Studies on the System Regulating Proton Movement across the Chloroplast Envelope : Effects of ATPase Inhibitors, Mg, and an Amine Anesthetic on Stromal pH and Photosynthesis

Effect of low-dose ionizing radiation on spatiotemporal parameters of functional responses induced by electrical signals in tobacco plants

Plants growing under an increased radiation background may be exposed to additional stressors. Plant acclimatization is formed with the participation of stress signals that cause systemic responses-a change in the activity of physiological processes. In this work, we studied the mechanisms of the effect of ionizing radiation (IR) on the systemic functional responses induced by electrical signals. Chronic β-irradiation (31.3 μGy/h) have a positive effect on the morphometric parameters and photosynthetic activity of tobacco plants (Nicotiana tabacum L.) at rest. An additional stressor causes an electrical signal, which, when propagated, causes a temporary change in chlorophyll fluorescence parameters, reflecting a decrease in photosynthesis activity. Irradiation did not significantly affect the electrical signals. At the same time, more pronounced photosynthesis responses are observed in irradiated plants: both the amplitude and the leaf area covered by the reaction increase. The formation of such responses is associated with changes in pH and stomatal conductance, the role of which was analyzed under IR. Using tobacco plants expressing the fluorescent pH-sensitive protein Pt-GFP, it was shown that IR enhances signal-induced cytoplasmic acidification. It was noted that irradiation also disrupts the correlation between the amplitudes of the electrical signal, pH shifts, changes in chlorophyll fluorescence parameters. Also stronger inhibition of stomatal conductance by the signal was shown in irradiated plants. It was concluded that the effect of IR on the systemic response induced by the electrical signal is mainly due to its effect on the stage of signal transformation into the response.

Genome-wide identification and expression analysis of magnesium transporter gene family in grape (Vitis vinifera)

BACKGROUND

Magnesium ion is one of the essential mineral elements for plant growth and development, which participates in a variety of physiological and biochemical processes. Since there is no report on the research of magnesium ion transporter in grape, the study of the structure and function of magnesium ion transporters (MGT) is helpful to understand the dynamic balance mechanism of intracellular magnesium ions and their inter- or intra-cellular activities.

RESULT

In this study, we identified the members of MGT protein family in grape and performed the phylogenetic and expression analysis. We have identified nine VvMGT genes in grape genome, which are distributed on eight different chromosomes. Phylogenetic analysis showed that MGT family members of grapes were divided into five subfamilies and had obvious homology with Arabidopsis, maize, and pear. Based on transcriptome data from the web databases, we analyzed the expression patterns of VvMGTs at different development stages and in response to abiotic stresses including waterlogging, drought, salinity, and copper. Using qRT-PCR method, we tested the expression of grape VvMGTs under magnesium and aluminum treatments and found significant changes in VvMGTs expression. In addition, four of the MGT proteins in grape were located in the nucleus.

CONCLUSION

Overall, in this study we investigated the structural characteristics, evolution pattern, and expression analysis of VvMGTs in depth, which laid the foundation for further revealing the function of VvMGT genes in grape.

