PRICKLY LETTUCE (Lactuca serriola) with GROUP B/2 resistance: (INHIBITION OF ACETOLACTATE SYNTHASE ) Inhibition of Acetolactate Synthase
MUTATION: PROLINE 197 to HISTIDINE
Prickly Lettuce (Lactuca serriola) is a dicot plant in the asteraceae family. A single amino acid substitution from Proline 197 to Histidine has led to resistance to Inhibition of Acetolactate Synthase as indicated in the table below.		Prickly Lettuce

Chemical Family	Example Herbicide	Resistance Level
Imidazolinones	Imazethapyr	Intermediate Resistance < 10 fold
Pyrimidinyl benzoates	Bispyribac-Na	Susceptible
Sulfonylureas	Chlorsulfuron	Resistant > 10 fold
Triazolopyrimidine - Type 1	Chloransulam-methyl	Intermediate Resistance < 10 fold
Triazolinones	Flucarbazone-Na	Not Determined

REFERENCES

Eberlein, C. V. ; Guttieri, M. J. ; Mallory-Smith, C. A. ; Thill, D. C. ; Baerg, R. J.. 1997. Altered acetolactate synthase activity in ALS-inhibitor resistant prickly lettuce (Lactuca serriola). Weed Science 45 : 212 - 217.

The effect of target site mutation for acetolactate synthase (ALS)-inhibitor resistance on ALS activity was evaluated in a sulfonylurea-resistant (R) biotype of prickly lettuce with a proline₁₇₃ to histidine substitution in domain A of the ALS enzyme. I₅₀ values for ALS inhibition by several ALS-inhibitor herbicides were determined for R and susceptible (S) biotypes. Results from both a standard ALS assay and a chloroplast assay for ALS activity showed that the R biotype was also cross-resistant to representatives of the imidazolinone (imazethapyr) and triazolopyrimidine (flumetsulam) families, but was not cross-resistant to the pyrimidinyl oxybenzoate (4,6-dimethoxypyrimidin-2-yl-oxy-2-benzoic 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 from 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 sensitivity to feedback inhibition was correlated with 70, 40 and 9% higher concentrations of valine, leucine and isoleucine, respectively, on a per seed basis in R vs. S seed..

Guttieri, M. J. ; Eberlein, C. V. ; Mallory-Smith, C. A. ; Thill, D. C. ; Hoffman, D. L.. 1992. DNA sequence variation in domain A of the acetolactate synthase genes of herbicide-resistant and -susceptible weed biotypes. Weed Science 40 : 670 - 676.

The DNA sequence of a 196-base pair (bp) region of the acetolactate synthase (ALS) genes of 3 weed species, kochia (Kochia scoparia), prickly lettuce (Lactuca serriola) and Russian thistle (Salsola iberica), was determined. This region encompasses the coding sequence for domain A, a region of the amino acid sequence previously demonstrated to play a pivotal role in conferring resistance to herbicides that inhibit ALS. The domain A DNA sequence from a chlorsulfuron-resistant (R) prickly lettuce biotype from Idaho differed from that of a chlorsulfuron-susceptible (S) biotype by a single point mutation, which substituted a histidine for a proline. The domain A DNA sequence from an R kochia biotype from Kansas also differed from that of an S biotype by a single point mutation in the same proline codon. This point mutation, however, conferred substitution of threonine for proline. Two different ALS-homologous sequences were isolated from an R biotype of Russian thistle. Neither sequence encoded amino acid substitutions in Domain A that differed from the consensus S sequence. The DNA sequence variation among the R and S kochia biotypes was used to characterize 6 Ada County (Idaho) kochia collections for correlation between phenotypic chlorsulfuron susceptibility and restriction digest patterns (RFLPs) of polymerase chain reaction amplification products. Most collections showed excellent correspondence between the RFLP patterns and the phenotypic response to chlorsulfuron application. However, one entirely R collection had the RFLP pattern of the S biotype, suggesting that resistance was not due to mutation in the proline codon..

This case was entered by Patrick Tranel Email: tranel@illinois.edu

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