Control of Ascorbate Synthesis by Respiration and Its Implications for Stress Responses

We show for the first time that respiration can control ascorbate (AA) synthesis in plants. Evidence for this control is provided by (a) the localization of l-galactono-1,4-lactone dehydrogenase (GalLDH), the terminal enzyme in AA biosynthesis, with mitochondrial complex I, and its regulation by electron transport through this complex, (b) the absolute requirement of the enzyme for oxidized cytochrome c (cyt c(ox)) as substrate, and (c) the coordinated response of respiration and AA synthesis to stress induced by hormone treatment. AA is a high abundance metabolite in plants with key roles in plant development and stress tolerance. Therefore, triggers that modulate respiration may also impact on AA production and hence stress responses. Unlike terminal enzymes of AA synthesis found in animals and fungi, plant GalLDH does not produce H2O2 as a by-product (Smirnoff, 2001). This might have evolved to avoid excessive oxidation of the mitochondrial electron transport chain (ETC) and damage to tricarboxylic acid cycle enzymes, because mitochondria are very susceptible to oxidative inhibition of function (Verniquet et al., 1991; Nulton-Persson and Szweda, 2001; Sweetlove et al., 2002). Moreover, recent evidence also suggests that the redox state of the plant mitochondrial ETC is critical in setting the whole cell redox stat (Dutilleul et al., 2003).