Quiz

• Photosynthesis


  Process that fixes atmospheric carbon dioxide and produces carbohydrates
• Respiration


  Process that catabolizes carbohydrates to produce ATP and reductants
• In illuminated leaves


  Intracellular metabolism is dynamically modulated depending on environmental changes
• The function of chloroplasts and mitochondria is closely coordinated in illuminated leaves
• Mitochondria in higher plants and green algae

  • Possess many unique components that do not exist in mammalian mitochondria
  • Contain a glycine decarboxylase complex (GDC) involved in the photorespiratory pathway
  • Respiratory electron transport chain consists of phosphorylating and non-phosphorylating pathways
• Non-phosphorylating pathways


  Involve alternative oxidase (AOX) and type II NAD(P)H dehydrogenases (NDs)
• The physiological significance of the energy-wasteful non-phosphorylating respiratory pathways is still not fully understood
• Many components of the non-phosphorylating pathways are induced and/or up-regulated under stress conditions, suggesting they play an important role for plant acclimation
• The non-phosphorylating pathways are considered to be efficient dissipation systems for excess reductants produced in the chloroplasts under high light conditions
• Excess light energy


  Induces generation of reactive oxygen species (ROS) that can cause photo-oxidative damage to the photosynthetic apparatus
• Dissipation systems for excess light energy in chloroplasts

  • Thermal dissipation of light energy by conformational changes in photosystem II
  • Xanthophyll cycle induced by photosystem I cyclic electron flow
  • Supplying reducing power of ferredoxin to nitrite reductase and/or glutamine:2-oxoglutarate aminotransferase
  • Exporting reducing power of NADPH to other cellular compartments
• Malate-oxaloacetate (Mal-OAA) shuttle


  Important system for transporting reductants out of the chloroplasts
• NADP-dependent malate dehydrogenase (NADP-MDH) in the chloroplast stroma is activated by the light-dependent ferredoxin-thioredoxin system
• Activated NADP-MDH


  Produces malate and NADP+ from NADPH and oxaloacetate
• Malate can be exported to the cytosol via malate transporters on the chloroplast membrane
• NADP-MDH over-expression


  Oxidizes the acceptor side of photosystem I
• High CO2 conditions


  Decreases NADP-MDH activation and the reduced fraction of the photosystem I acceptor side
• Mal-OAA shuttle


  An important system for reductant transport
• NADP-dependent malate dehydrogenase (NADP-MDH)

  • Activated by the light-dependent Fd-thioredoxin (Trx) system
  • Produces Mal and NADP+ from NADPH and OAA
• NADP-MDH over-expressing potato leaves

  Acceptor side of PSI was more oxidized than in wild type and NADP-MDH under-expressing ones
• Under high CO2 conditions in potato leaves

  NADPH is more efficiently consumed by the CO2 assimilation process, thereby decreasing the NADP-MDH activation state and the reduced fraction of the acceptor side of PSI
• NADP-MDH plays a key role in recycling NADPH, especially when its production is in excess of the requirement for the photosynthetic CO2 assimilation
• 1. oxoglutarate/malate transporter (OMT)


  Has a high affinity to Mal, OAA, and 2-oxoglutarate (2-OG), and is a suitable candidate as a Mal-OAA exchanging transporter
• Dicarboxylate transporter (DCT)


  Can subsequently re-import Mal by exporting glutamate (Glu)
• Glu export by DCT

  Essential for efficient assimilation of NH4+ generated from photorespiration and NO2- reduction
• Slow growth of OMT-deficient Arabidopsis can be recovered by growing the plants under high CO2 conditions
• Allocation of carbon to amino acid synthesis was impaired, and protein contents decreased, in OMT (DiT1)-antisense tobacco
• OMT has a function in the transport of the carbon skeleton to N assimilation
• It is still unclear whether OMT is truly involved in the Mal-OAA shuttle and functions as the exporter of reductants
• Although DCT (DiT2) can transport Mal and OAA, it is likewise unknown if this function is important in vivo
• Exported Mal is oxidized by NAD-dependent MDH (NAD-MDH) localized in the cytosol, peroxisomes, and mitochondria, which generates NADH
• This NADH is subsequently consumed by the reduction of NO3-, photorespiration, and mitochondrial respiration
• Leaves of a GDC-deficient barley mutant

  Have a lower capacity for photorespiration and chloroplast ATP/ADP and NADPH/NAD ratios were higher than those in wild type, suggesting that reductants are exported from the chloroplasts via a Mal-OAA shuttle
• In the Arabidopsis pgr5 mutant

  Non-photochemical quenching (NPQ) under high light condition was not induced, as a result of the lack of the Fd-dependent PSI-CEF pathway, and both the activity and activation state of NADP-MDH were higher when compared to wild-type plants
• The results of these two mutants indicate that the Mal-OAA shuttle plays a role in the dissipation of excess reductants in the chloroplasts
• Triose phosphate-phosphate transporter (TPT)


  Exchanges TP (e.g., dihydroxyacetone phosphate, DHAP) with phosphate (Pi)
• In illuminated leaves


  TP is exported from the chloroplasts to the cytosol and used for sucrose synthesis
• In leaves of an Arabidopsis TPT-lacking mutant

  Starch accumulated in the chloroplasts, and both photosynthesis and growth decreased
• TPT imports Pi into the chloroplast, and this is essential for optimal photo-phosphorylation in the chloroplasts of illuminated leaves
• TPT can also indirectly exchange TP with 3-phosphoglyceric acid (3-PGA)


  In this case, excess reductants are exported from the chloroplasts in the form of TP