SILKY WINDGRASS (Apera spica-venti) with GROUP B/2 resistance: (INHIBITION OF ACETOLACTATE SYNTHASE ) Inhibition of Acetolactate Synthase
MUTATION: TRYPTOPHAN 574 to LEUCINE
Silky Windgrass (Apera spica-venti) is a monocot plant in the poaceae 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.		Silky Windgrass

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	Resistant > 10 fold
Triazolinones	Flucarbazone-Na	Resistant > 10 fold

REFERENCES

Massa, D., B. Krenz, And R. Gerhards. 2011. Target-site resistance to ALS-inhibiting herbicides in Apera spica-venti populations is conferred by documented and previously unknown mutations. Weed Research 51 : 294 - 303.

In this study, whole-plant bioassays were performed on 72 Apera spica-venti populations that have survived application of acetolactate synthase (ALS)-inhibiting herbicides in recent years. Molecular genetic analysis of the ALS gene revealed a Thr mutation at Pro197 within 67 populations. Sequencing of the whole ALS gene from wild-type and resistant plants not carrying the above-mentioned mutation revealed the presence of a Leu mutation at Trp574 within two populations and an Asn mutation at Pro197 within two populations. As the Pro197-Asn amino acid substitution is reported for the first time in a field-selected weed population, a Cleaved Amplified Polymorphic Sequences (CAPS) marker was developed for its quick detection. In addition, one novel mutation was found within a population that coded for a His substitution at Arg377. Enzyme assays confirmed a significant reduction in inhibition of ALS activity compared with the wild type. This population showed resistance to sulfonylureas (SUs) and cross-resistance to sulfonylaminocarbonyltriazolinones (SCTs) and triazolopyrimidines (TPs) within the whole-plant bioassays. ALS protein sequence alignments from weedy and cultural plants revealed that the Arg377 is highly conserved among known wild-type enzymes. In agreement with existing literature concerning the structure and mechanisms of inhibition of plant ALS, this mutation is probably involved in target-site resistance to ALS inhibitors. Our results suggest that further single-nucleotide polymorphisms impairing proper herbicide performance might be selected within field populations in the near future, making the short- and long-range evolution of target-site resistance difficult to predict depending solely on herbicide use history.

Katerina Hamouzová, Pavlíina Košnarová, Jaroslav Salava, Josef Soukup and Pavel Hamouza. 2013. Mechanisms of resistance to acetolactate synthase-inhibiting herbicides in populations ofApera spica-ventifrom the Czech Republic. Pest Management Science : .

BACKGROUND: This study investigates the mechanisms of resistance to acetolactate synthase-inhibiting herbicides in populations of Apera spica-venti (L.) P.B. from the Czech Republic.

RESULTS: The proportion of resistance due to mutant acetolactate synthase (ALS) alleles was estimated by genotyping individuals from each of three populations for the eight ALS mutations known to confer resistance. Four resistance-conferring ALS mutations were identified: Pro-197-Ala, Pro-197-Thr, Trp-574-Leu and previously unreported Trp-574-Met substitution. Two populations (R1, R3) have amino acid substitution at positions Pro-197 and Trp-574. Individuals from the R3 population had two different resistance alleles. In the R2 population, only the resistant Trp-574-Met substitution was detected. Ten other single point mutations were identified, but these were not related to resistance.

The cytochrome malathion decreased chlorsulfuron resistance in the resistant populations that were examined. Although malathion increased mortality, the GR50 values were too high to conclude that non-target-based mechanism was the main one for the resistance in Apera spica-venti populations tested in this study.
CONCLUSIONS: Individuals of Apera spica-venti populations tested in this study possess the target-site ALS resistance mutation and an additional so far unknown resistance mechanism(s)..

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

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