Electron transport and photophosphorylation by Photosystem I in vivo in plants and cyanobacteria

Abstract

Recently, a number of techniques, some of them relatively new and many often used in combination, have given a clearer picture of the dynamic role of electron transport in Photosystem I of photosynthesis and of coupled cyclic photophosphorylation. For example, the photoacoustic technique has detected cyclic electron transport in vivo in all the major algal groups and in leaves of higher plants. Spectroscopic measurements of the Photosystem I reaction center and of the changes in light scattering associated with thylakoid membrane energization also indicate that cyclic photophosphorylation occurs in living plants and cyanobacteria, particularly under stressful conditions.In cyanobacteria, the path of cyclic electron transport has recently been proposed to include an NAD(P)H dehydrogenase, a complex that may also participate in respiratory electron transport. Photosynthesis and respiration may share common electron carriers in eukaryotes also. Chlororespiration, the uptake of O2 in the dark by chloroplasts, is inhibited by excitation of Photosystem I, which diverts electrons away from the chlororespiratory chain into the photosynthetic electron transport chain. Chlororespiration in N-starved Chlamydomonas increases ten fold over that of the control, perhaps because carbohydrates and NAD(P)H are oxidized and ATP produced by this process.The regulation of energy distribution to the photosystems and of cyclic and non-cyclic phosphorylation via state 1 to state 2 transitions may involve the cytochrome b 6-f complex. An increased demand for ATP lowers the transthylakoid pH gradient, activates the b 6-f complex, stimulates phosphorylation of the light-harvesting chlorophyll-protein complex of Photosystem II and decreases energy input to Photosystem II upon induction of state 2. The resulting increase in the absorption by Photosystem I favors cyclic electron flow and ATP production over linear electron flow to NADP and 'poises' the system by slowing down the flow of electrons originating in Photosystem II.Cyclic electron transport may function to prevent photoinhibition to the photosynthetic apparatus as well as to provide ATP. Thus, under high light intensities where CO2 can limit photosynthesis, especially when stomates are closed as a result of water stress, the proton gradient established by coupled cyclic electron transport can prevent over-reduction of the electron transport system by increasing thermal de-excitation in Photosystem II (Weis and Berry 1987). Increased cyclic photophosphorylation may also serve to drive ion uptake in nutrient-deprived cells or ion export in salt-stressed cells.There is evidence in some plants for a specialization of Photosystem I. For example, in the red alga Porphyra about one third of the total Photosystem I units are engaged in linear electron transfer from Photosystem II and the remaining two thirds of the Photosystem I units are specialized for cyclic electron flow. Other organisms show evidence of similar specialization.Improved understanding of the biological role of cyclic pho...

Photosynthetic performance of Salvinia natans exposed to chromium and zinc rich wastewater

“…This could well be an acclimation response to cope with the additional flow of electrons during stress (Makino et al, 2002). In general, plants exposed to abiotic stresses reveal downregulation of linear electron flow and activation of cyclic electron transport, when linear electron flow becomes saturated (Fork and Herbert, 1993;Golding et al, 2004; …”

Cyclic electron transport through photosystem I

“…In cyanobacteria, this measurement can be used to evaluate the activity of NDH-dependent PSI cyclic electron transport (Mi et al 1994). Fast re-reduction of P700 is also observed in C 4 plants, but it is slower in C 3 plants, probably due to lower NDH activity and the loss of favourable redox poising (Fork and Herbert 1993;Joët et al 2002). In C 3 plants, fast re-reduction is accelerated in anaerobiosis, drought conditions or after long dark adaptation (Golding et al 2004), but under anaerobic conditions, fast re-reduction shows little dependency on electron donation by NDH activity (Joët et al 2002).…”

“…Since the P700 oxidation level is determined from the balance between electron input and output through PSI, it is influenced not only by cyclic electron transport but also by other factors such as electron leakage to O 2 or electron storage in the inter-system chain and stroma. Furthermore, the activity of PSI cyclic electron transport has been suggested to require a high NADPH/NADP ϩ ratio or reduction of the plastoquinone pool (Arnon and Chain 1975;Fork and Herbert 1993;Mills et al 1979;Joët et al 2002). This favourable redox poising may occur in cyanobacteria and green algae via respiratory electron transport and C 4 plants via the malate pool, even under the experimental conditions of dark following farred illumination, but may not occur in C 3 plants (Fork and Herbert 1993;Joët et al 2002).…”

“…The re-reduction kinetics of P700 in the dark accelerates when electrons coming from the stroma are donated to PSI. This fast re-reduction has been observed in several species of algae and cyanobacteria (Maxwell and Biggins 1976;Fork and Herbert 1993). In cyanobacteria, this measurement can be used to evaluate the activity of NDH-dependent PSI cyclic electron transport (Mi et al 1994).…”

Short-term acclimation of the photosynthetic electron transfer chain to changing light: a mathematical model

“…The proton gradient generated by the activities of PSII and Cytb6f powers ATP synthase-catalysed phosphorylation of ADP. Cyclic electron flow (CEF) involves the flow of electrons from ferredoxin to the PQ pool, resulting in an increased acidification of the thylakoid lumen and increased production of ATP at the expense of NADPH [5,6]. A remarkable feature of oxygenic photosynthesis is the extremely wide range of reaction kinetics both within the electron transfer chain and the lightindependent reactions of carbon fixation [7].…”