acetohydroxybutyrate : acetolactat e produced by R ALS across a range of 2-oxobutyrate concentrations were sim ilar to acetohydroxybutyrate : acetolactate ratios produced by S ALS. Speci fic activity of ALS from R L. sativa was 46% of the specific activity from S L. sativa, suggesting that the resistance allele has detrimental effects on enzyme function, expression, or stability. ALS activity from R plants wa s less sensitive to feedback inhibition by valine, leucine, and isoleucine than ALS from S plants. Valine, leucine, and isoleucine concentrations were about 1.5 times higher in R seed than in S seed on a per gram of seed basi s, and concentrations of valine and leucine were 1.3 and 1.6 times higher, respectively, in R leaves than in S leaves. Findings suggest that the mutat ion for resistance results in altered regulation of branched-chain amino ac id synthesis.