Magdolna Droppa

Formation of the photosynthetic apparatus during greening of cadmium-poisoned barley leaves

The effect of cadmium on the formation of the photosynthetic apparatus of greening barley (Hordeum vulgare L. cv. Triangel) leaves has been investigated. Cadmium treatment of dark-grown leaves strongly reduced the extent of chlorophyll accumulation during greening. Low-temperature fluorescence emission showed, however, that neither the synthesis nor photoconversion of protochlorophyllide was inhibited, although a blue shift of the main fluorescence emission from 685 to 668 mm was found. Chlorophyll fluorescence lifetime was followed by measuring the phase-shift angle of modulated emission. Whereas this parameter normally decreases rapidly during greening, this change proceeded noticeably slower with increasing severity according to cadmium concentration. Cadmium also decreased the variable part of fluorescence induction. These results suggest that the cadmium in greening leaves, rather than interfering with chlorophyll biosynthesis, acts mainly by disturbing the integration of chlorophyll molecules into the stable complexes required for normal functional photoysnthetic activity.

The role of copper in photosynthesis

Direct involvement of Cu in the photosynthetic electron transport processes and effects of Cu which indirectly influence photosynthetic efficiency by modifying the lipid environment of electron transport, chloroplast ultrastructure and pigment and lipid biosynthesis. Cu deficiency and excess Cu addition are also discussed

Characteristics of Cu deficiency-induced inhibition of photosynthetic electron transport in spinach chloroplasts

Abstract In the present work we studied the effect of Cu deficiency on spinach chloroplasts. We found that in spinach the electron transport was inhibited as reported previously for sugar beet (Droppa, M., Terry, N. and Horvath, G. (1984) Proc. Natl. Acad. Sci. USA (1984) 81, 2369–2373). The breakpoint of the Arrhenius plot of the whole electron-transport activity was shifted from +6°C to +12°C in Cu-deficient chloroplasts. A similar effect could be observed with a spin-labelled probe, when the rotational correlation time was plotted vs. the reciprocal temperatures. This indicates that the membrane fluidity might be changed by Cu deficiency. The lipid/protein ratios were similar in both control and deficient chloroplasts. On the other hand, the saturated/unsaturated ratio of phosphatidylcholine (PC), phosphatidylglycerol (PG) and sulpholipids (SL) was increased but that of monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) decreased. We conclude that Cu deficiency does not change the entire membrane fluidity but rather the lipid composition of the microenvironment of some electron-transport components. The inhibition of Photosystem II electron transport in Cu-deficient chloroplasts was characterized by thermoluminescence and 2-dimensional gel electrophoresis. It was found that Cu deficiency shifted the main peak of the glow curve from +18°C to +8°C, similar to that of DCMU-poisoned chloroplasts. Two apoproteins of the 29 kDa polypeptide disappeared in Cu-deficient chloroplasts which indicates that this polypeptide has a regulatory role in ensuring the normal electron flow between Q A and Q B .

The effect of copper on chlorophyll organization during greening of barley leaves

The effect of copper on chlorophyll organization and function during greening of barley was examined, using chlorophyll fluorescence and photoacoustic techniques. Copper was found to inhibit pigment accumulation and to retard chlorophyll integration into the photosystems, as evident from low temperature (77 K) fluorescence spectra. Resolution of the minimal fluorescence (F0) into active and inactive parts, indicated a higher inactive fraction with copper treatment. This was attributed to chlorophyll molecules which failed to integrate normally, a conclusion supported by the longer fluorescence lifetime observed in copper treated plants. A lower ratio of chlorophyll a to b and fluorescence induction transients, showing accelerated Photosystem II closure, both indicate that copper treatment resulted in a larger light-harvesting antenna. Another effect of copper treatment was the suppression of oxygen evolution, indicating a decrease in photosynthetic capacity. We suggest that the non-integrated chlorophyll fraction sensitizes photodamage in the membrane, contributing to disruption of electron flow and pigment accumulation.

