Higher plants are equipped with a remarkably versatile system that protects them from the potentially phytotoxic actions of xenobiotics, i.e. synthetic chemicals present in the plant's environment. Particularly striking is the natural tolerance of certain plants toward herbicides that profoundly affect closely related species. This phenomenon of herbicide selectivity is widely exploited in agriculture to control competing weeds in a field of crop plants that are tolerant to the particular herbicide. Herbicide selectivity is, in most cases, based primarily on the differential ability of plant species to metabolically detoxify the herbicide (Lamoureux et al., 1991; Cole, 1994). Metabolic herbicide inactivation has been employed in the genetic engineering of crops for herbicide tolerance (Hinchee et al., 1993). Finally, enhanced herbicide detoxification is one of the mechanisms of herbicide resistance, in addition to altered target site susceptibility and yet-unknown mechanisms, that may emerge within formerly susceptible weed species upon continuous use of the same herbicide or herbicide class (Holt et al., 1993). It is well documented that plants are able to metabolize and detoxify herbicides by a variety of enzymatic reactions and with extraordinary diversity among species. Furthermore, recent research has revealed that transporters in the vacuolar membrane mediate the energydependent export of herbicide metabolites into the large central vacuole (Martinoia et al., 1993).