Chloroplast pH Homeostasis for the Regulation of Photosynthesis

The pH of various chloroplast compartments, such as the thylakoid lumen and stroma, is light-dependent. Light illumination induces electron transfer in the photosynthetic apparatus, coupled with proton translocation across the thylakoid membranes, resulting in acidification and alkalization of the thylakoid lumen and stroma, respectively. Luminal acidification is crucial for inducing regulatory mechanisms that protect photosystems against photodamage caused by the overproduction of reactive oxygen species (ROS). Stromal alkalization activates enzymes involved in the Calvin-Benson-Bassham (CBB) cycle. Moreover, proton translocation across the thylakoid membranes generates a proton gradient (ΔpH) and an electric potential (ΔΨ), both of which comprise the proton motive force (pmf) that drives ATP synthase. Then, the synthesized ATP is consumed in the CBB cycle and other chloroplast metabolic pathways. In the dark, the pH of both the chloroplast stroma and thylakoid lumen becomes neutral. Despite extensive studies of the above-mentioned processes, the molecular mechanisms of how chloroplast pH can be maintained at proper levels during the light phase for efficient activation of photosynthesis and other metabolic pathways and return to neutral levels during the dark phase remain largely unclear, especially in terms of the precise control of stromal pH. The transient increase and decrease in chloroplast pH upon dark-to-light and light-to-dark transitions have been considered as signals for controlling other biological processes in plant cells. Forward and reverse genetic screening approaches recently identified new plastid proteins involved in controlling ΔpH and ΔΨ across the thylakoid membranes and chloroplast proton/ion homeostasis. These proteins have been conserved during the evolution of oxygenic phototrophs and include putative photosynthetic protein complexes, proton transporters, and/or their regulators. Herein, we summarize the recently identified protein players that control chloroplast pH and influence photosynthetic efficiency in plants.

Plastidial transporters KEA1 and KEA2 at the inner envelope membrane adjust stromal pH in the dark

Photosynthesis and carbon fixation depend critically on the regulation of pH in chloroplast compartments in the daylight and at night. While it is established that an alkaline stroma is required for carbon fixation, it is not known how alkaline stromal pH is formed, maintained or regulated. We tested whether two envelope transporters, AtKEA1 and AtKEA2, directly affected stromal pH in isolated Arabidopsis chloroplasts using the fluorescent probe 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF). External K⁺ -induced alkalinization of the stroma was observed in chloroplasts from wild-type (WT) plants but not from kea1kea2 mutants, suggesting that KEA1 and KEA2 mediate K⁺ uptake/H⁺ loss to modulate stromal pH. While light-stimulated alkalinization of the stroma was independent of KEA1 and KEA2, the rate of decay to neutral pH in the dark is delayed in kea1kea2 mutants. However, the dark-induced loss of a pH gradient across the thylakoid membrane was similar in WT and mutant chloroplasts. This indicates that proton influx from the cytosol mediated by envelope K⁺ /H⁺ antiporters contributes to adjustment of stromal pH upon light to dark transitions.

Electrical signals as mechanism of photosynthesis regulation in plants

This review summarizes current works concerning the effects of electrical signals (ESs) on photosynthesis, the mechanisms of the effects, and its physiological role in plants. Local irritations of plants induce various photosynthetic responses in intact leaves, including fast and long-term inactivation of photosynthesis, and its activation. Irritation-induced ESs, including action potential, variation potential, and system potential, probably causes the photosynthetic responses in intact leaves. Probable mechanisms of induction of fast inactivation of photosynthesis are associated with Ca²⁺- and (or) H⁺-influxes during ESs generation; long-term inactivation of photosynthesis might be caused by Ca²⁺- and (or) H⁺-influxes, production of abscisic and jasmonic acids, and inactivation of phloem H⁺-sucrose symporters. It is probable that subsequent development of inactivation of photosynthesis is mainly associated with decreased CO₂ influx and inactivation of the photosynthetic dark reactions, which induces decreased photochemical quantum yields of photosystems I and II and increased non-photochemical quenching of photosystem II fluorescence and cyclic electron flow around photosystem I. However, other pathways of the ESs influence on the photosynthetic light reactions are also possible. One of them might be associated with ES-connected acidification of chloroplast stroma inducing ferredoxin-NADP⁺ reductase accumulation at the thylakoids in Tic62 and TROL complexes. Mechanisms of ES-induced activation of photosynthesis require further investigation. The probable ultimate effect of ES-induced photosynthetic responses in plant life is the increased photosynthetic machinery resistance to stressors, including high and low temperatures, and enhanced whole-plant resistance to environmental factors at least during 1 h after irritation.

