RUSSIAN-THISTLE (Salsola tragus) with GROUP B/2 resistance: (INHIBITION OF ACETOLACTATE SYNTHASE ) Inhibition of Acetolactate Synthase
MUTATION: TRYPTOPHAN 574 to LEUCINE
Russian-thistle (Salsola tragus) is a dicot plant in the chenopodiaceae 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.		Russian-thistle

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

REFERENCES

Warwick, S. I. ; Sauder, C. A. ; Beckie, H. J.. 2010. Acetolactate synthase (ALS) target-site mutations in ALS inhibitor-resistant Russian thistle (Salsola tragus). Weed Science 58 : 244 - 251.

ALS inhibitor-resistant biotypes are the fastest growing class of herbicide-resistant (HR) weeds. A Canadian ALS inhibitor-resistant biotype of Russian thistle was first reported in 1989. The molecular basis for ALS-inhibitor resistance is unknown for Canadian populations of this polyploid weed species, and was determined in this study for one Alberta and two Saskatchewan HR Russian thistle populations. HR plants survived spray application of the ALS-inhibitor mixture thifensulfuron:tribenuron in the greenhouse. All three HR Russian thistle populations were heterogeneous and contained both HR and herbicide-susceptible (HS) individuals. The molecular basis for resistance was determined by sequencing the ALS gene and/or conducting a TaqMan genotyping assay for single nucleotide polymorphism (SNP) for the Trp₅₇₄Leu mutation. Two target-site mutations were observed: Trp₅₇₄Leu in all three biotypes (554 individuals) and Pro₁₉₇Gln in one biotype (one individual), suggesting multiple-founding events for Russian thistle HR populations in western Canada. Segregation patterns among F1 and F2 progeny arrays of HR lines sprayed under greenhouse conditions varied; some segregated (i.e., had HR and HS progeny), whereas other lines were exclusively HR. In contrast, no segregation of molecular types, i.e., Trp₅₇₄, Trp/Leu₅₇₄ and Leu₅₇₄, as would be expected with heterozygosity at a single locus Trp/Leu₅₇₄, was observed. Such lack of segregation is consistent with the polyploid genome structure of Russian thistle and the presence of two copies of the ALS gene. The presence of more than one ALS gene confounded the ability of the molecular techniques to accurately identify "true" heterozygotes in this study..

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

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