Affiliation:
1. Institute of Biophysics – CNR Via De Marini 6 16149 Genova Italy
2. Department of Pharmacy and Biotechnology (FaBiT) University of Bologna Via Irnerio 42 40126 Bologna Italy
3. Department of Earth, Environment and Life Sciences (DISTAV) University of Genoa Viale Benedetto XV 5 16132 Genova Italy
Abstract
Summary
Ascorbate (Asc) is a major redox buffer of plant cells, whose antioxidant activity depends on the ratio with its one‐electron oxidation product monodehydroascorbate (MDHA). The cytoplasm contains millimolar concentrations of Asc and soluble enzymes that can regenerate Asc from MDHA or fully oxidized dehydroascorbate. Also, vacuoles contain Asc, but no soluble Asc‐regenerating enzymes.
Here, we show that vacuoles isolated from Arabidopsis mesophyll cells contain a tonoplast electron transport system that works as a reversible, Asc‐dependent transmembrane MDHA oxidoreductase. Electron currents were measured by patch‐clamp on isolated vacuoles and found to depend on the availability of Asc (electron donor) and ferricyanide or MDHA (electron acceptors) on opposite sides of the tonoplast.
Electron currents were catalyzed by cytochrome b561 isoform A (CYB561A), a tonoplast redox protein with cytoplasmic and luminal Asc binding sites. The Km for Asc of the luminal (4.5 mM) and cytoplasmic site (51 mM) reflected the physiological Asc concentrations in these compartments. The maximal current amplitude was similar in both directions.
Mutant plants with impaired CYB561A expression showed no detectable trans‐tonoplast electron currents and strong accumulation of leaf anthocyanins under excessive illumination, suggesting a redox‐modulation exerted by CYB561A on the typical anthocyanin response to high‐light stress.
Cited by
3 articles.
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