Decreased content of leaf ferredoxin changes electron distribution and limits photosynthesis in transgenic potato plants

Influence of the photoperiod on redox regulation and stress responses in Arabidopsis thaliana L. (Heynh.) plants under long- and short-day conditions

Arabidopsis thaliana L. (Heynh.) plants were grown in low light (150 micromol photons m(-2) s(-1) and 20 degrees C) either in short days (7.5 h photoperiod) or long days (16 h photoperiod), and then transferred into high light and low temperature (350-800 micromol photons m(-2) s(-1) at 12 degrees C). Plants grown in short days responded with a rapid increase in NADP-malate dehydrogenase (EC 1.1.1.82) activation state. However, persisting overreduction revealed a new level of regulation of the malate valve. Activity measurements and Northern-blot analyses indicated that NADP-malate dehydrogenase transcript and protein levels increased within a few hours. Using macroarrays, additional changes in gene expression were identified. Transcript levels for several enzymes of glutathione metabolism and of some photosynthetic genes increased. The cellular glutathione level increased, but its redox state remained unchanged. A different situation was observed in plants grown in long-day conditions. Neither NADP-malate dehydrogenase nor glutathione content changed, but the expression of several antioxidative enzymes increased strongly. We conclude that the endogenous systems that measure day length interact with redox regulation, and override the interpretation of the signals, i.e. they redirect redox-mediated acclimation signals to allow for more efficient light usage and redox poising in short days to systems for the prevention of oxidative damages when grown under long-day conditions.

Effects of light intensity on cyclic electron flow around PSI and its relationship to non-photochemical quenching of Chl fluorescence in tobacco leaves

We tested the hypothesis that plants grown under high light intensity (HL-plants) had a large activity of cyclic electron flow around PSI (CEF-PSI) compared with plants grown under low light (LL-plants). To evaluate the activity of CEF-PSI, the relationships between photosynthesis rate, quantum yields of both PSII and PSI, and Chl fluorescence parameters were analyzed simultaneously in intact leaves of tobacco plants which had been grown under different light intensities (150 and 1,100 micromol photons m(-2) s(-1), respectively) and with different amounts of nutrients supplied. HL-plants showed a larger value of non-photochemical quenching (NPQ) of Chl fluorescence at the limited activity of photosynthetic linear electron flow. Furthermore, HL-plants had a larger activity of CEF-PSI than LL-plants. These results suggested that HL-plants dissipated the excess photon energy through NPQ by enhancing the ability of CEF-PSI to induce acidification of the thylakoid lumen.

Strategies to maintain redox homeostasis during photosynthesis under changing conditions

Plants perform photosynthesis and assimilatory processes in a continuously changing environment. Energy production in the various cell compartments and energy consumption in endergonic processes have to be well adjusted to the varying conditions. In addition, dissipatory pathways are required to avoid any detrimental effects caused by over-reduction. A large number of short-term and long-term mechanisms interact with each other in a flexible way, depending on intensity and the type of impact. Therefore, all levels of regulation are involved, starting from energy absorption and electron flow events through to post-transcriptional control. The simultaneous presence of strong oxidants and strong reductants during oxygenic photosynthesis is the basis for regulation. However, redox-dependent control also interacts with other signal transduction pathways in order to adapt metabolic processes and redox-control to the developmental state. Examples are given here for short-term and long-term control following changes of light intensity and photoperiod, focusing on the dynamic nature of the plant regulatory systems. An integrating network of all these mechanisms exists at all levels of control. Cellular homeostasis will be maintained as long as the mechanisms for acclimation are present in sufficiently high capacities. If an impact is too rapid, and acclimation on the level of gene expression cannot occur, cellular damage and cell death are initiated.

CO2 response of cyclic electron flow around PSI (CEF-PSI) in tobacco leaves--relative electron fluxes through PSI and PSII determine the magnitude of non-photochemical quenching (NPQ) of Chl fluorescence

We hypothesized that cyclic electron flow around photosystem I (CEF-PSI) participates in the induction of non-photochemical quenching (NPQ) of chlorophyll (Chl) fluorescence when the rate of photosynthetic linear electron flow (LEF) is electron-acceptor limited. To test this hypothesis, the relationships among photosynthesis rate, electron fluxes through both PSI and PSII [Je(PSI) and Je(PSII)] and Chl fluorescence parameters were analyzed simultaneously in intact leaves of tobacco plants at several light intensities and partial pressures of ambient CO2 (Ca). At low light intensities, decreasing Ca lowered the photosynthesis rate, but Je(PSI) and Je(PSII) remained constant. Je(PSI) was larger than Je(PSII), indicating the existence of CEF-PSI. Increasing the light intensity enhanced photosynthesis and both Je(PSI) and Je (PSII). Je(PSI)/Je(PSII) also increased at high light and at high light and low Ca combined, showing a strong, positive relationship with NPQ of Chl fluorescence. These results indicated that CEF-PSI contributed to the dissipation of photon energy in excess of that consumed by photosynthesis by driving NPQ of Chl fluorescence. The main physiological function of CEF-PSI in photosynthesis of higher plants is discussed.

