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item Koger Iii, Clifford
item Shaner, Dale

Submitted to: Proceedings of Southern Weed Science Society
Publication Type: Abstract Only
Publication Acceptance Date: 3/1/2005
Publication Date: 5/1/2005
Citation: Koger III, C.H., Shaner, D.L. 2005. Rapid, non-destructive assays for screening potential glyphosate-resistant weed populations. Proceedings of Southern Weed Science Society.

Interpretive Summary:

Technical Abstract: Repeated use of glyphosate over years in GR crops and non-crop areas has resulted in the selection of weeds resistant to glyphosate. Resistance to glyphosate has been documented in six species worldwide. In the U.S., biotypes of horseweed (Conyza canadensis) resistant to glyphosate have been confirmed in thirteen states east of the Mississippi river. The mechanism of glyphosate resistance has been found to be limited translocation or an alteration of 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) in resistant plants depending on biotype and species. Glyphosate blocks the EPSPS pathway in susceptible plants, resulting in an increase in the amino acid precursor shikimate. A quick and relatively simple method to confirm glyphosate resistance in suspected populations would be useful to growers and consultants, and would help to reduce the spread of resistant populations through quicker implementation of alternative weed management practices. Two rapid, non-destructive assays were developed and tested for their potential in differentiating glyphosate-resistant from glyphosate-susceptible plants. In one assay, leaves of glyphosate-resistant and -susceptible corn (Zea mays), cotton (Gossypium hirsutum), and soybean (Glycine max) plants as well as glyphosate-resistant and -susceptible horseweed plants were dipped in solutions of 0, 300, 600, and 1200 mg ae L-1 glyphosate for 3 d and subsequent injury was evaluated. In the second assay, the sensitivity of the EPSPS enzyme was evaluated in vivo by incubating excised leaf-disc tissue from the same plants used in the first assay in 0.7, 1.3, 2.6, 5.3, 10.6, 21.1, 42.3, and 84.5 mg ae L-1 glyphosate solutions for 16 h and measuring shikimate levels with a spectrophotometer. The leaf-dip assay differentiated between glyphosate-resistant and -susceptible crops and horseweed biotypes. The 600 mg L-1 rate of glyphosate was more consistent in differentiating resistant and susceptible plants compared with the 300 and 1200 mg L-1 rates. The in vivo EPSPS assay detected significant differences between susceptible and glyphosate-resistant plants of all species. Shikimate accumulated in a glyphosate dose-dependent manner in leaf discs from susceptible crops, but shikimate did not accumulate in leaf discs from resistant crops and levels were similar to nontreated leaf-discs. Shikimate accumulated at high (> 21.1 mg ae L-1) concentrations of glyphosate in leaf discs from all horseweed biotypes. Shikimate accumulated at low glyphosate concentrations (< 10.6 mg L-1) in leaf discs from susceptible horseweed biotypes, but not in resistant biotypes. Both assays were able to differentiate resistant from susceptible biotypes of horseweed and may have utility for screening other weed populations for resistance to glyphosate.