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Herbicide resistance among summer cypress likely caused by several genetic variations

A new study finds different gene variations that cause herbicide resistance in a common agricultural weed in Canada, suggesting the genetic mutation has multiple origins and may present additional weed-control challenges.

The research focused on kochia, an annual species also known as summer cypress, that is among the 10 most abundant agricultural weeds in Canadian prairies, according to an article in the November-December 2008 issue of Weed Science, published by the Weed Science Society of America. Kochia also has the highest spread rate among weeds in the last 100 years in the northwestern United States and was the first weed species to evolve herbicide-resistant biotypes in western Canada in 1988.

The research examined the acetolactate synthase gene in kochia plants growing in western Canada and clarified the plant's genetic basis for resistance to herbicides, known as ALS inhibitors. The study also looked at levels of DNA sequence variation in the ALS gene among kochia plants that are herbicide-susceptible or herbicide-resistant.

"The broad cross-resistance in these populations suggests a target mutation--which imparts plants with a high-level, broad-spectrum resistance," write Suzanne I. Warwick, Renlin Xu, Connie Sauder, and Hugh J. Beckie in their article "Acetolactate Synthase Target-Site Mutations and Single Nucleotide Polymorphism Genotyping in ALS-Resistant Kochia (Kochia scoparia)."

Researchers studied six herbicide-susceptible and 24 herbicide-resistant kochia seeds from Alberta, Manitoba, and Saskatchewan. The herbicide-resistant seeds included some from a 2003 field survey of weeds in Saskatchewan and some from samples that growers submitted to the Crop Protection Lab there, plus seeds from two control populations. The seeds were sprayed with the ALS-inhibitor herbicide Refine Extra.

The research detected two resistance levels among the seeds to the herbicide mixture: level 2, which involves some injury to the kochia plant followed by recovery, and level 3, when the kochia plant becomes fully herbicide resistant.

"The two levels of herbicide resistance observed in this study may be due to different target-site mutations in the ALS gene," according to the article.

Sixteen DNA sequence variations--known as single nucleotide polymorphisms--were identified in the plants, including five that caused amino acid changes that confer ALS-herbicide resistance. These DNA variations correspond to three target-site mutations in the kochia. Because these three mutations, and combinations of them, were found among kochia growing in geographically separate regions, the researchers say the mutations likely have multiple origins, so they are unable to pinpoint a single culprit.

"The variability of the ALS gene observed in this study suggests that the broad geographic establishment of ALS-inhibitor (herbicide-resistant) kochia is likely due to multiple founding events rather than to spread of a single resistance allele," the researchers write.

To read the entire article, Acetolactate Synthase Target-Site Mutations and Single Nucleotide Polymorphism Genotyping in ALS-Resistant Kochia (Kochia scoparia) (Vol. 56(6):797-806), visit http://www.allenpress.com/pdf/WEES_56.6_797.pdf.

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