ALTERED ACETOLACTATE SYNTHASE ACTIVITY IN ALS-INHIBITOR RESISTANT PRICKLY LETTUCE (LACTUCA-SERRIOLA)

The effect of target site mutation for acetolactate synthase (ALS)-inh ibitor resistance on ALS activity was evaluated in a sulfonylurea-resi stant (R) biotype of prickly lettuce with a proline(173) to histidine substitution in Domain A of the ALS enzyme. I-50 values for ALS inhibi tion by several ALS-inhibitor herbicides were determined for R and sus ceptible (S) biotypes. Results from both a standard ALS assay and a ch loroplast assay for ALS activity showed that the R biotype also was cr oss-resistant to representatives of the imidazolinone (imazethapyr) an d triazolopyrimidine (flumetsulam) families, but was not cross-resista nt to the pyrimidinyl oxybenzoate (4,6-dimethoxypyrimidin-2-yl-oxy-2-b enzoic acid) tested. The K-m (pyruvate) was similar for ALS extracted from the R and S biotypes, suggesting that mutation for resistance did not alter pyruvate binding on the enzyme. However, specific activity of ALS from the R biotype was 57% less than specific activity of ALS f rom the S biotype, suggesting that the resistance mutation may affect enzyme function, expression, or stability. ALS from the R biotype was less sensitive to inhibition by the branched chain amino acids, valine , leucine, and isoleucine, than ALS from the S biotype. Reduced sensit ivity to feedback inhibition was correlated with 70, 40, and 9% higher concentrations of valine, leucine, and isoleucine, respectively, on a per seed basis in Rvs. S seed.