Overexpression of a Native Gene Encoding 5-Enolpyruvylshikimate-3-Phosphate Synthase (EPSPS) May Enhance Fecundity in Arabidopsis thaliana in the Absence of Glyphosate

Premise of research. Strong environmental selection pressures can lead to rapid adaptation and the opportunity to study evolutionary dynamics in real time. A prime example is the recent evolution of resistance to the herbicide glyphosate, the active ingredient in Roundup, in more than 35 weed species. Mechanisms for glyphosate resistance include gene amplification and overproduction of its target enzyme, 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), but little is known about whether these genetic changes are associated with differing fitness in glyphosate-free environments. Such fitness effects could have major implications for anticipated changes in the frequency of resistance traits without continued exposure to the selective pressure. Methodology. We used transgenic Arabidopsis thaliana as a model system to test for the effects of overproducing EPSPS on plant growth and reproduction. In a previous study, we developed six independent transgenic lines that overexpress a native EPSPS gene driven by the CaMV35S promoter (designated OX) and seven independent empty vector lines (designated EV). Here, we compared phenotypic traits among these lines and their wild-type parents in greenhouse experiments. Pivotal results. Two of the OX lines produced 23%–37% more seeds per plant than the wild-type line, respectively, and none showed evidence of a fitness penalty. In contrast, the performance of the EV lines was similar to, or somewhat worse than, that of the wild-type line. Despite considerable variation among lines, the OX lines had greater fecundity than the wild-type or EV lines overall. Conclusions. Our results suggest that overproduction of EPSPS in Arabidopsis does not have a fitness cost and might confer a fitness benefit under the examined growth conditions. Further basic research on how surplus EPSPS affects plant growth is warranted. We hypothesize that similar effects could occur in weed species that overproduce EPSPS, but the few studies that address this question have shown mixed results and no evidence for a fitness benefit to date.