Distinct Energetics and Regulatory Functions of the Two Major Cyclic Electron Flow Pathways in Chloroplasts
Deserah D. Strand, Nicholas Fisher and David M. Kramer
from: Chloroplasts: Current Research and Future Trends (Edited by: Helmut Kirchhoff). Caister Academic Press, U.K. (2016) Pages: 89-100.
The output of the light reactions of photosynthesis, i.e. ATP and NADPH, must be finely controlled to meet the varying metabolic demands of the plant. Deleterious side reactions, including the production of reactive oxygen species (ROS), can occur if this balance is not maintained. In this Chapter, we review recent advances in understanding of cyclic electron flow around photosystem I (CEF), a process that evolved to correct for such energy imbalances. CEF in higher plants has been proposed to function primarily through two pathways: the ferredoxin:plastoquinone reductase (FQR) and the NADPH dehydrogenase complex (NDH). Because these pathways appear to support the same function, they are often thought to be redundant. However, it was recently shown that the NDH complex is a type I proton pumping quinone reductase, making the complex a more efficient route for ATP generation via CEF than the FQR. In addition, these pathways are differentially regulated, the FQR through chloroplast redox status, and the NDH thermodynamically and, in part, by hydrogen peroxide. These observations imply that the two pathways are distinct in terms of their energetics and regulation and thus it is clear that the FQR and the NDH work under different conditions to provide with rapid or efficient energy balancing of the chloroplast. Specifically, we suggest a CEF model in which the FQR is rapidly activated to restore balance when an ATP deficit leads to a buildup of reducing power in the stroma, and the NDH activated when a prolonged deficit leads to closure of the PSI acceptor side with associated ROS production read more ...