Transmembrane movements of artificial redox mediators in relation to electron transport and ionic currents in chloroplasts

Electrochemical data obtained with TMPD⁺-sensitive electrodes indicate that ammonium-uncoupled chloroplasts retain TMPD (N,N,N',N'-tetramethyl- p -phenylenediamine) mainly in the reduced form during illumination, whereas uncoupled DCMU-treated chloroplasts accumulate TMPD in the oxidized form (TMPD⁺). This observation indicates that the reduced plastoquinol is the preferred electron donor for photosystem I (PSI) and TMPD can only compete efficiently when plastoquinone reduction is blocked. After adding DCMU the formation of a transmembrane gradient for TMPD⁺ is reflected by a slow-down of the electrogenic electron transport and by the emerging of the overshoot of the membrane current in the light-off response. A light-dependent increase in photoelectric current generated by chloroplasts in the presence of NH₄Cl and TMPD is observed and considered to be caused by a reversible release of current limitation in the interfacial conductance barriers in the lumen.     

Suppression of flash-induced PSII-dependent electrogenesis caused by proton pumping in chloroplasts

Pre-illumination of the thylakoid membrane of Peperomia metallica chloroplasts leads to a reversible suppression of the flash-induced electrical potential as measured either with the electrochromic bandshift (P515), microelectrode impalement or patch-clamp technique. The energization-dependent potential suppression was not observed in the presence of 1 μ M nigericin suggesting the involvement of proton and/or cation gradients. Energization in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and N,N,N',N'-tetramethylphenylenediamine (TMPD), i.e. cyclic electron flow around photosystem (PS) I, results in the accumulation of TMPD⁺ in the thylakoid lumen. The reversible suppression of the flash-induced membrane potential was not observed in these conditions indicating that it is not a general cation-induced increase of membrane capacitance. Cyclic electron flow around PSI in the presence of DCMU and phenazine methosulfate (PMS) results in the accumulation of PMS⁺ and H⁺ in the thylakoid lumen. The absence of reversible suppression of the flash-induced membrane potential for this condition shows that accumulation of protons does not lead to (1) a reversible increase of membrane capacitance and (2) a reversible suppression of PSI-dependent electrogenesis. Reversible inactivation of PSII by a low pH in the thylakoid lumen is therefore proposed to be the cause for the temporary suppression of the flash-induced electrical potential. The flash-induced PSII-dependent membrane potential, as measured after major oxidation of P700 in far-red background light, was indeed found to be suppressed at low assay pH (pH 5) in isolated spinach ( Spinacia oleracea ) chloroplasts.     

Introduction of polyphosphate as a novel phosphate pool in the chloroplast of transgenic potato plants modifies carbohydrate partitioning

Potato plants (Solanum tuberosum L., cv. Désirée) were transformed with the polyphosphate kinase gene from Escherichia coli fused to the leader sequence of the ferredoxin oxidoreductase gene (FNR) from Spinacea oleracea under the control of the leaf specific St-LS1 promoter to introduce a novel phosphate pool in the chloroplasts of green tissues. Transgenic plants (cpPPK) in tissue culture developed necrotic lesions in older leaves and showed earlier leaf senescence while greenhouse plants showed no noticeable phenotype. Leaves of cpPPK plants contained less starch but higher concentrations of soluble sugars. The presence of polyphosphate in cpPPK leaves was demonstrated by toluidine blue staining and unambiguously verified and quantified by in vitro 31P-NMR of extracts. Polyphosphate accumulated during leaf development from 0.06 in juvenile leaves to 0.83 mg P g-1 DW in old leaves and had an average chain length of 18 residues in mature leaves. In situ 31P-NMR on small leaf pieces perfused with well-oxygenated medium showed only 0.036 mg P g-1 DW polyphosphate that was, however, greatly increased upon treatment with 50 mM ammonium sulfate at pH 7.3. This phenomenon along with a yield of 0.47 mg P g-1 DW polyphosphate from an extract of the same leaf material suggests that 93% of the polyphosphate pool is immobile. This conclusion is substantiated by the observation that no differences in polyphosphate pool sizes could be discerned between darkened and illuminated leaves, leaves treated with methylviologen or anaerobis and control leaves, treatments causing a change in the pool of ATP available for polyPi synthesis. Results are discussed in the context of the chelating properties of polyphosphates for cations and its consequences for the partitioning of photoassimilate between starch and soluble sugars.     

Transmembrane movements of artificial redox mediators in relation to electron transport and ionic currents in chloroplasts

Electrochemical data obtained with TMPD⁺-sensitive electrodes indicate that ammonium-uncoupled chloroplasts retain TMPD (N,N,N',N'-tetramethyl- p -phenylenediamine) mainly in the reduced form during illumination, whereas uncoupled DCMU-treated chloroplasts accumulate TMPD in the oxidized form (TMPD⁺). This observation indicates that the reduced plastoquinol is the preferred electron donor for photosystem I (PSI) and TMPD can only compete efficiently when plastoquinone reduction is blocked. After adding DCMU the formation of a transmembrane gradient for TMPD⁺ is reflected by a slow-down of the electrogenic electron transport and by the emerging of the overshoot of the membrane current in the light-off response. A light-dependent increase in photoelectric current generated by chloroplasts in the presence of NH₄Cl and TMPD is observed and considered to be caused by a reversible release of current limitation in the interfacial conductance barriers in the lumen.     

Remarks on the internal structure of the shells of Ammonoidea,Gastropoda and Foraminifera — with a short introduction on biostratigraphy and paleo-ecology

The modern principles of marine biostratigraphy ask for more exact fossil determinations. In view of the increased activity in nearly all countries concerning this modern trend in geology more uniformity in the methods used for the determination of fossils and especially of microfossils, is needed. In this way a better understanding of the stratigraphical range of important species and their natural relations to other species in all parts of the world would be obtained. For this purpose the study of the internal structures is also important: siphuncular structures of the ammonite shell—heterostrophical oldest volutions of gastropods—canal systems of foraminifers. A few examples are given and figured. A few remarks on biostratigraphy, paleo-ecology, facies and guide fossils serve as an introduction.     

Temporary suppression of the flash-induced electrical potential across the thylakoid membrane upon energization

Energization of the chloroplast thylakoid membrane causes a temporary decrease in the amplitude of the flash-induced transmembrane electrical potential as monitored by the micro-electrode technique and by the electrochromic absorbance band shift at 518 nm in chloroplasts of Peperomia metallica. This energization-dependent decrease of the flash-induced potential has a relaxation time of recovery in the dark of about 23±4 s. The phenomenon can neither be explained by a decrease of the intrinsic efficiency of photosystem I and II (PSI and PSII) nor by a partial closure of reaction centers of PSI and PSII. This leads us to propose that the energization-dependent decrease of the amplitude of the flash-induced electrical potential is caused by either the formation of a fraction of PSI and/or PSII reaction centers with fast charge recombination or by an increase of the membrane capacitance. The dark recovery after energization of the amplitude of the transmembrane electrical potential and that of non-photochemical fluorescence quenching were found to be comparable, which suggests a common cause for both phenomena.