Characterization of metabolic genes for multiple herbicide resistance in Palmer amaranth and optimization of metribuzin rates to control pigweed in soybean

Abstract

Weed infestation is a significant threat to sustainable crop production, causing both direct and indirect crop losses. The most problematic weeds in the U.S. include Palmer amaranth (Amaranthus palmeri S. Watson) and waterhemp [Amaranthus tuberculautus (Moq). J. D. Sauer], known for their aggressive growth habits, competitiveness, and ability to evolve herbicide resistance. Currently, Palmer amaranth populations have evolved resistance to ten sites of action (SOA) of herbicides while waterhemp populations were documented to have resistance to at least seven SOAs. Herbicide resistance can evolve because of both target-site and non- target-site-based mechanisms. One of the most challenging non-target site resistance mechanisms is metabolism of herbicides due to increased activity of enzymes such as cytochrome P450s (P450) or glutathione-S-transferases (GSTs). Recently, a Palmer amaranth population (KCTR) from Kansas was found resistant to six SOAs of herbicides. Moreover, a predominance of metabolic resistance possibly mediated by P450 or GST enzyme activity was reported in this population. In this research, KCTR Palmer amaranth was used as a model to assess the molecular basis of metabolic resistance to multiple herbicides, which is the focus of Chapter 2 of this thesis. Rapidly evolving multiple herbicide-resistant (MHR) populations of both Palmer amaranth and waterhemp are already a challenge to growers, leaving limited chemical weed control options. However, metribuzin (photosystem II-inhibitor) can effectively control small-seeded broadleaf weeds including Amaranthus spp. when applied as a pre- emergence (PRE) herbicide. Despite the known activity of metribuzin growers are reluctant to use it as they are either unaware of metribuzin’s efficacy or due to suspected crop injury. In Chapter 3, the use of metribuzin was evaluated as a potential PRE option to control MHR Amaranthus weeds, while documenting early-season soybean response. Overall the major objectives of this thesis were to 1) identify the genes involved in metabolic resistance to multiple herbicides in Palmer amaranth (Chapter 2); 2) confirm and validate the activity of candidate genes in mediating herbicide metabolism (Chapter 2); 3) evaluate early season soybean growth and development following PRE applied metribuzin (Chapter 3); and 4) optimize metribuzin application rates for control of MHR Palmer amaranth and waterhemp populations (Chapter 3). Transcriptome analysis (Chapter 2) revealed 414, 129, 529, 688, and 152 genes differentially expressed in resistant plants following application of chlorsulfuron, 2,4-D, atrazine, lactofen, and mesotrione, respectively. Overexpression of CYP72A219 gene isoforms was observed following mesotrione, chlorsulfuron, and lactofen treatment while CYP704B1 overexpression was observed following 2,4-D and atrazine treatment. Additionally, isoforms of C-terminal domain GST were upregulated post-treatment with all herbicides except lactofen. Quantitative RT-PCR showed 3.4- to 6.6-fold and 5.9- to 12.4-fold overexpression of CYP72A219 and CYP704B1 transcripts, respectively, in KCTR Palmer amaranth. Field experiments evaluating metribuzin’s efficacy (Chapter 3) revealed no significant impact on soybean height and yield even at application rates of 841 g ai ha⁻¹ of metribuzin. Metribuzin at 630 and 315 g ai ha⁻¹ delayed weed emergence more effectively than sulfentrazone and S-metolachlor, while rates of 525 and 315 g ai ha⁻¹ were more effective in reducing weed density and biomass than sulfentrazone and S-metolachlor, respectively. Higher metribuzin rates (578 to 841 g ai ha⁻¹) achieved more than 95%, 90%, and 80% control of weeds at 14, 28, and 42 days after application, respectively. Overall, the outcome of this thesis uncovers key genes driving metabolic resistance to multiple herbicides in Palmer amaranth and demonstrates that higher rates of metribuzin can be safely used to control multiple herbicide-resistant Palmer amaranth and waterhemp in soybean, providing growers with a viable option for weed management.