Different energization mechanisms drive the vacuolar uptake of a flavonoid glucoside and a herbicide glucoside

Glycosylation of endogenous secondary plant products and abiotic substances such as herbicides increases their water solubility and enables vacuolar deposition of these potentially toxic substances. We characterized and compared the transport mechanisms of two glucosides, isovitexin, a native barley flavonoid C-glucoside and hydroxyprimisulfuron-glucoside, a herbicide glucoside, into barley vacuoles, Uptake of isovitexin 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 inhibitor of vacuolar H+-ATPase, but not by vanadate, Transport of isovitexin is strongly inhibited after dissipation of the Delta pH or the Delta psi across the vacuolar membrane. Uptake experiments with the heterologue flavonoid orientin and competition experiments with other phenolic compounds suggest that transport of flavonoid glucosides into barley vacuoles is specific for apigenin derivatives. In contrast, transport 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, Uptake 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'-adenylyl-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 glucoside may therefore be transported by an additional member of the ABC transporters.