DIFFERENT ENERGIZATION MECHANISMS DRIVE THE VACUOLAR UPTAKE OF A FLAVONOID GLUCOSIDE AND A HERBICIDE GLUCOSIDE

Glycosylation of endogenous secondary plant products and abiotic subst ances such as herbicides increases their water solubility and enables vacuolar deposition of these potentially toxic substances. We characte rized and compared the transport mechanisms of two glucosides, isovite xin, a native barley flavonoid C-glucoside and hydroxyprimisulfuron-gl ucoside, a herbicide glucoside, into barley vacuoles, Uptake of isovit exin is saturable (K-m=82 mu M) and stimulated by MgATP 1.3-1.5-fold. ATP-dependent uptake was inhibited by bafilomycin Al, a specific inhib itor of vacuolar H+-ATPase, but not by vanadate, Transport of isovitex in is strongly inhibited after dissipation of the Delta pH or the Delt a psi across the vacuolar membrane. Uptake experiments with the hetero logue flavonoid orientin and competition experiments with other phenol ic compounds suggest that transport of flavonoid glucosides into barle y vacuoles is specific for apigenin derivatives. In contrast, transpor t of hydroxyprimisulfuron-glucoside is strongly stimulated by MgATP (2 .5-3 fold), not sensitive toward bafilomycin, and much less sensitive to dissipation of the Delta pH, but strongly inhibited by vanadate, Up take of hydroxyprimisulfuron-glucoside is also stimulated by MgGTP or MgUTP by about 2-fold. Transport of both substrates is not stimulated by ATP or Mg2+ alone, ADP, or the nonhydrolyzable ATP analogue 5'-aden ylyl-beta,gamma-imido-diphosphate. Our results suggest that different uptake mechanisms exist in the vacuolar membrane, a Delta pH-dependent uptake mechanism for specific endogenous flavonoid-glucosides, and a directly energized mechanism for abiotic glucosides, which appears to be the main transport system for these substrates. The herbicide gluco side may therefore be transported by an additional member of the ABC t ransporters.