Changes in H(+)-ATP Synthase Activity, Proton Electrochemical Gradient, and pH in Pea Chloroplast Can Be Connected with Variation Potential

Local stimulation induces generation and propagation of electrical signals, including the variation potential (VP) and action potential, in plants. Burning-induced VP changes the physiological state of plants; specifically, it inactivates photosynthesis. However, the mechanisms that decrease photosynthesis are poorly understood. We investigated these mechanisms by measuring VP-connected systemic changes in CO2 assimilation, parameters of light reactions of photosynthesis, electrochromic pigment absorbance shifts, and light scattering. We reveal that inactivation of photosynthesis in the pea, including inactivation of dark and light reactions, was connected with the VP. Inactivation of dark reactions decreased the rate constant of the fast relaxation of the electrochromic pigment absorbance shift, which reflected a decrease in the H(+)-ATP synthase activity. This decrease likely contributed to the acidification of the chloroplast lumen, which developed after VP induction. However, VP-connected decrease of the proton motive force across the thylakoid membrane, possibly, reflected a decreased pH in the stroma. This decrease may be another mechanism of chloroplast lumen acidification. Overall, stroma acidification can decrease electron flow through photosystem I, and lumen acidification induces growth of fluorescence non-photochemical quenching and decreases electron flow through photosystem II, i.e., pH decreases in the stroma and lumen, possibly, contribute to the VP-induced inactivation of light reactions of photosynthesis.

Proton cellular influx as a probable mechanism of variation potential influence on photosynthesis in pea

Electrical signals (action potential and variation potential, VP) caused by environmental stimuli are known to induce various physiological responses in plants, including changes in photosynthesis; however, their functional mechanisms remain unclear. In this study, the influence of VP on photosynthesis in pea (Pisum sativum L.) was investigated and the proton participation in this process analysed. VP, induced by local heating, inactivated photosynthesis and activated respiration, with the initiation of the photosynthetic response connected with inactivation of the photosynthetic dark stage; however, direct VP influence on the light stage was also probable. VP generation was accompanied with pH increases in apoplasts (0.17-0.30 pH unit) and decreases in cytoplasm (0.18-0.60 pH unit), which probably reflected H(+) -ATPase inactivation and H(+) influx during this electrical event. Imitation of H(+) influx using the protonophore carbonyl cyanide m-chlorophenylhydrazone (CCCP) induced a photosynthetic response that was similar with a VP-induced response. Experiments on chloroplast suspensions showed that decreased external pH also induced an analogous response and that its magnitude depended on the magnitude of pH change. Thus, the present results showed that proton cellular influx was the probable mechanism of VP's influence on photosynthesis in pea. Potential means of action for this influence are discussed.

Phosphorylation-related modification at the dimer interface of 14-3-3ω dramatically alters monomer interaction dynamics

14-3-3 proteins are generally believed to function as dimers in a broad range of eukaryotic signaling pathways. The consequences of altering dimer stability are not fully understood. Phosphorylation at Ser58 in the dimer interface of mammalian 14-3-3 isoforms has been reported to destabilise dimers. An equivalent residue, Ser62, is present across most Arabidopsis isoforms but the effects of phosphorylation have not been studied in plants. Here, we assessed the effects of phosphorylation at the dimer interface of Arabidopsis 14-3-3ω. Protein kinase A phosphorylated 14-3-3ω at Ser62 and also at a previously unreported residue, Ser67, resulting in a monomer-sized band on native-PAGE. Phosphorylation at Ser62 alone, or with additional Ser67 phosphorylation, was investigated using phosphomimetic versions of 14-3-3ω. In electrophoretic and chromatographic analyses, these mutants showed mobilities intermediate between dimers and monomers. Mobility was increased by detergents, by reducing protein concentration, or by increasing pH or temperature. Urea gradient gels showed complex structural transitions associated with alterations of dimer stability, including a previously unreported 14-3-3 aggregation phenomenon. Overall, our analyses showed that dimer interface modifications such as phosphorylation reduce dimer stability, dramatically affecting the monomer-dimer equilibrium and denaturation trajectory. These findings may have dramatic implications for 14-3-3 structure and function in vivo.

