Progress 10/01/19 to 09/30/20
Outputs
Target Audience:The results of this research are shared with weed management practitioners in Illinois and across the United States. The target audiences include producers, agrichemical input suppliers, input distributors, Extension specialists, commodity organizations, crop consultants, professional weed science societies, and various agricultural media platforms. Changes/Problems:
Nothing Reported
What opportunities for training and professional development has the project provided?This research has provided additional data on the evolution of herbicide resistance in waterhemp. These results have been used in various training sessions with weed management practitioners that are designed to educate them on the evolution of herbicide resistance in weed populations. How have the results been disseminated to communities of interest?These results have been shared in myriad presentations to weed management practitioners to illustrate the increasing occurrence of multiple-resistant weed populations in order to foster changes in weed control practices that speed the evolution of resistance. What do you plan to do during the next reporting period to accomplish the goals?We plan to continue research to further characterize the mechanisms of herbicide resistance within this population. Preliminary research suggests that these populations can rapidly metabolize certain Group 15 herbicides via mechanisms not known to occur in tolerant corn.
Impacts
What was accomplished under these goals?
Previous research demonstrated that two Illinois waterhemp (Amaranthus tuberculatus) populations (CHR, SIR) are resistant to certain very-long-chain fatty acid (VLCFA)-inhibiting herbicides. The mechanism of resistance to the VLCFA-inhibitor, S-metolachlor, within the CHR and SIR populations was then documented enhanced metabolism when compared to sensitive waterhemp populations. Moreover, CHR and SIR metabolized S-metolachlor at the same rate as corn, which is naturally tolerant due to glutathione-S-transferase (GST)-mediated metabolism. The previous research did not, however, determine the enzymes responsible for metabolic resistance to S-metolachlor in CHR and SIR. Current experiments were designed to expand upon previous findings and directly investigate the metabolic enzymes involved in S-metolachlor resistance. GST- assays were initiated to compare the specific activities of GSTs from CHR, SIR, a sensitive waterhemp population (WUS), and corn when utilizing radiolabeled S-metolachlor as a substrate. Results found that protein extracts from resistant waterhemp did have increased GST-activity when compared to sensitive waterhemp, but much less than corn. Specific activities where 1.7-3 fold higher than WUS for CHR and SIR, respectively. Corn, however, possessed 2.6-3 fold higher GST-activity than SIR and CHR, respectively. Assays investigating P450 activity from waterhemp and corn microsomes were then initiated since the CHR and SIR waterhemp populations metabolize S-metolachlor as rapidly as corn, but do not have the GST-activity to match. Microsomes from the CHR and SIR populations possessed 21-28 fold higher specific activity than WUS with radiolabeled S-metolachlor as a substrate. CHR and SIR microsomal protein also oxidized S-metolachlor 30-39 fold more efficiently than corn. In addition, microsomes from the CHR and SIR populations formed a single major metabolite that was determined O-demethylated S-metolachlor via co-chromatography with a synthetic standard. Results demonstrate that the resistant CHR and SIR waterhemp populations have enhanced GST-activity in comparison to WUS as well as significantly higher abilities to oxidize S-metolachlor than WUS or corn. Overall, metabolic resistance to S-metolachlor in the resistant waterhemp populations involves both GST-mediated and oxidative metabolism, but the initial formation of O-demethylated S-metolachlor appears to be the predominant mechanism. Future research is planned to further investigate the metabolome of resistant waterhemp populations following treatment with S-metolachlor. A further understanding of the intricacies of S-metolachlor metabolism in waterhemp will be beneficial for the entire weed science community by aiding herbicide discovery efforts to avoid the discovered resistance mechanisms and guide future waterhemp management recommendations.
Publications
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Progress 10/01/18 to 09/30/19
Outputs
Target Audience:Results from this research have been shared with various weed management practitioners, including agronomic crop producers, agrichemical retail applicators, certified crop advisors, agronomic commodity organizations, professional weedscience societies, and several agricultural media organizations. Changes/Problems:
Nothing Reported
What opportunities for training and professional development has the project provided?This research has provided additional data on the evolution of herbicide resistance in waterhemp. These results have beenused in various training sessions with weed management practitioners that are designed to educate them on the evolution ofherbicide resistance in weed populations. How have the results been disseminated to communities of interest?These results have been shared in myriad presentations to weed management practitioners to illustrate the increasingoccurrence of multiple-resistant weed populations in order to foster changes in weed control practices that speed theevolution of resistance. What do you plan to do during the next reporting period to accomplish the goals?We plan to continue research to further characterize the mechanisms of herbicide resistance within this population. Current research suggests that these populations can rapidly metabolize certain Group 15 herbicides.
Impacts
What was accomplished under these goals?
