Location: Crop Production Systems Research Unit
Title: Characterization of glyphosate resistance in cloned Amaranthus palmeri plants Authors
Submitted to: Weed Biology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 10, 2013
Publication Date: March 1, 2014
Citation: Teaster, N.D., Hoagland, R.E. 2014. Characterization of glyphosate resistance in cloned Amaranthus palmeri plants. Weed Biology and Management. 14(1):1-10. Interpretive Summary: Palmer amaranth has evolved resistance to the herbicide glyphosate and is a serious problem of soybean, corn, and cotton in the southern U.S., causing substantial increased weed control costs and reductions in quality and yield. The scientists at the USDA-ARS, Crop Production Systems Research Unit, Stoneville, MS have examined and characterized individual plants of Palmer amaranth populations from MS and GA to gain insight on resistance mechanisms. A technique for cloning Palmer amaranth plants was developed so plants with the same genetic composition could be studied over multiple generations. A leaf disk bioassay and a digital imaging method were also designed to assess herbicidal injury. Using 40 cloned individual seedlings from four MS and GA populations, the sex and gene copy number of the molecular target site of glyphosate were determined. Glyphosate-resistant plants from GA possess multiple copies of this target site, as reported by others. We found that the copy numbers of the MS clones were higher than those of resistant plants from GA, but this did not result in elevated glyphosate resistance in the MS plants. Also there was no correlation of the sex of plants with increased resistance. Results suggest that although an elevated copy number of the target site gene can instill resistance, other factors may contribute to the overall glyphosate resistance level of this weed. Results also demonstrate that cloning and bioassay techniques are useful research tools to study herbicide resistance mechanisms in weeds.
Technical Abstract: Glyphosate resistant Palmer amaranth from Georgia (GA) possesses multiple copies of the target site, 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) of this herbicide. Cloned plants of glyphosate-resistant Palmer amaranth biotypes from Mississippi (MS) were compared with GA populations using leaf disk bioassays. Injury caused by exposure to various concentrations of glyphosate was analyzed visually and by digital imaging software. EPSPS copy number of the clones was determined by Q-PCR and showed that the resistant MS populations had higher copy numbers than those from the resistant GA population. However, higher copy number did not result in increased glyphosate resistance in the MS populations above that of the resistant GA population. Since this weed is dioecious, the sex of the cloned plants from MS was compared for possible interaction with glyphosate resistance, but no correlation of plant sex and the presence or level of resistance was found. Although an elevated copy number of the EPSPS gene instills resistance to glyphosate, other factors may contribute to the overall glyphosate resistance of Palmer amaranth in the MS populations tested.