POWELL AMARANTH (Amaranthus powellii) with GROUP B/2 resistance: (INHIBITION OF ACETOLACTATE SYNTHASE ) Inhibition of Acetolactate Synthase
MUTATION: SERINE 653 to THREONINE
Powell Amaranth (Amaranthus powellii) is a dicot plant in the amaranthaceae family. A single amino acid substitution from Serine 653 to Threonine has led to resistance to Inhibition of Acetolactate Synthase as indicated in the table below.		Powell Amaranth

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

REFERENCES

McNaughton, K. E. ; Letarte, J. ; Lee, E. A. ; Tardif, F. J.. 2005. Mutations in ALS confer herbicide resistance in redroot pigweed (Amaranthus retroflexus) and Powell amaranth (Amaranthus powellii). Weed Science 53 : 17 - 22.

A number of redroot pigweed and Powell amaranth populations from various locations in Ontario, Canada, have distinct patterns of resistance to the acetolactate synthase-inhibiting herbicides imazethapyr and thifensulfuron. This suggested the presence of diverse ALS gene mutations among these populations. Seven polymerase chain reaction primer pairs were used to amplify the gene to obtain full sequence information and to determine the identity of resistance-conferring mutations. There was a high degree of similarity in the ALS gene of the two species with only five nucleotides and one amino acid differing. A total of four herbicide resistance-conferring mutations were identified in the two species. The Ala₁₂₂Thr, Ala₂₀₅Val, and Trp₅₇₄Leu amino acid substitutions were found in redroot pigweed whereas Ala₁₂₂Thr, Trp₅₇₄Leu, and Ser₆₅₃Thr were detected in Powell amaranth. The pattern of resistance known to be conferred by the mutations concurred with the resistance level observed at the whole plant level. Distinct mutations being found in geographically separated populations suggest that selection for resistance occurred simultaneously in different locations. It reinforces the fact that resistance to ALS inhibitors is easily selected and that growers need to take this into account when formulating weed management strategies..

Diebold, R. S. ; McNaughton, K. E. ; Lee, E. A. ; Tardif, F. J.. 2003. Multiple resistance to imazethapyr and atrazine in Powell amaranth (Amaranthus powellii). Weed Science 51 : 312 - 318.

Multiple-herbicide resistance represents an added weed management challenge to growers as it can considerably reduce their options for weed control. The widespread nature of triazine resistance in Ontario, Canada, coupled with the more recent appearance of resistance to acetolactate synthase (ALS) inhibitors in Amaranthus species warranted documenting biotypes with multiple resistance. A collection of Powell amaranth (A. powellii) and redroot pigweed (A. retroflexus) biotypes that had previously been characterized for resistance to ALS inhibitors was therefore screened with atrazine. Dose-response analysis with atrazine and imazethapyr was also conducted. High-level resistance to imazethapyr and atrazine was determined in a Powell amaranth biotype from Perth County, Ontario. This biotype had a >1860-fold and 109-fold resistance to atrazine and imazethapyr, respectively. Sequence analysis was conducted for the psbA and ALS genes that code for the target sites of the triazines and imidazolinones, respectively. A mutation in the psbA gene was identified that coded for an amino acid substitution of glycine for serine at residue 264 of the D1 protein. This mutation is the most likely cause for triazine resistance in this biotype. Similarly, a nucleotide substitution was identified that codes for threonine in place of serine at position 652 of the ALS protein. This mutation in the ALS gene has only been observed previously in laboratory-selected mutants of arabidopsis and tobacco and is known to endow resistance to imidazolinones in plants. It is concluded that multiple resistance in this Powell amaranth biotype is due to the presence of altered target sites for triazine and imidazolinone herbicides..

Mcnaughton, K. E., E. Lee A., And F. Tardif J.. 2001. Mutations in the ALS gene confering resistance to group II herbicides in redroot pigweed (Amaranthus retroflexus) and green pigweed (A. powellii). Weed Science Society of America Abstracts 41 : 97.

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

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