Flavone Glucoside Uptake into Barley Mesophyll and ArabidopsisCell Culture Vacuoles. Energization Occurs by H+-Antiportand ATP-Binding Cassette-Type Mechanisms

摘要

In many cases, secondary plant products accumulate in the large central vacuole of plant cells. However, the mechanisms involved in the transport of secondary compounds are only poorly understood. Here, we demonstrate that the transport mechanisms for the major barley (Hordeum vulgare) flavonoid saponarin (apigenin 6-C-glucosyl-7-O-glucoside) are different in various plant species: Uptake into barley vacuoles occurs via a proton antiport and is competitively inhibited by isovitexin (apigenin 6-C-glucoside), suggesting that both flavone glucosides are recognized by the same transporter. In contrast, the transport into vacuoles from Arabidopsis, which does not synthesize flavone glucosides, displays typical characteristics of ATP-binding cassette transporters. Transport of saponarin into vacuoles of both the species is saturable with a Km of 50 to 100 μm. Furthermore, the uptake of saponarin into vacuoles from a barley mutant exhibiting a strongly reduced flavone glucoside biosynthesis is drastically decreased when compared with the parentvariety. Thus, the barley vacuolar flavone glucoside/H⁺antiporter could be modulated by the availability of the substrate. Wepropose that different vacuolar transporters may be responsible for thesequestration of species-specific/endogenous and nonspecific/xenobioticsecondary compounds in planta.