The Role of Ascorbate Free Radical as an Electron Acceptor to Cytochrome 6-Mediated Trans-Plasma Membrane Electron Transport in Higher Plants'

In highly purified plasma membrane fractions of at least six higher plant species, a specific high-potential b-type Cyt (redox potential at pH 7.0 between +120 and +160 mV, a band at 560-561 nm) has been detected (Asard et al., 1989; Askerlund et al., 1989). This component is reducible in vitro by sodium ascorbic acid and constitutes 60 to 80% of the total Cyt amount detectable in plant plasma membranes. Suggestions have been made conceming the possible role of this redox component in plasma membrane redox reactions (Buckhout and Luster, 1991; Crane et al., 1991); however, little experimental evidence is available so far to support these ideas. Recently we have used plant plasma membrane vesicles loaded with the electron donor sodium ascorbate to investigate the possible involvement of the Cyt in trans-membrane electron transport reactions (Asard et al., 1992). a-Band absorption measurements demonstrated that the Cyt was reduced by ascorbate trapped inside the vesicles and that it was likely to act as a carrier in electron transfer from intemal ascorbate to an artificial extemal electron acceptor such as ferricyanide (Asard et al., 1992). This transmembrane electron transport model gives rise to new ideas conceming the physiological role of the Cyt and, in particular, raises questions regarding the possible nature of the in vivo electron acceptor. Recent demonstrations of the production of active (reduced) oxygen molecules upon elicitation of plant cells point to a possible role of molecular oxygen as an electron acceptor during plasma membrane electron transport (Doke and