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Multiple mutations may cause herbicide resistance

A new study of herbicide-resistant kochia Prairie-wide suggests more than one genetic pathway for weeds to develop some level of resistance, which in turn suggests more challenges ahead in controlling it.

A team of Agriculture and Agri-Food Canada researchers in Saskatoon and Ottawa released their findings in the November-December issue of the journal Weed Science, published by the Weed Science Society of America and announced in a release Monday.

Kochia, the researchers wrote, came to Canada from Europe and Asia and is one of the top 10 most-abundant agricultural weeds in Canada’s Prairies. It’s also reported to have the highest rate of spread among 40 alien weed species during the past 100 years in the northwestern U.S.

Kochia was also the first weed species to evolve herbicide-resistant biotypes in Western Canada, in 1988, according to researchers Suzanne Warwick, Renlin Xu, Connie Sauder, and Hugh Beckie.

The study focused on kochia’s resistance to ALS inhibitors, which are members of herbicide Group 2. It looked at the ALS gene in kochia plants, its genetic basis for resistance, and the levels of variation in the DNA sequence governing the ALS gene.

“The broad cross-resistance in these populations suggests a target mutation… which imparts plants with a high-level, broad-spectrum resistance,” the researchers wrote.

Studying six herbicide-susceptible and 24 herbicide-resistant kochia seeds sprayed with Refine Extra (a Group 2 ALS inhibitor), the researchers found two resistance levels among the seeds: level 2, in which the kochia plant is injured but recovers, and level 3, in which the plant becomes fully resistant.

“The two levels of herbicide resistance observed in this study may be due to different target-site mutations in the ALS gene,” the researchers wrote. “Multiple events”

DNA variations were found that corresponded to three target-site mutations in the kochia. What’s more, all three target-site mutations, as well as both combinations, were found in at least one population in each of the three Prairie provinces.

Since the three mutations, and combinations of them, were found in kochia growing in “geographically separate” regions, the researchers wrote, the mutations likely have multiple origins, so they can’t pinpoint a single culprit gene.

“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 wrote.

(An allele is one of a group of two or more variations of a gene that’s found in a specific place on a chromosome.)

That suggests transmission not only by pollen flow — which in kochia can produce outcrossing per plant of 13.1 per cent at 1.5 metres’ distance — but also in the seed flow of kochia, a tumbling weed species.

Both pollen and seed movement can introduce resistance alleles into a population, the researchers said, and pollen movement may even allow the accumulation of different resistance alleles in the same plant.

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