title: Gene amplification confers glyphosate resistance in Amaranthus palmeri.
abstract: The herbicide glyphosate became widely used in the United States and other parts of the world after the commercialization of glyphosate-resistant crops. These crops have constitutive overexpression of a glyphosate-insensitive form of the herbicide target site gene, 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS). 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 with 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 more than 160-fold more copies of the EPSPS gene than did genomes of susceptible plants. Quantitative RT-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-F(2) populations, and is proposed to be the molecular basis of glyphosate resistance. FISH revealed that EPSPS genes were present on every chromosome and, therefore, gene amplification was likely not caused by unequal chromosome crossing over. This occurrence of gene amplification as an herbicide resistance mechanism in a naturally occurring weed population is particularly significant because it could threaten the sustainable use of glyphosate-resistant crop technology.
authors.affiliation: Department of Bioagricultural Sciences and Pest Management, Colorado State University, Fort Collins, CO 80523, USA.
authors.forename: Todd A
authors.initials: TA
authors.lastname: Gaines
authors.forename: Wenli
authors.initials: W
authors.lastname: Zhang
authors.forename: Dafu
authors.initials: D
authors.lastname: Wang
authors.forename: Bekir
authors.initials: B
authors.lastname: Bukun
authors.forename: Stephen T
authors.initials: ST
authors.lastname: Chisholm
authors.forename: Dale L
authors.initials: DL
authors.lastname: Shaner
authors.forename: Scott J
authors.initials: SJ
authors.lastname: Nissen
authors.forename: William L
authors.initials: WL
authors.lastname: Patzoldt
authors.forename: Patrick J
authors.initials: PJ
authors.lastname: Tranel
authors.forename: A Stanley
authors.initials: AS
authors.lastname: Culpepper
authors.forename: Timothy L
authors.initials: TL
authors.lastname: Grey
authors.forename: Theodore M
authors.initials: TM
authors.lastname: Webster
authors.forename: William K
authors.initials: WK
authors.lastname: Vencill
authors.forename: R Douglas
authors.initials: RD
authors.lastname: Sammons
authors.forename: Jiming
authors.initials: J
authors.lastname: Jiang
authors.forename: Christopher
authors.initials: C
authors.lastname: Preston
authors.forename: Jan E
authors.initials: JE
authors.lastname: Leach
authors.forename: Philip
authors.initials: P
authors.lastname: Westra
mesh_terms: D051229:3-Phosphoshikimate 1-Carboxyvinyltransferase / Q000235:genetics; D027721:Amaranthus / Q000201:enzymology / Q000235:genetics*; D018076:DNA, Complementary; D005784:Gene Amplification; D018628:Gene Dosage; D005998:Glycine / Q000031:analogs & derivatives*; D006540:Herbicides; D008969:Molecular Sequence Data; D012765:Shikimic Acid / Q000378:metabolism; D000097797:Glyphosate
chemical_list: D051229:3-Phosphoshikimate 1-Carboxyvinyltransferase; D018076:DNA, Complementary; D005998:Glycine; D006540:Herbicides; D012765:Shikimic Acid; D000097797:Glyphosate