GREEN FOXTAIL (Setaria viridis) with GROUP B/2 resistance: (INHIBITION OF ACETOLACTATE SYNTHASE ) Inhibition of Acetolactate Synthase
MUTATION: SERINE 653 to THREONINE
Green Foxtail (Setaria viridis) is a monocot plant in the poaceae 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.		Green Foxtail

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

REFERENCES

Laplante, J., I. Rajcan, And F. J. Tardif.. 2009. Multiple allelic forms of acetohydroxyacid synthase are responsible for herbicide resistance in Setaria viridis. Theoretical and Applied Genetics 119 : 577 - 585.

In weed species, resistance to herbicides inhibiting acetohydroxyacid synthase (AHAS) is often conferred by genetic mutations at one of six codons in the AHAS gene. These mutations provide plants with various levels of resistance to different chemical classes of AHAS inhibitors. Five green foxtail [Setaria viridis (L.) Beauv.] populations were reported in Ontario with potential resistance to the AHAS-inhibiting herbicide imazethapyr. The objectives of this study were to confirm resistance, establish the resistance spectrum for each of the five populations, and determine its genetic basis. Dose response curves were generated for whole plant growth and enzyme activity, and the AHAS gene was sequenced. Resistance was confirmed by determining the resistance factor to imazethapyr in the five resistant green foxtail populations for whole plant dose response experiments (21- to 182-fold) and enzyme assays (15- to 260-fold). All five imazethapyr-resistant populations showed cross-resistance to nicosulfuron and flucarbazone while only three populations had cross-resistance to pyrithiobac. Sequence analyses revealed single base-pair mutations in the resistant populations of green foxtail. These mutations were coded for Thr, Asn, or Ile substitution at Ser653. In addition, a new mutation was found in one population that coded for an Asp substitution at Gly654. There is an agreement between the spectra of resistance observed and the type of resistance known to be conferred by these substitutions. Moreover, it indicates that, under similar selection pressure (imazethapyr), a variety of mutations can be selected for different populations, making the resistance pattern difficult to predict from herbicide exposure history.

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

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