Role of lipids in the organization and function of Photosystem II studied by homogeneous catalytic hydrogenation of thylakoid membranes in situ

Abstract The effects of homogeneous catalytic hydrogenation on the organization and function of Photosystem II was investigated by fluorescence induction kinetic measurements with isolated chloroplasts. The results showed that saturation of the double bonds of fatty acyl constituents of membrane lipids had distinct effects on PS II. We found that progressive hydrogenation resulted in a gradual decrease of PS II electron transport with a concomitant increase of the initial fluorescence rise from F0, the non-variable level, to Fp1, the intermediate plateau level, indicating that an optimal saturation level of fatty acids of membrane lipids is important in maintaining the efficient electron transfer from Q−A to plastoquinone pool. In addition, the fluorescence induction kinetics of DCMU-poisoned chloroplasts showed that the proportion of PS IIβ in the membrane was increased by hydrogenation, suggesting that the saturation level of fatty acids may play an important role in regulating the association of the peripheral chlorophyll a b light-harvesting complex II (LHC II-peripheral) with PS IIβ, thus determining the PS II heterogeneity in PS IIα and PS IIβ.

Molecular rearrangements of thylakoids after heavy metal poisoning, as seen by Fourier transform infrared (FTIR) and electron spin resonance (ESR) spectroscopy

The specific effects exerted by different heavy metals on both the function and the structure of the photosynthetic apparatus were addressed. The functional analysis performed via the fluorescence induction kinetics revealed that the applied toxic heavy metals can be classified into two groups: Cd and Ni had no significant effect on the photosynthetic electron transport, while Cu, Pb and Zn strongly inhibited the Photosystem II (PS II) activity, as evidenced by the dramatic decreases in both the variable (Fv) and the maximal (Fm) fluorescence. The structural effects of the heavy metal ions on the thylakoid membranes were considered in three relations: (1) lipids, (2) proteins — studied by Fourier transform infrared (FTIR) spectroscopy, and (3) lipid—protein interactions — investigated by electron spin resonance (ESR) spectroscopy using spin-labeled probe molecules. The studied heavy metal ions had only a non-specific rigidifying effect on the thylakoid lipids. As regards proteins, Cd and Ni had no effect on the course of their heat denaturation. The heat denaturation of the proteins was accompanied by a decrease in the α-helix content (1656 cm-1), a parallel increase in the disordered segments (1651 cm-1), a decrease in the intramolecular β-sheet (1636 cm-1) content and the concomitant appearance of an intermolecular β-structure (1621 cm-1). In contrast with Cd and Ni, Cu and Zn blocked the appearance of the intermolecular β-structure. Pb represented an intermediate case. It seems that these heavy metals alter the native membrane structure in such a way that heat-induced aggregation becomes more limited. The ESR data revealed that certain heavy metals also affect the lipid—protein interactions. While Cd and Ni had hardly any effect on the solvation fraction of thylakoid lipids, Cu, Pb and Zn increased the fraction of lipids solvating the proteins. On the basis of the FTIR and ESR data, it seems that Cu, Pb, and Zn increase the surfaces available for lipid—protein interactions by dissociating membrane protein complexes, and that these ‘lipidated’ proteins have a smaller chance to aggregate upon heat denaturation. The data presented here indicate that the damaging effects of poisonous heavy metals are element-specific, Cu, Pb and Zn interact directly with the thylakoid membranes of the photosynthetic apparatus, while Cd and Ni interfere rather with other metabolic processes of plants.

