Abstract
In cyanobacteria, the photosynthetic prokaryotes, direct interaction between photosynthesis and respiration exists at plastoquinone (PQ) pool, which is shared by the two electron transport chains. Another possible point of intersection of the two electron transport chains is NADPH, which is the major electron donor to the respiratory chain as well as the final product of the photosynthetic chain. Here, we showed that the redox state of NADPH in the dark affected chlorophyll fluorescence induction in the cyanobacterium Synechocystis sp. PCC 6803 in a quantitative manner. Accumulation of the reduced NADPH in the dark due to the defect in type 1 NAD(P)H dehydrogenase complex in the respiratory chain resulted in the faster rise to the peak in the dark-to-light induction of chlorophyll fluorescence, while depletion of NADPH due to the defect in pentose phosphate pathway resulted in the delayed appearance of the initial peak in the induction kinetics. There was a strong correlation between the dark level of NADPH determined by its fluorescence and the peak position of the induction kinetics of chlorophyll fluorescence. These results indicate that photosynthesis interacts with respiration through NADPH, which enable us to monitor the redox condition of the acceptor side of photosystem I by simple measurements of chlorophyll fluorescence induction in cyanobacteria.
Highlights
Metabolic pathways are often separately described in simple schematic diagrams in textbooks
We used two cyanobacterial strains, in which the redox states of NADPH are different but those of the PQ pool are similar after dark-acclimation, in order to verify the effect of NADPH on chlorophyll fluorescence
In Δgnd, both NADPH and the PQ pool should be oxidized during dark-acclimation, because nicotinamide adenine dinucleotide phosphate (NADP)+ cannot be reduced without oxidative pentose phosphate (OPP) pathway and the PQ pool cannot be reduced as well in the absence of the electron donor to the respiratory chain
Summary
Metabolic pathways are often separately described in simple schematic diagrams in textbooks. Interactions between different metabolic pathways through common metabolites are universally observed. These interactions may not be so conspicuous in eukaryote, much stronger interaction is observed in prokaryotes, which have no organelle to compartment metabolic pathways. Respiration Affects Photosynthesis via NADPH by a wide range of other metabolic pathways. To examine such metabolic interaction, the dark-to-light induction curve of chlorophyll fluorescence, so-called the Kautsky curve (Kautsky and Hirsch, 1931; Govindjee, 1995), is useful. The disruption of “photosynthesis-related” genes and many genes that are not directly related to photosynthesis affects the condition of photosynthesis
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