SMOOTH PIGWEED (Amaranthus hybridus (syn: quitensis)) with GROUP B/2 resistance: (INHIBITION OF ACETOLACTATE SYNTHASE ) Inhibition of Acetolactate Synthase
MUTATION: SERINE 653 to ASPARAGINE
Smooth Pigweed (Amaranthus hybridus (syn: quitensis)) is a dicot plant in the amaranthaceae family. A single amino acid substitution from Serine 653 to Asparagine 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	Resistant > 10 fold
Sulfonylureas	Chlorsulfuron	Intermediate Resistance < 10 fold
Triazolopyrimidine - Type 1	Chloransulam-methyl	Intermediate Resistance < 10 fold
Triazolinones	Flucarbazone-Na	Not Determined

NOTE

Different biotypes harboring the Ser653Asn mutation varied substantially in response to some herbicides. In particular, resistance ratios for chloransulam-methyl ranged from 0.08 (sensitive) to 10. See Whaley et al. (2006) for details.

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..

Berger, S., P. T. Madeira, J. Ferrell, L. Gettys, S. Morichetti, J. J. Cantero, and C. Nunez. 2016. Palmer amaranth (Amaranthus palmeri) identification and documentation of ALS-resistance in Argentina.
. Weed Science 64 : 312 - 320.

Palmer amaranth has greatly disrupted agricultural practices in the United States with its rapid growth and rapid evolution of herbicide resistance. This weed species is now suspected in Argentina. To document whether the suspected plant populations are indeed Palmer amaranth, molecular comparisons to known standards were conducted. Additionally, these same plant populations were screened for possible herbicide resistance to several acetolactate synthase (ALS)-inhibiting herbicides. Sequencing data confirmed that suspected populations (A2, A3, A4) were indeed Palmer amaranth. Another population (A1) was tested to determine whether hybridization had occurred between Palmer amaranth and mucronate amaranth the native amaranth species of the region. Tests confirmed that no hybridization had occurred and that A1 was simply a unique phenotype of mucronate amaranth. Each population was screened for resistance to imazapic, nicosulfuron, and diclosulam. All Palmer amaranth populations from Argentina were shown to be resistant to at least one ALS-inhibiting herbicide. The populations were then subjected to further testing to identify the mutation responsible for the observed ALS resistance. All mucronate amaranth populations exhibited a mutation previously documented to confer ALS resistance (S₆₅₃N). No known resistance-conferring mutations were found in Palmer amaranth..

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

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