Abstract
BackgroundIt is known that excess reducing equivalents in the form of NADPH in chloroplasts can be transported via shuttle machineries, such as the malate-oxaloacetate (OAA) shuttle, into the mitochondria, where they are efficiently oxidised by the mitochondrial alternative oxidase (AOX) respiratory pathway. Therefore, it has been speculated that the AOX pathway may protect plants from photoinhibition, but the mechanism by which this protection occurs remains to be elucidated.ResultsThe observation that the malate-OAA shuttle activity and the AOX pathway capacity increased markedly after intense light treatment in Rumex K-1 leaves indicates that excess NADPH was transported from the chloroplasts and oxidised by the AOX pathway. The inhibition of the AOX pathway by salicylhydroxamic acid (SHAM) caused the over-reduction of the photosystem I (PSI) acceptor side, as indicated by the increases in the extent of reduction of P700+. Furthermore, the photosynthetic linear electron flow was restricted, which was indicated by the decreases in the PSII electron transport rate (ETR) and the photosynthetic O2 evolution rate. The restriction of the photosynthetic linear electron flow, which generates the thylakoid ΔpH, inevitably decreased the de-epoxidation of the xanthophyll cycle (ΔPRI). Therefore, the induction of non-photochemical quenching (NPQ) was suppressed when the AOX pathway was inhibited. The effect of the inhibition of the AOX pathway on NPQ induction was less at 20 mM NaHCO3 than at 1 mM NaHCO3. The suppression of NPQ induction by the inhibition of the AOX pathway was also observed during the induction phase of photosynthesis. In addition, the inhibition of the AOX pathway increased the accumulation of hydrogen peroxide (H2O2), suggesting that the AOX pathway functions as an antioxidant mechanism.ConclusionsThe inhibition of the AOX pathway resulted in the rapid accumulation of NADPH in the chloroplasts, which caused the over-reduction of the PSI acceptor side. Furthermore, the restriction of the photosynthetic linear electron flow due to the inhibition of the AOX pathway limited the generation of the thylakoid ΔpH and suppressed the induction of NPQ. Therefore, the mitochondrial AOX pathway protected the photosynthetic apparatus against photodamage by alleviating the over-reduction of the PSI acceptor side and accelerating the induction of NPQ in Rumex K-1 leaves.
Highlights
It is known that excess reducing equivalents in the form of NADPH in chloroplasts can be transported via shuttle machineries, such as the malate-oxaloacetate (OAA) shuttle, into the mitochondria, where they are efficiently oxidised by the mitochondrial alternative oxidase (AOX) respiratory pathway
The results demonstrated that the treatments with 0, 0.2, 0.6 or 1 mM salicylhydroxamic acid (SHAM) had no direct effects on the actual PSII photochemical efficiencies (FPSII), the photochemical quenching coefficients or the nonphotochemical quenching (NPQ) in intact chloroplasts isolated from Rumex K-1 leaves (Figure 2), suggesting that the concentrations of SHAM used in this study had no direct effects on photosynthetic behaviours
The effects of SHAM treatments on the AOX pathway capacities and the NADP-malate dehydrogenase (NADP-MDH) initial activities in Rumex K-1 leaves To determine a working concentration of the inhibitor for this study, we examined the effects of the inhibitor on the capacities of the AOX pathways in Rumex K-1 leaves in the dark or in intense light
Summary
It is known that excess reducing equivalents in the form of NADPH in chloroplasts can be transported via shuttle machineries, such as the malate-oxaloacetate (OAA) shuttle, into the mitochondria, where they are efficiently oxidised by the mitochondrial alternative oxidase (AOX) respiratory pathway. It has been shown that excess reducing equivalents in the form of NADPH generated by photosynthesis can be transported to the cytosol, peroxisomes and mitochondria via shuttle machineries, such as the malate-oxaloacetate (OAA) shuttle [17,18,19,20], where they are oxidised in metabolic pathways under photoinhibitory conditions. The AOX pathway is a non-phosphorylating pathway and can efficiently oxidise the reducing equivalents generated in the chloroplasts without being restricted by the proton gradient across the mitochondrial inner membrane or the cellular ATP/ADP ratio. It has been speculated that the AOX pathway may play a particular role in protecting plants from photoinhibition [16,18,25,26,27,28]; the exact mechanism by which the mitochondrial AOX pathway alleviates photoinhibition remains to be elucidated
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