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

Abstract

In many cases, the vacuolar uptake of secondary metabolites has been demonstrated to be strictly specific for a given compound and plant species. While most plants contain glycosylated secondary substances, few cases are known where flavonoids may also carry negative charges, e.g. as glucuronide conjugates. Vacuolar transport of glucosylated phenylpropanoid derivatives has been shown to occur by proton–substrate antiport mechanisms (Klein, M., Weissenböck, G., Dufaud, A., Gaillard, C., Kreuz, K., Martinoia, E., 1996. Different energization mechanisms drive the vacuolar uptake of a flavonoid glucoside and a herbicide glucoside. J. Biol. Chem. 271, 29 666–29 671). In contrast, flavone glucuronides appearing specifically in rye mesophyll vacuoles are taken up by direct energisation utilising MgATP, strongly arguing for the presence of an ATP-binding cassette (ABC) transporter belonging to the subfamily of multidrug resistance-associated proteins (MRP) on the rye vacuolar membrane (Klein, M., Martinoia, E., Hoffmann-Thoma, G., Weissenböck, G., 2000. A membrane-potential dependent, ubiquitous ABC-like transporter mediates the vacuolar uptake of rye flavone glucuronides — regulation of glucuronide uptake by glutathione and its conjugates. Plant Journal 21, 289–304). MRPs are known to transport negatively charged organic anions. Results presented here suggest that the vacuolar directly energised MRP-like glucuronate pump for plant-specific flavone glucuronides is ubiquitously present in diverse plant species since rye flavone glucuronides are taken up into vacuoles isolated from the barley mesophyll or from the broccoli stalk parenchyma representing two species which do not synthesise glucuronidated secondary compounds. According to the transport characteristics and inhibition profile observed we propose the existence of a high-capacity, uncoupler-insensitive vacuolar ABC transporter for flavone glucuronides and possibly other negatively charged organic compounds — plant-born or xenobiotic — irrespective of the plant’s capability to endogenously produce glucuronidated compounds.