Submitted to: Proceedings of the National Academy of Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/14/2009
Publication Date: 1/19/2010
Citation: Gaines, T., Preston, C., Leach, J., Chisholm, S., Shaner, D.L., Nissen, S., Bukin, B., Patzoldt, W., Tranel, P., Webster, T.M., Vencill, W.K., Sammons, D., Wang, D., Westra, P. 2010. Weed Evolution To Herbicide Resistance. Proceedings of the National Academy of Sciences. Gene Amplification Confers Glyphosate Resistance in Amaranthuis palmerii. PNAS 1007:1029-1034. Interpretive Summary: Weed management is critical for crop production and glyphosate plays a major role in controlling weeds in conventional and glyphosate-resistant crops. The continuous use of glyphosate in glyphosate resistant cotton has led to the selection of populations of Palmer amaranth, a serious weed in cotton production in the U.S., that are resistant to glyphosate. The mechanism of glyphosate resistance in this Palmer amaranth biotype is greatly elevated levels of the target site for the herbicide due to gene duplication. Although this is the first known occurrence of gene amplification as an herbicide resistance mechanism in a naturally occurring weed population, it is significant since it could threaten the sustainable use of glyphosate resistant crop technology.
Technical Abstract: The herbicide glyphosate became widely used in the U.S. and other parts of the world following the introduction of glyphosate-resistant crops. These crops were created by introduction of a modified 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene, the herbicide target site. Increased use of glyphosate over multiple years imposes selective genetic pressure on weed populations. We investigated recently discovered glyphosate-resistant Amaranthus palmeri populations from Georgia, in comparison to normally sensitive populations. EPSPS enzyme activity from resistant and susceptible plants was equally inhibited by glyphosate, which led us to use quantitative PCR to measure relative copy numbers of the EPSPS gene. Genomes of resistant plants contained from 5-fold to over 160-fold more copies of the EPSPS gene than did genomes of susceptible plants. Quantitative reverse-transcription PCR on cDNA revealed that EPSPS expression was positively correlated with genomic EPSPS relative copy number. Immunoblot analyses showed that increased EPSPS protein level also correlated with EPSPS genomic copy number. EPSPS gene amplification was heritable, correlated with resistance in pseudo-F2 populations, and is proposed to be the molecular basis of glyphosate resistance. This is the first known occurrence of gene amplification as an herbicide resistance mechanism in a naturally occurring weed population, and is particularly significant because it could threaten the sustainable use of glyphosate resistant crop technology.