Effects of Mutation for ALS-Inhibitor Resistance on ALS Activity in Resistant and Susceptible Near-Isonuclear Lactuca Lines

Abstract

The biochemical and physiological effects of target site mutation for ALS-inhibitor resistance were evaluated using chlorsulfuron-resistant (R) and chlorsulfuronsusceptible (S) near isonuclear ‘Bibb’ lettuce (Lactuca sativa L.) lines. The near isonuclear lines (isolines) were developed by backcrossing the resistance allele from a chlorsulfuronresistant prickly lettuce (Lactuca serriola L.) biotype into Bibb lettuce. The specific activity of ALS from the R Bibb isoline was 46% of the specific activity of ALS isolated from the S Bibb isoline, suggesting that there are detrimental effects of the mutation for resistance on enzyme function, expression, or stability. The Km (pyruvate) values of ALS isolated from R Bibb and S Bibb were similar, 7.3 and 11.1 mM, respectively, suggesting that the mutation for resistance did not alter pyruvate binding on the ALS enzyme. ALS activity from both R and S Bibb isolines was inhibited by valine, leucine, and isoleucine. However, ALS from R plants was less sensitive to feedback inhibition by the individual amino acids than ALS from S plants. Concentrations of valine, leucine, and isoleucine were about 1.5 times higher in R seed than in S seed, and concentrations of valine and leucine in R Bibb lettuce leaves were 1.3 and 1.6 times higher, respectively, than in leaves of the S isoline. Therefore, mutation for resistance to ALS-inhibiting herbicides may alter regulation of branched chain amino acid synthesis.