A probable Na+(K+)/H+ exchanger on the chloroplast envelope functions in pH homeostasis and chloroplast development in Arabidopsis thaliana

Electroneutral monovalent cation/proton antiport across the chloroplast envelope has been shown previously to have an important regulatory effect on stromal pH and thereby on photosynthetic carbon reduction. Here we report that an Arabidopsis nuclear gene, AtCHX23, encodes a putative Na(+)(K(+))/H(+) exchanger and functions in the adjustment of pH in the cytosol and possibly in maintaining a high pH level in the chloroplast stroma. The AtCHX23 protein is localized in the chloroplast envelope. Plastids from chx23 mutants had straight thylakoids but lacked grana lamellae. chx23 mutant leaves were pale yellow and had a much reduced chlorophyll content. The chlorophyll content of chx23 was increased by growing in medium at low (4.0) pH and decreased by growing at high (7.0) pH. The cytosolic pH in the leaves of the mutant was significantly higher than that in the wild type. chx23 mutants displayed a high sensitivity to NaCl. Together, these data indicate that CHX23 is a probable chloroplast Na(+)(K(+))/H(+) exchanger important for pH homeostasis and chloroplast development and function.

A probable Na+(K+)/H+ exchanger on the chloroplast envelope functions in pH homeostasis and chloroplast development in Arabidopsis thaliana

Electroneutral monovalent cation/proton antiport across the chloroplast envelope has been shown previously to have an important regulatory effect on stromal pH and thereby on photosynthetic carbon reduction. Here we report that an Arabidopsis nuclear gene, AtCHX23, encodes a putative Na(+)(K(+))/H(+) exchanger and functions in the adjustment of pH in the cytosol and possibly in maintaining a high pH level in the chloroplast stroma. The AtCHX23 protein is localized in the chloroplast envelope. Plastids from chx23 mutants had straight thylakoids but lacked grana lamellae. chx23 mutant leaves were pale yellow and had a much reduced chlorophyll content. The chlorophyll content of chx23 was increased by growing in medium at low (4.0) pH and decreased by growing at high (7.0) pH. The cytosolic pH in the leaves of the mutant was significantly higher than that in the wild type. chx23 mutants displayed a high sensitivity to NaCl. Together, these data indicate that CHX23 is a probable chloroplast Na(+)(K(+))/H(+) exchanger important for pH homeostasis and chloroplast development and function.

Solute pores, ion channels, and metabolite transporters in the outer and inner envelope membranes of higher plant plastids

All plant cells contain plastids. Various reactions are located exclusively within these unique organelles, requiring the controlled exchange of a wide range of solutes, ions, and metabolites. In recent years, several proteins involved in import and/or export of these compounds have been characterized using biochemical and electrophysiological approaches, and in addition have been identified at the molecular level. Several solute channels have been identified in the outer envelope membrane. These porin-like proteins in the outer envelope membrane were formerly thought to be quite unspecific, but have now been shown to exhibit significant substrate specificity and to be highly regulated. Therefore, the inter-envelope membrane space is not as freely accessible as previously thought. Transport proteins in the inner envelope membrane have been characterized in more detail. It has been proved unequivocally that a family of proteins (including triose phosphate-/phosphoenolpyruvate-, and glucose 6-phosphate-specific transporters) permit the exchange of inorganic phosphate and phosphorylated intermediates. A new type of plastidic 2-oxoglutarate/malate transporter has been identified and represents the first carrier with 12 putative transmembrane domains, to be located in the inner envelope membrane. The plastidic ATP/ADP transporter also contains 12 putative transmembrane domains and possesses striking structural similarity to ATP/ADP transporters found in intracellular, human pathogenic bacteria.

