The ABC-like vacuolar transporter for rye mesophyll flavone glucuronides is not species-specific

In many cases, the vacuolar uptake of secondary metabolites has been demons trated to be strictly specific for a given compound and plant species. Whil e most plants contain glycosylated secondary substances, few cases are know n where flavonoids may also carry negative charges, e.g. as glucuronide con jugates. Vacuolar transport of glucosylated phenylpropanoid derivatives has been shown to occur by proton-substrate antiport mechanisms (Klein, M., We issenbock, G., Dufaud, A., Gaillard, C., Kreuz, K., Martinoia, E., 1996. Di fferent energization mechanisms drive the vacuolar uptake of a flavonoid gl ucoside and a herbicide glucoside. J. Biol. Chem. 271, 29 666-29 671). In c ontrast, flavone glucuronides appearing specifically in rye mesophyll vacuo les are taken up by direct energisation utilising MgATP, strongly arguing f or the presence of an ATP-binding cassette (ABC) transporter belonging to t he subfamily of multidrug resistance-associated proteins (MRP) on the rye v acuolar membrane (Klein, M., Martinoia, E., Hoffmann-Thoma, G., Weissenbock , G., 2000. A membrane-potential dependent, ubiquitous ABC-like transporter mediates the vacuolar uptake of rye flavone glucuronides - regulation of g lucuronide uptake by glutathione and its conjugates. Plant Journal 21, 289- 304). MRPs are known to transport negatively charged organic anions. Result s presented here suggest that the vacuolar directly energised MRP-like gluc uronate pump for plant-specific flavone glucuronides is ubiquitously presen t in diverse plant species since rye flavone glucuronides are taken up into vacuoles isolated from the barley mesophyll or from the broccoli stalk par enchyma representing two species which do not synthesise glucuronidated sec ondary compounds. According to the transport characteristics and inhibition profile observed we propose the existence of a high-capacity, uncoupler-in sensitive vacuolar ABC transporter for flavone glucuronides and possibly ot her negatively charged organic compounds - plant-born or xenobiotic irrespe ctive of the plant's capability to endogenously produce glucuronidated comp ounds. (C) 2001 Elsevier Science Ltd. All rights reserved