Since its commercialization in the 1990's, S-metolachlor has been widely used preemergence (PRE) in crops such as corn, soybean, and cotton to control annual grasses and small-seeded dicot weed speciessuch as waterhemp (Amaranthus tuberculatus). Previously, we reported two multiple herbicide-resistant (MHR) waterhemp populations (MCR and CHR) from Illinois were not controlled with S-metolachlor under field conditions. Greenhouse dose-response experiments with S-metolachlor supported field observations and generated R/S ratios ranging from 18-64 fold compared with two sensitive populations. We hypothesized that a physiological mechanism within the plant, such as enhanced herbicide metabolism, was responsible for the reduced efficacy of S-metolachlor. Radiolabeled S-metolachlor was utilized to investigate herbicide metabolism in seedlings from the CHR and MCR populations in comparison to sensitive waterhemp (WUS and ACR) populations and corn. Thin-layer chromatography (TLC) experiments revealed that CHR and MCR seedlings metabolized S-metolachlor faster than either sensitive population between 2 and 24 hours after treatment. High-performance liquid chromatography (HPLC) experiments determined the times to degrade 50% (DT50) and 90% (DT90) of parent S-metolachlor in CHR and MCR were shorter than either sensitive waterhemp population but equal to corn. The calculated DT90 values for CHR, MCR, and corn are 3.2, 2.7, and 2.7 hours, respectively. In contrast, more than six hours were required for either WUS or ACR to metabolize 90% of the parent herbicide. TLC and HPLC experiments also revealed that metabolite profiles in CHR and MCR differ from sensitive waterhemp or corn. The current corroboration of field, greenhouse, and laboratory experiments suggests CHR and MCR have evolved metabolic resistance to S-metolachlor. Research is underway to further quantify and identify initial metabolites formed and investigate the putative enzyme(s) and metabolic pathway(s) involved in S-metolachlor detoxification in waterhemp.
Publications
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Progress 04/20/18 to 09/30/18
Outputs
Target Audience:Results from this research have been shared with various weed management practitioners, including agronomic crop producers, agrichemical retail applicators, certified crop advisors, agronomic commodity organizations, professional weed science societies,and several agricultural media organizations. Changes/Problems:
Nothing Reported
What opportunities for training and professional development has the project provided?This research has provided additional data on the evolution of herbicide resistance in waterhemp. These results have been used in various training sessions with weed management practitioners that are designed to educate them on the evolution of herbicide resistance in weed populations. How have the results been disseminated to communities of interest?These results have been shared in myriad presentations to weed management practitioners to illustrate the increasing occurrence of multiple-resistant weed populations in order to foster changes in weed control practices that speed the evolution of resistance. What do you plan to do during the next reporting period to accomplish the goals?We plan to continue research to further characterize the mechanisms of herbicide resistance within this population. Preliminary research suggests that these populations can rapidly metabolize certain Group 15 herbicides.
Impacts
What was accomplished under these goals?
Group 15 herbicides, although discovered in the 1950's, remain an important resource for preemergence (PRE) control of annual grasses and small-seeded broadleaves. Previous and ongoing research with a five-way resistant population of waterhemp (Amaranthus tuberculatus) from Champaign County, Illinois (designated CHR) demonstrates that Group 15 herbicides alone are not effective for PRE control of the population. Acetochlor, alachlor, and pyroxasulfone provide the greatest PRE control of CHR under field conditions, while S-metolachlor and dimethenamid-P provided significantly less control. A similar observation had been previously reported for another multiple herbicide-resistant (MHR) waterhemp population from Mclean County, Illinois (designated MCR). Since both CHR and MCR are resistant to s-triazine, HPPD-, and ALS-inhibiting herbicides, the objectives of this research were to compare CHR and MCR to other waterhemp populations in a controlled growth environment and investigate a possible association among the various known resistances and Group 15 efficacy. Progeny generated from each MHR population (CHR-M6 and MCR-NH40) were compared to another MHR waterhemp population from Illinois (ACR; s-triazine, ALS- and PPO-inhibitor resistant) and a known herbicide-sensitive population (WUS) under greenhouse conditions for their responses to four Group 15 active ingredients. Based on biomass reduction (GR50) values, calculated resistant-to-sensitive ratios (R/S) between CHR-M6 and WUS were 7.5, 6.1, 5.5, and 2.9 for S-metolachlor, acetochlor, dimethenamid-P, and pyroxasulfone, respectively. R/S ratios between CHR-M6 and WUS were larger when calculated using seedling survival (LD50) and values were greater for MCR-NH40 than CHR-M6. ACR was the most sensitive to all Group 15 herbicides tested. Results from these greenhouse studies complement and corroborate previous findings from the field. Future research is planned to further investigate the CHR and MCR populations and determine whether an edaphic factor or a physiological factor, such as rapid metabolism, is responsible for the differences in activity among the Group 15 active ingredients tested under both environments.
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