INTERNATIONAL HERBICIDE-RESISTANT WEED DATABASE

PALMER AMARANTH (Amaranthus palmeri) with GROUP B/2 resistance: (INHIBITION OF ACETOLACTATE SYNTHASE ) Inhibition of Acetolactate Synthase

MUTATION: SERINE 653 to ASPARAGINE

Palmer Amaranth (Amaranthus palmeri) 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.		Palmer Amaranth

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

NOTE

The Molin et al. 2016 paper reported this mutation in Palmer amaranth but it was not characterized.

Berger et al. 2016 also identified but did not characterize this mutation.

The mutation was also identified by Larran et al. 2017, but was present within a segregating population, so the conferred levels of resistance to the different chemical families are unknown.

Mutation also identified by Singh et al. 2018, but present in plant that also had Trp574Leu.

REFERENCES

Molin, W. T., V. K. Nandula, A. A. Wright, and J. A. Bond. 2016. Transfer and expression of ALS inhibitor resistance from Palmer amaranth (Amaranthus palmeri) to an A. spinosus × A. palmeri hybrid.. Weed Science 64 : 240 - 247.

Transfer of herbicide resistance among closely related weed species is a topic of growing concern. A spiny amaranth × Palmer amaranth hybrid was confirmed resistant to several acetolactate synthase (ALS) inhibitors including imazethapyr, nicosulfuron, pyrithiobac, and trifloxysulfuron. Enzyme assays indicated that the ALS enzyme was insensitive to pyrithiobac and sequencing revealed the presence of a known resistance conferring point mutation, Trp574Leu. Alignment of the ALS gene for Palmer amaranth, spiny amaranth, and putative hybrids revealed the presence of Palmer amaranth ALS sequence in the hybrids rather than spiny amaranth ALS sequences. In addition, sequence upstream of the ALS in the hybrids matched Palmer amaranth and not spiny amaranth. The potential for transfer of ALS inhibitor resistance by hybridization has been demonstrated in the greenhouse and in field experiments. This is the first report of gene transfer for ALS inhibitor resistance documented to occur in the field without artificial/human intervention. These results highlight the need to control related species in both field and surrounding noncrop areas to avoid interspecific transfer of resistance genes..

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..

Larran, A. S., V. E. Palmieri, V. E. Perotti, L. Lieber, D. Tuesca, and H. R. Permingeat. 2017. Target-site resistance to acetolactate synthase (ALS)-inhibiting herbicides in Amaranthus palmeri from Argentina.. Pest Management Science doi:10.1002/ps.4662 : .

BACKGROUND Herbicide-resistant weeds are a serious problem worldwide. Recently, two populations of Amaranthus palmeri with suspected cross-resistance to acetolactate synthase (ALS)-inhibiting herbicides (R1 and R2) were found by farmers in two locations in Argentina (Vicuña Mackenna and Totoras, respectively). We conducted studies to confirm and elucidate the mechanism of resistance.

RESULTS We performed in vivo dose–response assays, and confirmed that both populations had strong resistance to chlorimuron-ethyl, diclosulam and imazethapyr when compared with a susceptible population (S). In vitro ALS activity inhibition tests only indicated considerable resistance to imazethapyr and chlorimuron-ethyl, indicating that other non-target mechanisms could be involved in diclosulam resistance. Subsequently, molecular analysis of als nucleotide sequences revealed three single base-pair mutations producing substitutions in amino acids previously associated with resistance to ALS inhibitors, A122, W574, and S653.

CONCLUSION This is the first report of als resistance alleles in A. palmeri in Argentina. The data support the involvement of a target-site mechanism of resistance to ALS-inhibiting herbicides. © 2017 Society of Chemical Industry

Singh, S., V. Singh, R. A. Salas-Perez, M. V. Bagavathiannan, A. Lawton-Rauh, and N. Roma-Burgos. 2019. Target-site mutation accumulation among ALS inhibitor-resistant Palmer amaranth. Pest Manag Sci 75 : 1131 - 1139.

BACKGROUND: Palmer amaranth (Amaranthus palmeri S. Wats) is one of the most common and troublesome weeds in the USA. Palmer amaranth resistance to acetolactate synthase (ALS) inhibitors is widespread in the USA, as in Arkansas. The cross-resistance patterns and mechanism of resistance are not known. Experiments were conducted to determine cross-resistance to ALS inhibitors and identify target-site mutations in 20 Palmer amaranth localities from 13 counties in Arkansas.

RESULTS: All Palmer amaranth localities tested had plants cross-resistant to imazethapyr, flumetsulam, primisulfuron, pyrithiobac and trifloxysulfuron. The dose of trifloxysulfuron that caused 50% control was 21–56-fold greater for resis- tant accessions than for susceptible ones. All but three resistant plants analyzed had one or two relative copies of ALS; one plant had seven relative copies. All resistant plants tested (18 localities) carried the Trp574Leu mutation, which is known to confer broad resistance to ALS inhibitors, supporting the cross-resistance pattern observed. Besides the Trp574Leu mutation, 30% of localities had individuals with one additional resistance-conferring mutation including Ala122Thr, Pro197Ala or Ser653Asn.

CONCLUSION: The Trp574Leu mutation in ALS is the primary mechanism of resistance to ALS inhibitors in Palmer amaranth from Arkansas, USA. In some localities, multiple mutations have accumulated in one plant. All localities tested contained plants with resistance to five families of ALS inhibitors. Localities with extremely high resistance to ALS inhibitors, and those outside the subset we studied, may harbor non-target site resistance mechanisms. ALS inhibitors are generally no longer effective on Palmer amaranth in these localities from the US mid-south.

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

Heap, I. The International Herbicide-Resistant Weed Database. Online. Tuesday, September 29, 2026. Available at www.weedscience.org

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