Photosynthesis Research (v.129, #3)

Cyclic electron flow: facts and hypotheses by Giovanni Finazzi; Giles N. Johnson (227-230).
Over the last 15 years, research into the process of cyclic electron flow in photosynthesis has seen a huge resurgence. Having been considered by some in the early 1990s as a physiologically unimportant artefact, it is now recognised as essential to normal plant growth. Here, we provide an overview of the major developments covered in this special issue of photosynthesis research.
Keywords: Photosynthesis; Electron flow; Photoprotection; ATP synthesis

The antimycin A-sensitive pathway of cyclic electron flow: from 1963 to 2015 by Mathias Labs; Thilo Rühle; Dario Leister (231-238).
Cyclic electron flow has puzzled and divided the field of photosynthesis researchers for decades. This mainly concerns the proportion of its overall contribution to photosynthesis, as well as its components and molecular mechanism. Yet, it is irrefutable that the absence of cyclic electron flow has severe effects on plant growth. One of the two pathways mediating cyclic electron flow can be inhibited by antimycin A, a chemical that has also widely been used to characterize the mitochondrial respiratory chain. For the characterization of cyclic electron flow, antimycin A has been used since 1963, when ferredoxin was found to be the electron donor of the pathway. In 2013, antimycin A was used to identify the PGRL1/PGR5 complex as the ferredoxin:plastoquinone reductase completing the last puzzle piece of this pathway. The controversy has not ended, and here, we review the history of research on this process using the perspective of antimycin A as a crucial chemical for its characterization.
Keywords: Antimycin A; Cyclic electron flow; Photosynthesis; PGRL1; PGR5; NDH; FQR

Obstacles in the quantification of the cyclic electron flux around Photosystem I in leaves of C3 plants by Da-Yong Fan; Duncan Fitzpatrick; Riichi Oguchi; Weimin Ma; Jiancun Kou; Wah Soon Chow (239-251).
Sixty years ago Arnon and co-workers discovered photophosphorylation driven by a cyclic electron flux (CEF) around Photosystem I. Since then understanding the physiological roles and the regulation of CEF has progressed, mainly via genetic approaches. One basic problem remains, however: quantifying CEF in the absence of a net product. Quantification of CEF under physiological conditions is a crucial prerequisite for investigating the physiological roles of CEF. Here we summarize current progress in methods of CEF quantification in leaves and, in some cases, in isolated thylakoids, of C3 plants. Evidently, all present methods have their own shortcomings. We conclude that to quantify CEF in vivo, the best way currently is to measure the electron flux through PS I (ETR1) and that through PS II and PS I in series (ETR2) for the whole leaf tissue under identical conditions. The difference between ETR1 and ETR2 is an upper estimate of CEF, mainly consisting, in C3 plants, of a major PGR5–PGRL1-dependent CEF component and a minor chloroplast NDH-dependent component, where PGR5 stands for Proton Gradient Regulation 5 protein, PGRL1 for PGR5-like photosynthesis phenotype 1, and NDH for Chloroplast NADH dehydrogenase-like complex. These two CEF components can be separated by the use of antimycin A to inhibit the former (major) component. Membrane inlet mass spectrometry utilizing stable oxygen isotopes provides a reliable estimation of ETR2, whilst ETR1 can be estimated from a method based on the photochemical yield of PS I, Y(I). However, some issues for the recommended method remain unresolved.
Keywords: Cyclic electron flux; Linear electron flux; Membrane inlet mass spectrometry; P700; Photosystem I; Photosystem II

Cyclic electron transport around photosystem I (PSI) generates ∆pH across the thylakoid membrane without net production of NADPH. In angiosperms, two pathways of PSI cyclic electron transport operate. The main pathway depends on PGR5/PGRL1 proteins and is likely identical to the historical Arnon’s pathway. The minor pathway depends on chloroplast NADH dehydrogenase-like (NDH) complex. In assays of their rates in vivo, the two independent pathways are often mixed together. Theoretically, linear electron transport from water to NADP+ cannot satisfy the ATP/NADPH production ratio required by the Calvin-Benson cycle and photorespiration. PGR5/PGRL1-dependent PSI cyclic electron transport contributes substantially to the supply of ATP for CO2 fixation, as does linear electron transport. Also, the contribution of chloroplast NDH cannot be ignored, especially at low light intensity, although the extent of the contribution depends on the plant species. An increase in proton conductivity of ATP synthase may compensate ATP synthesis to some extent in the pgr5 mutant. Combined with the decreased rate of ∆pH generation, however, this mechanism sacrifices homeostasis of the thylakoid lumen pH, seriously disturbing the pH-dependent regulation of photosynthetic electron transport, induction of qE, and downregulation of the cytochrome b 6 f complex. PGR5/PGRL1-dependent PSI cyclic electron transport produces sufficient proton motive force for ATP synthesis and the regulation of photosynthetic electron transport.
Keywords: Cyclic electron transport; NDH; NPQ; PGR5; Photosystem I; Proton motive force

