Submitted to: American Journal of Plant Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/19/2013
Publication Date: 12/23/2013
Citation: Teaster, N.D., Hoagland, R.E. 2013. Varying tolerance to glyphosate in a population of Palmer amaranth with low epsps copy number. American Journal of Plant Sciences. 4(12):2400-2408, DOI:10.4236/ajps.2013.412297.
Interpretive Summary: Glyphosate-resistance in Palmer amaranth (Amaranthus palmeri) was first reported in 2006 and now is widespread in the southern U.S. where it has become the most troublesome weed in agronomic crops. Recently, elevated copy number of the EPSPS (target site of glyphosate) gene as was reported as the resistance mechanism in this weed. A Palmer amaranth population collected in the year 2000 in Mississippi (MS) was thought to be susceptible to glyphosate. Scientists in the USDA-ARS Crop Production Systems Research Unit, Stoneville, MS examined seedlings from this population in greenhouse tests, bioassays and individual plant testing and found that individual seedlings exhibited varying degrees of resistance to glyphosate. Whole plant spraying of glyphosate at field rates revealed that about 40 % of the plants were resistant. After initial bioassays and spray application, five plants with a range of tolerance to glyphosate were selected for asexual reproductive propagation (cloning) so that further studies could be performed on these individuals. Q-PCR analysis of these resistant clones indicated that EPSPS gene copy number was not elevated. Shikimate levels were lower in the resistant clones and higher in the susceptible ones, which correlated with variation of resistance demonstrated in bioassays and spray application to seedlings. Results demonstrate that individuals within a population can vary widely with respect to glyphosate resistance, suggesting that uptake, translocation, sequestration, metabolism or altered target site may contribute to the resistance in some individuals of this population as reported in some other resistant populations of Palmer amaranth. This knowledge is important to the elucidation of and characterization of herbicide resistance mechanisms in weeds.
Technical 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 was 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.