Resistance to Glyphosate and Paraquat in Conyza bonariensis and Conyza canadensis from California Orchards: Management, Distribution, and Mechanism of Resistance

Conyza bonariensis and Conyza canadensis are weeds found in orchards and other areas of California. These species are problematic in orchards because of populations that have evolved resistance to glyphosate, and glyphosate and paraquat in the case of C. bonariensis. Due to the need for management of these resistant weeds, a series of greenhouse and field experiments were conducted to evaluate alternative herbicides for managing these populations. The herbicides glufosinate and the mixtures of glyphosate with saflufenacil and glyphosate with 2,4-D were effective in controlling susceptible, glyphosate-resistant, and glyphosate-paraquat-resistant populations of C. bonariensis, and thus could be used as a tool to mitigate the spread of the resistance. Although management options were in place, no information was available on the distribution of the multiple-resistant populations in the state. Distribution and presence of glyphosate and paraquat resistance was evaluated in field populations of both Conyza sp. throughout the Central Valley, as well as in accessions previously grouped into different genetic groups based on genetic population structure. Glyphosate-resistant Conyza sp. was commonly found in all surveyed areas. Glyphosate-paraquat resistance was confirmed in accessions from distinct genetic groups that were originally collected from three counties in the state: Merced, Fresno, and Kern Counties. One C. canadensis accession was confirmed to be glyphosate-paraquat-resistant. Resistance to paraquat was always associated with glyphosate resistance. Because multiple resistance was found across a wide geographical range and in accessions from distinct genetic groups, multiple-resistant Conyza sp. likely evolved independently several times in California. In the next phase of research, the mechanism of resistance in these Conyza populations was investigated. Using radiolabeled herbicides, the absorption and translocation of the glyphosate and paraquat was monitored over time in glyphosate-resistant, glyphosate-paraquat-resistant, and susceptible biotypes of both species. Resistance to glyphosate or paraquat was associated with reduced translocation of the herbicides in both Conyza sp. The mechanism of reduced translocation was hypothesized to be related to the sequestration of herbicides into the vacuoles probably through tonoplast transporters. To investigate genes that could be involved in the mechanism of resistance, a transcription study using real time PCR was performed. First, eight candidate reference genes were evaluated for their stability after stress with glyphosate or paraquat in both Conyza sp. The genes actin, heat-shock-protein 70, and cyclophilin were identified as stable under herbicide stress in both species. Second, transcription of two putative tonoplast transporters from the ATP binding cassette (ABC) transporter family, one putative cationic amino acid transporter (CAT), and 5-enolpyruvylshikimate-3-phosphatase synthase (EPSPS) genes were evaluated. No changes in the transcription of CAT or EPSPS genes were observed after herbicide treatments. Transcription of ABC transporters increased after herbicide treatment, but the changes could not be associated with resistant biotypes, but rather suggests a stress response. This work was the first study of glyphosate-paraquat resistance in Conyza sp., and the findings highlighted herein can advance the understanding on this matter.