Physiology of glyphosate-resistant johnsongrass and implications for management
Authors:   , Dixon Sarah E. 
Published: February 2023

[EMBARGOED UNTIL 12/1/2023] Glyphosate, as the formulated commercial herbicide Roundup�, was introduced by Monsanto in 1974. The broad-spectrum activity of glyphosate made it an excellent candidate for engineering herbicide-resistant crops. Following the introduction and widespread adoption of glyphosate resistant crops, the amount of glyphosate applied to agricultural land increased dramatically. In 2016, a producer reported a failure of glyphosate to control johnsongrass within an agronomic field in Buchanan County, MO to the herbicide manufacturer. In response to the reported failure of glyphosate to control johnsongrass, plant material was collected, and the objectives of the following research were to 1) determine if glyphosate resistance is present in the Buchanan County, MO population and 2) determine the mechanism(s) imparting resistance, if found; and 3) evaluate prospective management practices for this johnsongrass population to mitigate the spread of herbicide resistance. Johnsongrass plants were subjected to a greenhouse dose-response assay to assess the potential for glyphosate resistance, which was confirmed when the dose required to reduce aboveground biomass by 50 percent (GR50) was 1,073 g ae ha-1 for the putative glyphosate-resistant (gly-R) population versus 230 g for a comparative glyphosate-susceptible (gly-S) population. Potential non-target site resistance was assessed with a growth chamber study utilizing 14C-glyphosate applied to potted plants to determine potential differences in herbicide absorption and translocation. Following the application of a solution containing 14C-glyphosate to gly-R and gly-S plants in a growth chamber experiment, glyphosate absorption was not different between the two populations. However, at 96 hours after treatment (HAT), the gly-R population had translocated 36 percent of the absorbed herbicide out of the treated leaf versus 62 percent for the gly-S population, and 42 percent less 14C-glyphosate than the gly-S population to the root system. Potential target site mechanisms of glyphosate resistance were examined by sequencing fragments of the gene encoding 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS). EPSPS expression, and expression of the glyphosate efflux transporter EcABCC8 were evaluated by quantitative PCR (qPCR). Results of EPSPS sequencing identified no point mutations. One gly-R individual had 4.5-fold EPSPS expression relative to three gly-S plants. No evidence for EPSPS amplification was observed in the remaining four gly-R plants examined. Two plants were identified that overexpressed EcABCC8 2.2-fold compared to gly-S. Differential translocation of glyphosate is the primary mechanism of resistance to glyphosate in johnsongrass isolated from Buchanan County, MO. In addition, overexpression of EPSPS and EcABCC8 may also contribute to glyphosate resistance within this population. In field experiments with soybean conducted in Buchanan County, Missouri in 2020 and 2021, weed density and biomass data confirmed that grass weed control from glufosinate consistently outperformed glyphosate at both study locations. While some treatments resulted in excellent control of gly-R johnsongrass, POST only programs place significant pressure on single herbicide modes of action, and the consequences of such an approach are discussed. Additional field studies examined the sensitivity of roadside johnsongrass populations to glyphosate. The GR50 of roadside populations was plotted against the Euclidean distance of each population from the gly-R population in distance-decay models. The distance of roadside johnsongrass from the original gly-R population predicted the GR50 of roadside populations within 173 g ha-1 of glyphosate, and an increase in sensitivity to glyphosate was predicted when populations were collected at further distances from the gly-R source. A simulated roadside management study assessed the response of mature (1.5 m height) gly-R johnsongrass to fall-applied chemical weed control options suitable for roadside applications. When herbicides were applied to mature plants in the fall, results indicated that all treatments reduced the number of shoots from rhizome in the spring. Rhizomes from plants treated with glyphosate alone at 1,736 or 3,473 g ai ha-1 produced biomass reduced only 27 to 39 percent compared to non-treated controls. While effective chemical control options for johnsongrass were identified in both crop and non-crop field studies, a combination of chemical, mechanical, and cultural practices is likely to be the most successful practice to limit the tolerance of johnsongrass populations to glyphosate.