Vacuolar glyphosate-sequestration correlates with glyphosate resistance in ryegrass (Lolium spp.) from Australia, South America and Europe: a 31P-NMR investigation

Glyphosate resistant (GR) ryegrass, Lolium spp., has now been found on five continents. While reduced translocation of glyphosate and altered target enzyme have been reported as the two main GR mechanisms in ryegrass, the cellular level mechanism responsible for reduced translocation remains to be elucidated. Glyphosate sensitive (GS) and GR ryegrass variants from Australia, Brazil, Chile, and Italy showing differing levels of glyphosate resistance were examined by 31P NMR. All variants showed similar cellular uptake of glyphosate. However, for all GR variants, glyphosate sequestration occurred within the cell vacuole while all GS variants showed minimal or no vacuole sequestration. Further, the extent of vacuole sequestration correlated qualitatively with the level of resistance. Previous 31P-NMR studies of horseweed (Conyza canadensis) revealed that glyphosate sequestration imparted glyphosate resistance. Data presented herein suggest that glyphosate vacuolar sequestration is a strongly contributing, if not the major contributing, resistance mechanism in ryegrass as well.