Molecular characterization of imidazolinone-resistant Brassica rapa × B. napus hybrids.

Herbicide-resistant oilseed rape (Brassica napus) cultivation in our country entails the risk of gene transfer to related wild species. One of these species is the wild turnip (B. rapa), an important weed of winter crops widely distributed in the Pampas region. Despite hybridization risks, Clearfield ® oilseed rape is available in Argentina. In 2008, a B. rapa population, which was sympatric to an imidazolinone-resistant and a conventional oilseed rape cultivar, was located on a farm in the main cropping area of the country. Herbicide-resistant individuals were found in the progeny of this population in a herbicide screening test. Therefore, a molecular characterization using cleaved amplified polymorphic sequence (CAPS) and simple sequence repeat (SSR) markers was conducted on these plants to determine their hybrid nature and to establish the origin of the imidazolinone resistance trait. The results of this study, along with information of field records, confirmed that the resistant plants were first generation interspecific hybrids. Imidazolinone resistance had been effectively transferred from the herbicide-resistant oilseed rape, even in the particular situation of pollen competition. Oilseed rape resistant cultivars are becoming more common in the country. So, considering that seed loss and crop volunteers are common in these species, it is crucial to avoid the dispersion of new resistant weed biotypes as they reduce the effectiveness of chemical control technologies.

Transgene escape and persistence in an agroecosystem: the case of glyphosate-resistant Brassica rapa L. in central Argentina.

Brassica rapa L. is an annual Brassicaceae species cultivated for oil and food production, whose wild form is a weed of crops worldwide. In temperate regions of South America and especially in the Argentine Pampas region, this species is widely distributed. During 2014, wild B. rapa populations that escaped control with glyphosate applications by farmers were found in this area. These plants were characterized by morphology and seed acidic profile, and all the characters agreed with B. rapa description. The dose-response assays showed that the biotypes were highly resistant to glyphosate. It was also shown that they had multiple resistance to AHAS-inhibiting herbicides. The transgenic origin of the glyphosate resistance in B. rapa biotypes was verified by an immunological test which confirmed the presence of the CP4 EPSPS protein and by an event-specific GT73 molecular marker. The persistence of the transgene in nature was confirmed for at least 4 years, in ruderal and agrestal habitats. This finding suggests that glyphosate resistance might come from GM oilseed rape crops illegally cultivated in the country or as a seed contaminant, and it implies gene flow and introgression between feral populations of GM B. napus and wild B. rapa. The persistence and spread of the resistance in agricultural environments was promoted by the high selection pressure imposed by intensive herbicide usage in the prevalent no-till farming systems.

Transgenic glyphosate-resistant oilseed rape (Brassica napus) as an invasive weed in Argentina: detection, characterization, and control alternatives.

The presence of glyphosate-resistant oilseed rape populations in Argentina was detected and characterized. The resistant plants were found as weeds in RR soybeans and other fields. The immunological and molecular analysis showed that the accessions presented the GT73 transgenic event. The origin of this event was uncertain, as the cultivation of transgenic oilseed rape cultivars is prohibited in Argentina. This finding might suggest that glyphosate resistance could come from unauthorized transgenic oilseed rape crops cultivated in the country or as seed contaminants in imported oilseed rape cultivars or other seed imports. Experimentation showed that there are alternative herbicides for controlling resistant Brassica napus populations in various situations and crops. AHAS-inhibiting herbicides (imazethapyr, chlorimuron and diclosulam), glufosinate, 2,4-D, fluroxypyr and saflufenacil proved to be very effective in controlling these plants. Herbicides evaluated in this research were employed by farmers in one of the fields invaded with this biotype and monitoring of this field showed no evidence of its presence in the following years.