Abstract
At the end of the linear photosynthetic electron transfer (PET) chain, the small soluble protein ferredoxin (Fd) transfers electrons to Fd:NADP(H) oxidoreductase (FNR), which can then reduce NADP+ to support C assimilation. In addition to this linear electron flow (LEF), Fd is also thought to mediate electron flow back to the membrane complexes by different cyclic electron flow (CEF) pathways: either antimycin A sensitive, NAD(P)H complex dependent, or through FNR located at the cytochrome b6f complex. Both Fd and FNR are present in higher plant genomes as multiple gene copies, and it is now known that specific Fd iso-proteins can promote CEF. In addition, FNR iso-proteins vary in their ability to dynamically interact with thylakoid membrane complexes, and it has been suggested that this may also play a role in CEF. We will highlight work on the different Fd-isoproteins and FNR-membrane association found in the bundle sheath (BSC) and mesophyll (MC) cell chloroplasts of the C4 plant maize. These two cell types perform predominantly CEF and LEF, and the properties and activities of Fd and FNR in the BSC and MC are therefore specialized for CEF and LEF respectively. A diversity of Fd isoproteins and dynamic FNR location has also been recorded in C3 plants, algae and cyanobacteria. This indicates that the principles learned from the extreme electron transport situations in the BSC and MC of maize might be usefully applied to understanding the dynamic transition between these states in other systems.
Highlights
In photosynthetic electron transfer (PET), electrons are originally generated by the splitting of water at photosystem II (PSII), transferred to the cytochrome b6f complex (Cytb6f) by plastoquinone (PQ), and from there through the thylakoid lumen to photosystem I (PSI), via plastocyanin (PC)
Considering the differentiation of MC and between the cells conducting CEF (BSC) in PET, it seems possible that tight Fd:NADP(H) oxidoreductase (FNR) binding to the membrane favors cyclic electron flow (CEF), while weak binding / soluble FNR is necessary for linear electron flow (LEF), with the capacity for dynamic regulation
Data comparing PET in the MC and BSC cells of maize, which are specialized for LEF and CEF respectively, allow us to speculate about how the transition between these two different PET pathways could be regulated in other photosynthetic organisms
Summary
In photosynthetic electron transfer (PET), electrons are originally generated by the splitting of water at photosystem II (PSII), transferred to the cytochrome b6f complex (Cytb6f) by plastoquinone (PQ), and from there through the thylakoid lumen to photosystem I (PSI), via plastocyanin (PC). It has been suggested that FNR could either directly reduce the PQ pool using electrons from Fd [37], or act as a Fd binding site on the thylakoid membrane during CEF, as it has been found to associate with both the NDH complex [38], and the Cytb6f [39]. We will focus on how different Fd and FNR forms could be differentially involved in partitioning electrons into either LEF or CEF.
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