SMOOTH PIGWEED (Amaranthus hybridus (syn: quitensis)) with GROUP B/2 resistance: (INHIBITION OF ACETOLACTATE SYNTHASE ) Inhibition of Acetolactate Synthase
MUTATION: ASPARTATE 376 to GLUTAMATE
Smooth Pigweed (Amaranthus hybridus (syn: quitensis)) is a dicot plant in the amaranthaceae family. A single amino acid substitution from Aspartate 376 to Glutamate 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	Resistant > 10 fold
Triazolopyrimidine - Type 1	Chloransulam-methyl	Resistant > 10 fold
Triazolinones	Flucarbazone-Na	Resistant > 10 fold

REFERENCES

Whaley, C. M. ; Wilson, H. P. ; Westwood, J. H.. 2007. A new mutation in plant Als confers resistance to five classes of Als-inhibiting herbicides. Weed Science 55 : 83 - 90.

Experiments were conducted to evaluate a biotype of smooth pigweed that had survived applications of sulfonylurea (SU) and imidazolinone (IMI) herbicides in a single season. The source field had a history of repeated acetolactate synthase (ALS)-inhibiting herbicide use over several years. Whole-plant response experiments evaluated the resistant (R11) biotype and an ALS-inhibitor susceptible (S) smooth pigweed biotype to herbicides from the SU, IMI, pyrimidinylthiobenzoate (PTB), and triazolopyrimidine sulfonanilide (TP) chemical families. The R11 biotype exhibited 60- to 3200-fold resistance to all four ALS-inhibiting herbicide chemistries compared with the S biotype. Nucleotide sequence comparison of Als genes from R11 and S biotypes revealed a single nucleotide difference that resulted in R11 having an amino acid substitution of aspartate to glutamate at position 376, as numbered relative to the protein sequence of mouseearcress. This is the first report of an amino acid substitution at this position of an Als gene isolated from a field-selected weed biotype. To verify the role of this mutation in herbicide resistance, the Als gene was cloned and expressed in Arabidopsis. Transgenic Arabidopsis expressing this Als gene exhibited resistance to SU, IMI, PTB, TP, and sulfonylaminocarbonyltriazolinone ALS-inhibiting herbicide classes..

Whaley, C. M., H. P. Wilson, And J. H. Westwood.. 2004. Characterization of a new ALS-inhibitor resistance mutation from the ALS gene of smooth pigweed (Amaranthus hybridus). Weed Science Society of America Abstracts : .

A smooth pigweed population collected from a field in southeastern Pennsylvania was confirmed resistant to acetolactate synthase (ALS)- inhibiting herbicide representatives from sulfonylurea, imidazolinone, pyrimidinyloxybenzoate, and triazolopyrimidine classes. Sequencing the ALS gene from the herbicide resistant (R) population revealed a single amino acid substitution of asp375 to glu, which corresponds to position 376 in Arabidopsis thaliana. This mutation has not previously been reported from herbicide resistant plants. To confirm resistance, the R ALS gene was overexpressed in normally susceptible Arabidopsis lines. Smooth pigweed ALS genes from the R and ALS inhibitor-susceptible (S) populations were fused to the cauliflower mosaic virus 35S promoter, cloned into a vector with a hygromycin resistance marker, and introduced into Arabidopsis using Agrobacterium-mediated transformation. Seeds collected from Arabidopsis following transformation were selected on hygromycin-containing media and grown for seed. Arabidopsis selection on imazethapyr-containing media confirms that transformants with the R ALS gene are resistant to this herbicide compared to transformants expressing the S ALS gene and wild-type plants. Both R and S ALS genes from smooth pigweed have been cloned into the pET32a protein expression vector and introduced into E. coli to evaluate kinetics of the R enzyme under various concentrations of ALS-inhibiting herbicides, substrate, and cofactors.

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

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