A. Stanley Culpepper

Gene amplification confers glyphosate resistance in Amaranthus palmeri

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-F2 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.

Measuring Volatile Emissions from Mulch Covered Vegetable Beds

Reducing chloropicrin gas emissions has become a significant concern for the EPA. Buffer zones are being discussed that could reduce the amount of land available to produce commercial vegetables. A study was implemented to determine the amount of chloropicrin released through plastic mulch in commercial vegetable production in the southeastern United States. A method was developed to measure fumigant gases passing through plastic mulches. Various formulations and combinations of chloropicrin fumigants used in commercial vegetable production were evaluated. Low density polyethylene (LDPE) and low impermeable films were compared for chloropicrin gas emission. Chloropicrin gas emissions were not impacted by formulation but emissions were reduced 24 to 88% when using a high barrier film compared to LDPE film.