TITLE:
Varying Tolerance to Glyphosate in a Population of Palmer Amaranth with Low EPSPS Gene Copy Number
AUTHORS:
Neal D. Teaster, Robert E. Hoagland
KEYWORDS:
Amaranthus palmeri; EPSPS (5-Enolpyruvylshikimate-3-phosphate Synthase); EPSPS Gene Copy Number; Glyphosate-Resistance; Herbicide Tolerance; Population Variance; Pigweed
JOURNAL NAME:
American Journal of Plant Sciences,
Vol.4 No.12,
December
23,
2013
ABSTRACT: A Palmer amaranth population (seeds collected in the year 2000; Washington Co., MS) suspected to be susceptible to glyphosate was examined as a population and as individual plants and found to exhibit varying tolerance or resistance to glyphosate. Whole plant spraying of glyphosate (0.84 kg·ha?1) to the population revealed that approximately 40% of this population were resistant to glyphosate and an LD50 of 0.75 kg·ha?1 was determined. Spray application of glyphosate indicated that some plants displayed varying degrees of resistance 14 days after treatment. Initial tests using leaf disc bioassays on 10 individual plants selected randomly from the population, allowed characterization of glyphosate resistance using both visual ratings of injury and quantitative measurement via chlorophyll content analysis. After initial bioassays and spray application, five plants with a range of tolerance to glyphosate were selected for cloning so that further studies could be accomplished on these individuals. Q-PCR analysis of these clones showed that resistance was not due to elevated EPSPS gene copy number. Shikimate levels were lower in the resistant and higher in the susceptible clones which correlated with varying degrees of resistance demonstrated in bioassays and spray application of glyphosate of these clones. Results demonstrate that individuals in a population can vary widely with respect to herbicide resistance and suggest that uptake, translocation, sequestration, metabolism or altered target site may contribute to the resistance in some individuals of this population.