Glyphosate tolerance and gene flow in common lambsquarters (Chenopodium album L.)

Common lambsquarters (Chenopodium album L.) is one of the world's worst weeds. In recent years, tolerance to glyphosate has been confirmed in several accessions in the United States. Research was conducted from 2007 to 2011 to determine the mechanism of tolerance, the inheritance of the tolerance trait, the potential for gene flow of tolerance alleles, and the role of glyphosate use history on the sensitivity of common lambsquarters to glyphosate. Comparison of a glyphosate-tolerant Indiana accession with a sensitive Wisconsin accession showed that tolerance was conferred by altered translocation away from meristematic tissues where the target enzyme ( 5-enolpyruvylshikimate-3-phosphate synthase, EPSPS) is primarily expressed. The sensitivity, sequence, and protein expression for EPSPS did not differ between accessions. This is the first reported characterization of the mechanism of glyphosate tolerance in common lambsquarters. Furthermore, the response to glyphosate of an F1 accession heterozygous for the tolerance trait suggests that the inheritance of the trait may be recessive. If so, it would be the first documented case of recessive inheritance of decreased sensitivity to glyphosate. Chenopodium giganteum D. Donn., a member of the C. album aggregate with a dominant magenta leaf axil phenotypic marker, was used as a pollen parent in gene flow experiments to determine the potential for gene flow of nuclear traits, such as those conferring most mechanisms of herbicide tolerance. Results confirmed a primarily self-pollinating breeding system in a typical crop environment with low levels (< 1%) of cross-pollination at distances of 15 m. These results suggest that gene flow may contribute to initial tolerance allele frequencies in previously sensitive populations which may be favored by selective pressure by herbicides. To determine if glyphosate use history was a factor in the evolution of common lambsquarters sensitivity to glyphosate, six Wisconsin accessions were evaluated after 10 yr of differential glyphosate use. Results showed no consistent differences at the whole-plant and target site levels and no difference in fecundity. However, considerable variability in target-site sensitivity was observed, which may constitute a trait upon which selection by glyphosate may act to confer other mechanisms of tolerance in addition to altered translocation.