Redox enzymes in the plant plasma membrane and their possible roles

Purified plasma membrane (PM) vesicles from higher plants contain redox proteins with low-molecular-mass prosthetic groups such as flavins (both FMN and FAD), hemes, metals (Cu, Fe and Mn), thiol groups and possibly naphthoquinone (vitamin K-1), all of which are likely to participate in redox processes. A few enzymes have already been identified: Monodehydroascorbate reductase (EC 1.6.5.4) is firmly bound to the cytosolic surface of the PM where it might be involved in keeping both cytosolic and, together with a b-type cytochrome, apoplastic ascorbate reduced. A malate dehydrogenase (EC 1.1.1.37) is localized on the inner side of the PM. Several NAD(P)H-quinone oxidoreductases have been purified from the cytocolic surface of the PM, but their function is still unknown. Different forms of nitrate reductase (EC 1.6.6.1-3) are found attached to, as well as anchored in, the PM where they may act as a nitrate sensor and/or contribute to blue-light perception, although both functions are speculative. Ferric-chelate-reducing enzymes (EC 1.6.99.13) are localized and partially characterized on the inner surface of the PM but they may participate only in the reduction of ferric-chelates in the cytosol. Very recently a ferric-chelate-reducing enzyme containing binding sites for FAD, NADPH and hemes has been identified and suggested to be a trans-PM protein. This enzyme is involved in the reduction of apoplastic iron prior to uptake of Fe2+ and is induced by iron deficiency. The presence of an NADPH oxidase, similar to the so-called respiratory burst oxidase in mammals, is still an open question. An auxin-stimulated and cyanide-insensitive NADH oxidase (possibly a protein disulphide reductase) has been characterized but its identity is still awaiting independent confirmation. Finally, the only trans-PM redox protein which has been partially purified from plant PM so far is a high-potential and ascorbate-reducible b-type cytochrome. In co-operation with vitamin K-1 and an NAD(P)H-quinone oxidoreductase, it may participate in trans-PM electron transport.