COMMON COCKLEBUR (Xanthium strumarium) with GROUP B/2 resistance: (INHIBITION OF ACETOLACTATE SYNTHASE ) Inhibition of Acetolactate Synthase
MUTATION: ALANINE 122 to THREONINE
Common cocklebur (Xanthium strumarium) is a dicot plant in the asteraceae family. A single amino acid substitution from Alanine 122 to Threonine has led to resistance to Inhibition of Acetolactate Synthase as indicated in the table below.		Common cocklebur

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

NOTE

REFERENCES

Bernasconi, P.; Woodworth, A. R.; Rosen, B. A.; Subramanian, M. V.; Siehl, D. L.. 1995. A naturally occurring point mutation confers broad range tolerance to herbicides that target acetolactate synthase. Journal of Biological Chemistry 270 : 17381 - 1738.

Acetolactate synthase (ALS) inhibitors are among the most commonly used herbicides. They fall into four distinct families of compounds: sulfonylureas, imidazolinones, triazolopyrimidine sulfonanilides, and pyrimidinyl oxybenzoates. The molecular basis of imidazolinone tolerance of two field isolates of cocklebur (Xanthium sp.) from Mississippi and Missouri was investigated. In both cases, tolerance was conferred by a form of ALS that was less sensitive to inhibitors than the wild type. The insensitivity pattern of the Mississippi isolate was similar to that of a commercial maize mutant, generated in the laboratory, ICI 8532 IT. Sequencing revealed that the same residue (Ala57 right arrow Thr) was mutated in both Mississippi cocklebur and ICI 8532 IT. ALS from the Missouri isolate was highly insensitive to all the ALS herbicide families, and was similar in this respect to another commercial maize mutant, Pioneer 3180 IR. Sequencing of ALS from both plants revealed a common mutation that changed Trp552 to Leu. The sensitive cocklebur ALS cDNA, fused with a glutathione S -transferase, was functionally expressed in Escherichia coli . The recombinant protein had enzymatic properties similar to those of the plant enzyme. All the possible point mutations affecting Trp552 were investigated by site-directed mutagenesis. Only the Trp right arrow Leu mutation yielded an active enzyme. This mutation conferred a dramatically reduced sensitivity toward representatives of all 4 chemical families, demonstrating its role in herbicide tolerance. This study indicates that mutations conferring herbicide tolerance, obtained in an artificial environment, also occur in nature, where the selection pressure is much lower. Thus, this study validates the use of laboratory models to predict mutations that may develop in natural populations.

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

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