title: The mitochondrial alternative oxidase pathway protects the photosynthetic apparatus against photodamage in Rumex K-1 leaves.
abstract: BACKGROUND
  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. 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.
  
  
  RESULTS
  The 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.
  
  
  CONCLUSIONS
  The 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.
authors.affiliation: State Key Laboratory of Crop Biology, Shandong Agricultural University, Tai'an 271018, Shandong, China.
authors.forename: Li-Tao
authors.initials: LT
authors.lastname: Zhang
authors.forename: Zi-Shan
authors.initials: ZS
authors.lastname: Zhang
authors.forename: Hui-Yuan
authors.initials: HY
authors.lastname: Gao
authors.forename: Xiang-Long
authors.initials: XL
authors.lastname: Meng
authors.forename: Cheng
authors.initials: C
authors.lastname: Yang
authors.forename: Jian-Guo
authors.initials: JG
authors.lastname: Liu
authors.forename: Qing-Wei
authors.initials: QW
authors.lastname: Meng
mesh_terms: D002734:Chlorophyll / Q000378:metabolism; D002736:Chloroplasts / Q000187:drug effects / Q000378:metabolism; D004579:Electron Transport; D004789:Enzyme Activation; D006861:Hydrogen Peroxide / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D008027:Light; D050538:Malate Dehydrogenase (NADP+) / Q000378:metabolism; D008928:Mitochondria / Q000201:enzymology*; D024101:Mitochondrial Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D009249:NADP / Q000378:metabolism; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D010100:Oxygen / Q000378:metabolism; D010788:Photosynthesis; D045331:Photosystem I Protein Complex / Q000378:metabolism; D045332:Photosystem II Protein Complex / Q000378:metabolism; D018515:Plant Leaves / Q000187:drug effects / Q000201:enzymology / Q000528:radiation effects*; D010940:Plant Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D031804:Rumex / Q000187:drug effects / Q000201:enzymology* / Q000528:radiation effects; D012457:Salicylamides / Q000494:pharmacology; D017693:Sodium Bicarbonate / Q000494:pharmacology; D000099188:Alternative Oxidase
chemical_list: D002734:Chlorophyll; D006861:Hydrogen Peroxide; D050538:Malate Dehydrogenase (NADP+); D024101:Mitochondrial Proteins; D009249:NADP; D010088:Oxidoreductases; D010100:Oxygen; D045331:Photosystem I Protein Complex; D045332:Photosystem II Protein Complex; D010940:Plant Proteins; D012457:Salicylamides; D017693:Sodium Bicarbonate; D000099188:Alternative Oxidase; C005703:salicylhydroxamic acid