By concentrating CO2, C4 photosynthesis can suppress photorespiration and achieve high photosynthetic efficiency, especially under conditions of high light, high temperature, and drought. To concentrate CO2, extra ATP is required, which would also require a change in photosynthetic electron transport in C4 photosynthesis from that in C3 photosynthesis. Several analyses have shown that the accumulation of the components of cyclic electron flow (CEF) around photosystem I, which generates the proton gradient across thylakoid membranes (ΔpH) and functions in ATP production without producing NADPH, is increased in various NAD-malic enzyme and NADP-malic enzyme C4 plants, suggesting that CEF may be enhanced to satisfy the increased need for ATP in C4 photosynthesis. However, in C4 plants, the accumulation patterns of the components of two partially redundant pathways of CEF, NAD(P)H dehydrogenase-like complex and PROTON GRADIENT REGULATION5–PGR5-like1 complex, are not identical, suggesting that these pathways may play different roles in C4 photosynthesis. Accompanying the increase in the amount of NDH, the expression of some genes which encode proteins involved in the assembly of NDH is also increased at the mRNA level in various C4 plants, suggesting that this increase is needed to increase the accumulation of NDH. To better understand the relation between CEF and C4 photosynthesis, a reverse genetic approach to generate C4 transformants with respect to CEF will be necessary.
Keywords: Cyclic electron flow (CEF) around photosystem I (PS I); C4 photosynthesis; Photosynthetic electron transport (PET); NAD(P)H dehydrogenase-like complex (NDH); PROTON GRADIENT REGULATION5 (PGR5)–PGR5-like1 (PGRL1) complex; ATP requirement

To elucidate the molecular mechanism to oxidize the reaction center chlorophyll, P700, in PSI, we researched the effects of partial pressure of O2 (pO2) on photosynthetic characteristic parameters in sunflower (Helianthus annuus L.) leaves. Under low CO2 conditions, the oxidation of P700 was stimulated; however the decrease in pO2 suppressed its oxidation. Electron fluxes in PSII [Y(II)] and PSI [Y(I)] showed pO2-dependence at low CO2 conditions. H+-consumption rate, estimated from Y(II) and CO2-fixation/photorespiration rates (JgH+), showed the positive curvature relationship with the dissipation rate of electrochromic shift signal (V H + ), which indicates H+-efflux rate from lumen to stroma in chloroplasts. Therefore, these electron fluxes contained, besides CO2-fixation/photorespiration-dependent electron fluxes, non-H+-consumption electron fluxes including Mehler-ascorbate peroxidase (MAP)-pathway. Y(I) that was larger than Y(II) surely implies the functioning of cyclic electron flow (CEF). Both MAP-pathway and CEF were suppressed at lower pO2, with plastoquinone-pool reduced. That is, photorespiration prepares the redox-poise of photosynthetic electron transport system for CEF activity as an electron sink. Excess Y(II), [ΔY(II)] giving the curvature relationship with V H + , and excess Y(I) [ΔCEF] giving the difference between Y(I) and Y(II) were used as an indicator of MAP-pathway and CEF activity, respectively. Although ΔY(II) was negligible and did not show positive relationship to the oxidation-state of P700, ΔCEF showed positive linear relationship to the oxidation-state of P700. These facts indicate that CEF cooperatively with photorespiration regulates the redox-state of P700 to suppress the over-reduction in PSI under environmental stress conditions.
Keywords: Cyclic electron flow; Electron sink; Mehler-ascorbate peroxidase (MAP)-pathway; Oxygen; Photosystem I; Photorespiration; P700

Photoacclimation of photosynthesis in the Eustigmatophycean Nannochloropsis gaditana by Andrea Meneghesso; Diana Simionato; Caterina Gerotto; Nicoletta La Rocca; Giovanni Finazzi; Tomas Morosinotto (291-305).
Nannochloropsis is an eukaryotic alga of the phylum Heterokonta, originating from a secondary endosymbiotic event. In this work, we investigated how the photosynthetic apparatus responds to growth in different light regimes in Nannochloropsis gaditana. We found that intense illumination induces the decrease of both photosystem I and II contents and their respective antenna sizes. Cells grown in high light showed a larger capacity for electron transport, with enhanced cyclic electron transport around photosystem I, contributing to photoprotection from excess illumination. Even when exposed to excess light intensities for several days, N. gaditana cells did not activate constitutive responses such as nonphotochemical quenching and the xanthophyll cycle. These photoprotection mechanisms in N. gaditana thus play a role in acclimation to fast changes in illumination within a time range of minutes, while regulation of the electron flow capacity represents a long-term response to prolonged exposure to excess light.
Keywords: Acclimation; Microalgae; Cyclic electron flow; Photoprotection; Nonphotochemical quenching

Both the structure and the protein composition of thylakoid membranes have an impact on light harvesting and electron transfer in the photosynthetic chain. Thylakoid membranes form stacks and lamellae where photosystem II and photosystem I localize, respectively. Light-harvesting complexes II can be associated to either PSII or PSI depending on the redox state of the plastoquinone pool, and their distribution is governed by state transitions. Upon state transitions, the thylakoid ultrastructure and lateral distribution of proteins along the membrane are subject to significant rearrangements. In addition, quinone diffusion is limited to membrane microdomains and the cytochrome b 6 f complex localizes either to PSII-containing grana stacks or PSI-containing stroma lamellae. Here, we discuss possible similarities or differences between green algae and C3 plants on the functional consequences of such heterogeneities in the photosynthetic electron transport chain and propose a model in which quinones, accepting electrons either from PSII (linear flow) or NDH/PGR pathways (cyclic flow), represent a crucial control point. Our aim is to give an integrated description of these processes and discuss their potential roles in the balance between linear and cyclic electron flows.
Keywords: Cyclic electron flow; Cytochrome b 6 f complex; Heterogeneity of thylakoid membranes; Membrane ultrastructure; Quinone microdomains; Redox sensing; State transitions