Electron currents mediated by tonoplast cytochromes b561.

Ascorbate (ASC) is a key redox buffer in plant cells, whose antioxidant capacity depends on its balance with monodehydroascorbate (MDHA), its one-electron oxidation product. In the cytoplasm of Arabidopsis mesophyll cells, ASC is present at high concentrations and interacts with enzymes that oxidize it to MDHA, such as ascorbate peroxidases, as well as with enzymes that regenerate it, like NAD(P)H-dependent MDHA oxidoreductases (MDHAR) and glutathione-dependent dehydroascorbate reductases (DHAR). In vacuoles, ASC is found at lower concentrations and vacuoles lack these enzymes, but it can still undergo non-enzymatic oxidation by phenoxy radicals generated by class III peroxidases. It has been discovered that vacuoles isolated from Arabidopsis mesophyll cells contain an electron transport system that functionally connects the cytoplasmic and vacuolar ASC pools, acting as a transmembrane MDHA oxidoreductase dependent on Asc. Patch-clamp measurements have shown that electron currents across the tonoplast depend on the presence of ASC as an electron donor and MDHA or ferricyanide as electron acceptors on opposite sides of the membrane. These electron currents are catalyzed by cytochrome b561 isoform A (CYB561A), a tonoplast redox protein with ASC-binding sites in both the cytoplasm and the vacuole, electrically connected by two heme b groups. The recent functional characterization of other members of the cytochrome b561 family underscores how these proteins are essential for cellular redox balance and metabolism, facilitating electron transport across membranes and supporting processes such as iron homeostasis, stress defence, and cell wall modifications, highlighting their fundamental role in plant physiology.