A voltage-dependent porin-like channel in the inner envelope membrane of plant chloroplasts

The electrical activity of a single channel of 525 +/- 12 picosiemens in 150 mM KCl was measured after fusion of the inner envelope membrane of the chloroplast with planar lipid bilayers. The reversal potentials measured in KCl gradients indicate that this channel is weakly anion selective (PCl/PK = 1.6 +/- 0.2). The gating mechanism of the pore is voltage dependent. The channel shifts from a fully open state to a substrate at positive electrical potentials and remains closed at negative electrical potentials. Succinylation of the protein increases the open probability of the fully open state and reverses the channel selectivity. Analysis of the single-channel conductance as a function of the salt concentration and of the open probability at various voltages suggests that this channel is a new membrane porin not previously identified.

K+-conducting ion channel of the chloroplast inner envelope: functional reconstitution into liposomes

Potassium flux between the chloroplast stroma and cytoplasm is known to be indirectly linked to H+ countertransport and, hence, stromal pH and photosynthetic capacity. The specific molecular mechanism that facilitates K+ flux across the chloroplast envelope is not known and has been a source of controversy for well over a decade. The objective of this study was to elucidate the nature of this envelope protein. To this end, solubilized protein in detergent extracts of purified chloroplast inner envelope vesicles was reconstituted into artificial liposomes, and cation fluxes into these proteoliposomes were measured. Results of inhibitor studies and counterflux experiments indicated that a K+-conducting ion channel was solubilized and functionally reconstituted into the proteoliposomes. This transport protein may be a nonspecific monovalent cation channel. This report represents a direct demonstration of ion channel activity associated with the limiting (inner) membrane of the chloroplast envelope.

K stimulation of ATPase activity associated with the chloroplast inner envelope

Studies were conducted to characterize ATPase activity associated with purified chloroplast inner envelope preparations from spinach (Spinacea oleracea L.) plants. Comparison of free Mg(2+) and Mg.ATP complex effects on ATPase activity revealed that any Mg(2+) stimulation of activity was likely a function of the use of the Mg.ATP complex as a substrate by the enzyme; free Mg(2+) may be inhibitory. In contrast, a marked (one- to twofold) stimulation of ATPase activity was noted in the presence of K(+). This stimulation had a pH optimum of approximately pH 8.0, the same pH optimum found for enzyme activity in the absence of K(+). K(+) stimulation of enzyme activity did not follow simple Michaelis-Menton kinetics. Rather, K(+) effects were consistent with a negative cooperativity-type binding of the cation to the enzyme, with the K(m) increasing at increasing substrate. Of the total ATPase activity associated with the chloroplast inner envelope, the K(+)-stimulated component was most sensitive to the inhibitors oligomycin and vanadate. It was concluded that K(+) effects on this chloroplast envelope ATPase were similar to this cation's effects on other transport ATPases (such as the plasmalemma H(+)-ATPase). Such ATPases are thought to be indirectly involved in active K(+) uptake, which can be facilitated by ATPase-dependent generation of an electrical driving force. Thus, K(+) effects on the chloroplast enzyme in vitro were found to be consistent with the hypothesized role of this envelope ATPase in facilitating active cation transport in vivo.

Stromal pH and Photosynthesis Are Affected by Electroneutral K and H Exchange through Chloroplast Envelope Ion Channels

