Date Published: July 10, 2017
Publisher: Public Library of Science
Author(s): Marcelo L. Moretti, Rocio Alárcon-Reverte, Stephen Pearce, Sarah Morran, Bradley D. Hanson, Dean E. Riechers.
Herbicide resistance is a challenge for modern agriculture further complicated by cases of resistance to multiple herbicides. Conyza bonariensis and Conyza canadensis are invasive weeds of field crops, orchards, and non-cropped areas in many parts of the world. In California, USA, Conyza populations resistant to the herbicides glyphosate and paraquat have recently been described. Although the mechanism conferring resistance to glyphosate and paraquat in these species was not elucidated, reduced translocation of these herbicides was observed under experimental conditions in both species. Glyphosate and paraquat resistance associated with reduced translocation are hypothesized to be a result of sequestration of herbicides into the vacuole, with the possible involvement of over-expression of genes encoding tonoplast transporters of ABC-transporter families in cases of glyphosate resistance or cationic amino acid transporters (CAT) in cases of paraquat resistance. However, gene expression in response to herbicide treatment has not been studied in glyphosate and paraquat resistant populations. In the current study, we evaluated the transcript levels of genes possibly involved in resistance using real-time PCR. First, we evaluated eight candidate reference genes following herbicide treatment and selected three genes that exhibited stable expression profiles; ACTIN, HEAT-SHOCK-PROTEIN-70, and CYCLOPHILIN. The reference genes identified here can be used for further studies related to plant-herbicide interactions. We used these reference genes to assay the transcript levels of EPSPS, ABC transporters, and CAT in response to herbicide treatment in susceptible and resistant Conyza spp. lines. No transcription changes were observed in EPSPS or CAT genes after glyphosate or paraquat treatment, suggesting that these genes are not involved in the resistance mechanism. Transcription of the two ABC transporter genes increased following glyphosate treatment in all Conyza spp. lines. Transcription of ABC transporters also increased after paraquat treatment in all three lines of C. bonariensis. However, in C. canadensis, paraquat treatment increased transcription of only one ABC transporter gene in the susceptible line. The increase in transcription of ABC transporters after herbicide treatment is likely a stress response based on similar response observed across all Conyza lines regardless of resistance or sensitivity to glyphosate or paraquat, thus these genes do not appear to be directly involved in the mechanism of resistance in Conyza spp.
Herbicide resistance is widespread in modern agriculture and challenges the long-term sustainability of herbicide use . Although many herbicides are commercially available, the market is dominated by a small number of widely used chemicals [2, 3]. Two of the most extensively used herbicides are glyphosate  and paraquat . The repeated use of these herbicides has selected for the evolution in many weed species of resistance to individual herbicide modes of action and, increasingly, to multiple herbicides. Reports of dual resistance to both glyphosate and paraquat in weedy species are limited, but cases in Conyza bonariensis and Conyza canadensis were recently reported in several locations in California, USA [6, 7]. Members of the Conyza genus are listed among the most problematic herbicide-resistant weeds globally based on number of countries where they are present, number of herbicide modes of action with documented resistance, and estimated infested sites and cropping systems [8, 9]; these weeds affect corn, soybean, vineyards, orchards, and non-cropped areas across five continents . Due to the agronomic and economic importance of C. bonariensis and C. canadensis in the U.S. and other parts of world, there is an urgent need to understand the mechanisms of glyphosate and paraquat resistance to help combat this threat.