The role of phospholipids in regulating photosynthetic electron transport activities: Treatment of thylakoids with phospholipase C

The involvement of phospholipids in the regulation of photosynthetic electron transport activities was studied by incubating isolated pea thylakoids with phospholipase C to remove the head-group of phospholipid molecules. The treatment was effective in eliminating 40–50% of chloroplast phospholipids and resulted in a drastic decrease of photosynthetic electron transport. Measurements of whole electron transport (H2O→methylviologen) and Photosystem II activity (H2O→p-benzoquinone) demonstrated that the decrease of electron flow was due to the inactivation of Photosystem II centers. The variable part of fluorescence induction measured in the absence of electron acceptor was decreased by the progress of phospholipase C hydrolysis and part of the signal could be restored on addition of 3-(3′,4′-dicholorophenyl)-1,1-dimethylurea. The B and Q bands of thermoluminescence corresponding to S2S3QB− and S2S3QA− charge recombination, respectively, was also decreased with a concomitant increase of the C band, which originated from the tyrosine D+QA− charge recombination. These results suggest that phospholipid molecules play an important role in maintaining the membrane organization and thus maintaining the electron transport activity of Photosystem II complexes.

Mode of action of Photosystem II herbicides studied by thermoluminescence

Abstract The mode of action of chemically different herbicides (ureas, pyridazinones, phenylcarbamates, triazines, hydroxyquinolines, hydroxybenzonitriles and dinitrophenols) on photosynthetic electron transport was investigated by measurements of oxygen evolution and thermoluminescence. Depending on the particular herbicide used the thermoluminescence band related to Q (the primary acceptor of Photosystem II) appears at +5, 0 or −14°C. It was shown that these three different peak positions can be ascribed to various redox states of Q, the shifts being due to the binding of herbicides to the chloroplast membrane. Both displacement experiments and additive inhibition of herbicide pairs measured by thermoluminescence and oxygen evolution suggested that the sites of action of these herbicides are on the same protein. However, herbicide treatment of trypsinized chloroplasts showed that there were three different binding sites on the same protein, in agreement with the classification of herbicides into three groups based on thermoluminescence measurements. Our results suggest that the primary and secondary acceptors of Photosystem II (Q and B, respectively) are in close proximity and form a common complex with the herbicide-binding protein within the chloroplast membrane.

Thermoluminescence characteristics of granal and agranal chloroplasts of maize

Materials illuminated at temperatures below 0°C emit light upon heating. This phenomenon is called thermoluminescence and was first observed with chloroplasts by Arnold and Sherwood [ 11. The emission bands of thermoluminescence are a result of charge recombination between the various positively charged electron donors and negatively charged electron acceptors of the photosystems [2-41. According to most authors thermoluminescence originates only from photosystem II [5-71 but Sane et al. [2] as well as Shuvalov and Litvin [S] attributed some of the glow peaks to photosystem I. Inoue and Shibata [3,9] demonstrated that the emission of the bands at +25”C and +4O”C is linked to the positively charged S states of the water-splitting enzyme. Few data, however, are available on the location of negative charges participating in the charge recombination [2-41. The aim of the present study was to obtain further information about the involvement of photosystem I in thermoluminescence and to correlate the main bands of the glow curve with the different components of the electron transport chain.

Functional characteristics of intact chloroplasts isolated from mesophyll protoplasts and bundle sheath cells of maize

A procedure was developed to isolate mesophyll and bundle sheath chloroplasts of a high degree of intactness and low cross-contamination. Light-induced 14CO2 fixation of isolated chloroplasts was similar to that of protoplasts and cells in that it was low and was stimulated by the addition of exogenous substrates. O2 evolution was absent in both bundle sheath chloroplasts and cells. The flash-induced 515 nm absorbance change of intact mesophyll chloroplasts showed a biphasic rise, previously known to be a characteristic only of intact algae. With bundle sheath chloroplasts or cells, no 515 nm signal could be detected. In the presence of 10 μmol l-1 phenazine methosulphate, bundle sheath chloroplasts exhibited a flash-induced 515 nm signal with a monophasic rise and amplitude comparable to that of the mesophyll chloroplasts. A similar signal was obtained with bundle sheath chloroplasts suspended in an extract prepared from the mesophyll tissue. Both the substrate stimulation of the CO2 fixation and the reconstitution of the 515 nm signal in bundle sheath chloroplasts by the mesophyll extract indicate the requirement of cooperation between the mesophyll and bundle sheath cells of maize leaves.