Potassium movement across the limiting membrane of the chloroplast inner envelope is known to be linked to counterex-change of protons. For this reason, K(+) efflux is known to facilitate stromal acidification and the resultant photosynthetic inhibition. However, the specific nature of the chloroplast envelope proteins that facilitate K(+) fluxes, and the biophysical mechanism which links these cation currents to H(+) counterflux, is not characterized. It was the objective of this work to elucidate the nature of the system regulating K(+) flux linked to H(+) counterflux across the chloroplast envelope. In the absence of external K(+), exposure of spinach (Spinacia oleracea) chloroplasts to the K(+) ionophore valinomycin was found to increase the rate of K(+) efflux and H(+) influx. These data were interpreted as suggesting that H(+) counterexchange must be indirectly linked to movement of K(+) across the envelope. Studies using the K(+) channel blocker tetraethylammonium indicated that K(+) likely moves, in a uniport fashion, into or out of the stroma through a monovalent cation channel in the envelope. Blockage of K(+) efflux from the stroma by exposure to tetraethylammonium was found to restrict H(+) influx, further substantiating an indirect linkage of these cation currents. Further studies comparing the effect of exogenous H(+) ionophores and K(+)/H(+) exchangers suggested that K(+) uniport through this ion channel likely is the main endogenous pathway for K(+) currents across the envelope. These experiments were also consistent with the presence of a proton channel in the envelope. Movement of H(+) through this channel was speculated to be regulated and rate limited by an electroneutral requirement for K(+) countercurrents through the separate K(+) uniport pathway. K(+) and H(+) fluxes across the chloroplast envelope were envisioned to be interrelated via this mechanism. The significant effect of cation currents across the envelope, as mediated by these channels, on photosynthetic capacity of the isolated chloroplast was also demonstrated.

Lidocaine and ATPase inhibitor interaction with the chloroplast envelope

Photosynthetic capacity of isolated intact chloroplasts is known to be sensitive to K(+) fluxes across the chloroplast envelope. However, little is known about the system of chloroplast envelope proteins that regulate this K(+) movement. The research described in this report focused on characterizing some of the components of this transport system by examining inhibitor effects on chloroplast metabolism. Digitoxin, an inhibitor of membrane-bound Na(+)/K(+) ATPases, was found to reduce stromal K(+) at a range of external K(+) and inhibit photosynthesis. Scatchard plot analysis revealed a specific protein receptor site with a K(m) for digitoxin binding of 13 nanomolar. Studies suggested that the receptor site was on the interior of the envelope. The effect of a class of amine anesthetics that are known to be K(+) channel blockers on chloroplast metabolism was also studied. Under conditions that facilitate low stromal pH and concomitant photosynthetic inhibition, the anesthetic, lidocaine, was found to stimulate photosynthesis. This stimulation was associated with the maintenance of higher stromal K(+). Comparison of the effects on photosynthesis of lidocaine analogs which varied in lipophilicity suggested a lipophilic pathway for anesthetic action. The results of experiments with lidocaine and digitoxin were consistent with the hypothesis that a K(+) channel and a K(+)-pumping envelope ATPase contribute to overall K(+) flux across the chloroplast envelope. Under appropriate assay conditions, photosynthetic capacity of isolated chloroplasts was shown to be much affected by the activity of these putative envelope proteins.

Surface Charge-Mediated Effects of Mg on K Flux across the Chloroplast Envelope Are Associated with Regulation of Stromal pH and Photosynthesis

Studies of Spinacia oleracea L. were undertaken to characterize further how Mg(2+) external to the isolated intact chloroplast interacts with stromal K(+), pH, and photosynthetic capacity. Data presented in this report were consistent with the previously developed hypothesis that millimolar levels of external, unchelated Mg(2+) result in lower stromal K(+), which somehow is linked to stromal acidification. Stromal acidification directly results in photosynthetic inhibition. These effects were attributed to Mg(2+) interaction (binding) to negative surface charges on the chloroplast envelope. Chloroplast envelope-bound Mg(2+) was found to decrease the envelope membrane potential (inside negative) of the illuminated chloroplast by 10 millivolts. It was concluded that Mg(2+) effects on photosynthesis were likely not mediated by this effect on membrane potential. Further experiments indicated that envelope-bound Mg(2+) caused lower stromal K(+) by restricting the rate of K(+) influx; Mg(2+) did not affect K(+) efflux from the stroma. Mg(2+) restriction of K(+) influx appeared consistent with the typical effects imposed on monovalent cation channels by polyvalent cations that bind to negatively charged sites on a membrane surface near the outer pore of the channel. It was hypothesized that this interaction of Mg(2+) with the chloroplast envelope likely mediated external Mg(2+) effects on chloroplast metabolism.