SMOOTH PIGWEED (Amaranthus hybridus (syn: quitensis)) with GROUP B/2 resistance: (INHIBITION OF ACETOLACTATE SYNTHASE ) Inhibition of Acetolactate Synthase
MUTATION: ALANINE 122 to THREONINE
Smooth Pigweed (Amaranthus hybridus (syn: quitensis)) is a dicot plant in the amaranthaceae 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.		Smooth Pigweed

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

NOTE

REFERENCES

Whaley, C. M. ; Wilson, H. P. ; Westwood, J. H.. 2006. ALS resistance in several smooth pigweed (Amaranthus hybridus) biotypes. Weed Science 54 : 828 - 832.

Experiments were conducted to identify acetolactate synthase (ALS, EC 2.2.1.6 (formerly EC 4.1.3.18)) mutation sites in eight biotypes of smooth pigweed (A. hydridus) and correlate these mutations with patterns of herbicide cross-resistance. Four herbicide-resistant smooth pigweed biotypes (R5, R6, R7 and R8), collected from fields in Virginia, Delaware and Maryland (USA), showed a similar response to post-emergence applications of the ALS-inhibitors imazethapyr, pyrithiobac, chlorimuron, thifensulfuron and cloransulam. These R biotypes were 261- to 537-fold resistant to imazethapyr and 29- to 88-fold resistant to pyrithiobac. The biotypes also had reduced sensitivity to chlorimuron and thifensulfuron of 2- to 14-fold and 10- to 25-fold, respectively, relative to a susceptible smooth pigweed biotype (S). Biotypes R6, R7 and R8 had reduced sensitivity of 3- to 10-fold to cloransulam relative to the S biotype, whereas R5 had increased sensitivity. All of these biotypes had a serine to asparagine substitution at amino acid position 653, as numbered relative to the protein sequence of Arabidopsis thaliana. This stands were in contrast to four other imidazolinone (IMI)-resistant smooth pigweed biotypes (R1, R2, R3 and R4) that were collected from fields in Somerset County, Maryland. These biotypes had an alanine to threonine substitution at position 122 of the ALS enzyme and were previously characterized at the whole-plant level with high-level resistance to IMI herbicides, increased sensitivity to pyrimidinylthiobenzoate and triazolopyrimidine sulfonanilide herbicides, and low to no cross-resistance to sulfonylurea herbicides..

Trucco, F. ; Hager, A. G. ; Tranel, P. J.. 2006. Acetolactate synthase mutation conferring imidazolinone-specific herbicide resistance in Amaranthus hybridus. Journal of Plant Physiology 163 : 475 - 479.

Acetolactate synthase (ALS) catalyzes the first common step in the biosynthesis of branched-chain amino acids in plants and is the target of several herbicides. ALS inhibitors have enjoyed popularity as herbicides due to numerous attributes, although their current adequacy in weed control programs is hampered by herbicide resistance. Most cases of ALS-inhibitor resistance have resulted from selection of an altered target site. The study herein reports on an alanine by threonine amino acid substitution at position 122 of ALS as the basis for imidazolinone-specific resistance in an A. hybridus population from Illinois. In vitro inhibition of enzymatic activity (I₅₀) required 1000-fold greater concentration of imazethapyr in the resistant population compared with a susceptible control. This mutation represents the second ALS alteration associated with herbicide resistance in a natural A. hybridus population..

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

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