Enhancement of cyclic electron flow around PSI at high light and its contribution to the induction of non-photochemical quenching of chl fluorescence in intact leaves of tobacco plants

Non-photochemical quenching (NPQ) of Chl fluorescence is a mechanism for dissipating excess photon energy and is dependent on the formation of a DeltapH across the thylakoid membranes. The role of cyclic electron flow around photosystem I (PSI) (CEF-PSI) in the formation of this DeltapH was elucidated by studying the relationships between O2-evolution rate [V(O2)], quantum yield of both PSII and PSI [Phi(PSII) and Phi(PSI)], and Chl fluorescence parameters measured simultaneously in intact leaves of tobacco plants in CO2-saturated air. Although increases in light intensity raised V(O2) and the relative electron fluxes through both PSII and PSI [Phi(PSII) x PFD and Phi(PSI) x PFD] only Phi(PSI) x PFD continued to increase after V(O2) and Phi(PSII) x PFD became light saturated. These results revealed the activity of an electron transport reaction in PSI not related to photosynthetic linear electron flow (LEF), namely CEF-PSI. NPQ of Chl fluorescence drastically increased after Phi(PSII) x PFD became light saturated and the values of NPQ correlated positively with the relative activity of CEF-PSI. At low temperatures, the light-saturation point of Phi(PSII) x PFD was lower than that of Phi(PSI) x PFD and NPQ was high. On the other hand, at high temperatures, the light-dependence curves of Phi(PSII) x PFD and Phi(PSI) x PFD corresponded completely and NPQ was not induced. These results indicate that limitation of LEF induced CEF-PSI, which, in turn, helped to dissipate excess photon energy by driving NPQ of Chl fluorescence.

Dynamic flexibility in the light reactions of photosynthesis governed by both electron and proton transfer reactions

Plant photosynthesis performs the remarkable feat of converting light energy into usable chemical forms, which involves taming highly reactive intermediates without harming plant cells. This requires an apparatus that is not only efficient and robust but also flexible in its responses to changing environmental conditions. It also requires that the output of the energy-storing reactions be matched with the demands of metabolism. This article addresses the mechanisms by which this flexibility is achieved for short-term environmental changes. We argue that chloroplasts need two types of flexible mechanisms: one for modulating the output ratio of ATP:NADPH, which involves cyclic electron flux around photosystem I; and another for changing the regulatory sensitivity of the light-harvesting antenna to electron (and proton) flow.

The Arabidopsis chloroplastic NifU-like protein CnfU, which can act as an iron-sulfur cluster scaffold protein, is required for biogenesis of ferredoxin and photosystem I

The biosynthesis of iron-sulfur clusters is a highly regulated process involving several proteins. Among them, so-called scaffold proteins play pivotal roles in both the assembly and delivery of iron-sulfur clusters. Here, we report the identification of two chloroplast-localized NifU-like proteins, AtCnfU-V and AtCnfU-IVb, from Arabidopsis (Arabidopsis thaliana) with high sequence similarity to a cyanobacterial NifU-like protein that was proposed to serve as a molecular scaffold. AtCnfU-V is constitutively expressed in several tissues of Arabidopsis, whereas the expression of AtCnfU-IVb is prominent in the aerial parts. Mutant Arabidopsis lacking AtCnfU-V exhibited a dwarf phenotype with faint pale-green leaves and had drastically impaired photosystem I accumulation. Chloroplasts in the mutants also showed a decrease in both the amount of ferredoxin, a major electron carrier of the stroma that contains a [2Fe-2S] cluster, and in the in vitro activity of iron-sulfur cluster insertion into apo-ferredoxin. When expressed in Escherichia coli cells, AtCnfU-V formed a homodimer carrying a [2Fe-2S]-like cluster, and this cluster could be transferred to apo-ferredoxin in vitro to form holo-ferredoxin. We propose that AtCnfU has an important function as a molecular scaffold for iron-sulfur cluster biosynthesis in chloroplasts and thereby is required for biogenesis of ferredoxin and photosystem I.