POWELL AMARANTH (Amaranthus powellii) with GROUP B/2 resistance: (INHIBITION OF ACETOLACTATE SYNTHASE ) Inhibition of Acetolactate Synthase
MUTATION: TRYPTOPHAN 574 to LEUCINE
Powell Amaranth (Amaranthus powellii) is a dicot plant in the amaranthaceae family. A single amino acid substitution from Tryptophan 574 to Leucine 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	Resistant > 10 fold
Triazolopyrimidine - Type 1	Chloransulam-methyl	Not Determined
Triazolinones	Flucarbazone-Na	Not Determined

NOTE

"Considerable pleiotrophic effects" have been associated with Trp574Leu in A. powellii. See Tardif et al. (2006) for details.

REFERENCES

Tardif, F. J. ; Rajcan, I. ; Costea, M.. 2006. A mutation in the herbicide target site acetohydroxyacid synthase produces morphological and structural alterations and reduces fitness in Amaranthus powellii. New Phytologist 169 : 251 - 264.

We investigated the effect of a herbicide resistance-conferring mutation on fitness in Amaranthus powellii. Morphological and histological observations were made. Growth and leaf appearance were recorded for six resistant and six susceptible populations. The competitiveness of a susceptible population was compared with that of a resistant population using a replacement series experiment. Leaves of the resistant plants were distorted and much smaller than those of susceptible plants. Additionally, they exhibited an abnormal morphological and structural pattern consisting of a mosaic of heterogeneous areas in the same leaf blade. The roots and stems had similar structures in susceptible and resistant plants, but the former were up to four times more developed. The resistant plants were slower to develop and produced 67% less biomass and 58% lower leaf area than susceptible plants. Under competitive conditions, one susceptible population outperformed one resistant population by 7-15 times. The Trp₅₇₄Leu acetohydroxyacid synthase (AHAS) mutation appears to have considerable pleiotropic effects on the early growth and development of the plants which, in competitive conditions, greatly reduce fitness..

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..

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

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