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

A complete ferredoxin (Fd) cDNA clone was isolated from potato (Solanum tuberosum L. cv Desiree) leaves. By molecular and immunoblot analysis, the gene was identified as the leaf-specific Fd isoform I. Transgenic potato plants were constructed by introducing the homologous potato fed 1 cDNA clone as an antisense construct under the control of the constitutive cauliflower mosaic virus 35S promoter. Stable antisense lines with Fd contents between 40% and 80% of the wild-type level were selected by northern- and western-blot analysis. In short-term experiments, the distribution of electrons toward their stromal acceptors was altered in the mutant plants. Cyclic electron transport, as determined by the quantum yields of photosystems I and II, was enhanced. The CO2 assimilation rate was decreased, but depending on the remaining Fd content, some lines showed photoinhibition. The leaf protein content remained largely constant, but the antisense plants had a lower total chlorophyll content per unit leaf area and an increased chlorophyll a/b ratio. In the antisense plants, the redox state of the quinone acceptor A in photosystem II (QA) was more reduced than that of the wild-type plants under all experimental conditions. Because the plants with lower Fd amounts reacted as if they were grown under a higher light intensity, the possibility that the altered chloroplast redox state affects light acclimation is discussed.

Dependence of the flash-induced oxygen evolution pattern on the chemically and far red light-modulated redox condition in cyanobacterial photosynthetic electron transport.

Flash-induced photosynthetic oxygen evolution was measured in cells and thylakoid preparations from the coccoid cyanobacteria Synechocystis sp. PCC 6803 and Synechococcus sp. PCC 7942 and from the filamentous cyanobacterium Oscillatoria chalybea. The resulting characteristic flash patterns from these cyanobacteria can be chemically altered by addition of exogenously added substances like CCCP, DCPiP and inorganic salts. Potassium chloride, manganese sulfate and calcium chloride affected the sequences by specific increases in the flash yield and/or effects on the transition parameters. Chloride appeared to exert the strongest stimulatory effect on the oxygen yield. In comparison to chloride, both manganese and calcium did not significantly stimulate the flash amplitudes as such, but improved the functioning of the oxygen evolving complex by decreasing the miss parameter alpha. Particular effects were observed with respect to the time constants of the relaxation kinetics of the first two flash signals Y1/Y2 of the cyanobacterial patterns. In the presence of the investigated chemicals the amplitudes of the first two flash signals (Y2 in particular) were increased and the relaxation kinetics were enhanced so that the time constant became about identical to the conditions of steady state oxygen flash amplitudes. The results provide further evidence against a possible participation of either PS I or respiratory processes to Y1/Y2 of cyanobacterial flash patterns. Dramatic effects were observed when protoplasts from Oscillatoria chalybea or cells from Synechocystis sp. PCC 6803 and Synechococcus sp. PCC 7942 were exposed to weak far red background illumination. Under these conditions, Y2 (and to a smaller extent Y1) of otherwise unchanged flash sequences were specifically modified. Y2 was substantially increased and again the relaxation kinetics were accelerated making the signal indistinguishable from a Y(SS) signal. From the mathematical fit of the sequences we conclude that S2 contributes to 10-20% of the S-state distribution (in comparison to 0% in the control). Thus, far red background illumination might represent a valuable means for photosynthetic investigations where high amounts of S2 are required like e. g. EPR measurements. In such experiments the corresponding EPR signals appeared substantially enhanced following far red preillumination (Ahrling and Bader, unpublished observations). Our results clearly show that the 'controversial results' from parts of the literature suggesting the participation of different mechanisms (net oxygen evolution, inhibited uptake processes etc.) are not required to explain the flash-induced oxygen evolution in cyanobacteria: the seemingly 'incompatible' conditions and conformations can be perfectly interconverted by different modulation techniques (chemicals, far red) of the respective redox condition within the water oxidation complex of photosynthesis.

18O isotope effect in the photosynthetic water splitting process.

In mass spectroscopic experiments of oxygen evolution in Photosystem II at 50% enrichment of H(2)18O, one expects equal signals of 18O(2) and 16O(2) unless one of the isotopes is favored by the oxygen evolving complex (OEC). We have observed a deviation from this expectation, being a clear indication of an isotope effect. We have measured the effect to be 1.14-1.30, which is higher than the theoretically predicted value of 1.014-1.06. This together with the strong temperature variation of the measured effect with a discontinuity at 11 degrees C observed for wild-type tobacco and at 9 degrees C for a yellow-green tobacco mutant suggest that an additional mechanism is responsible for the observed high isotope effect. The entry of a finite size of water clusters to the cleavage site of the OEC can explain the observation.

Mass Spectrometric Detection and Analysis of Nitrogen Fixation in Oscillatoria chalybea.

By means of mass spectroscopic measurements in an artifical gas atmosphere containing the stable nitrogen isotope 15N2 we were able to demonstrate nitrogen fixation capacity in the filamentous cyanobacterium Oscillatoria chalybea. Our technique proved to be wellsuited also for investigations on the light-induced nitrogen fixation in the purple bacteria Rhodobacter sphaeroides and Rhodobacter capsulatus. Oscillatoria chalybea grown without combined nitrogen showed a substantial 15N2-uptake which could clearly be correlated with nitrogen fixation. Nitrate grown cultures did not show this nitrogen uptake or only to a minimal extent. Addition of ammonium chloride resulted in a rapid deactivation of the nitrogenase system. Similar observations have been made with other so-called switch-off effectors like phenazine methosulfate. The structural integrity of the filaments appeared to be a prerequisite for nitrogen fixation also in this organism, as even mild mechanical homogenization strongly inhibited the N2-uptake signals. Illumination of the assays under conditions where the photooxidition of water is not operational (Bader, K. P. (1994), Biochim. Biophys. Acta 1188, 213 -219) did not affect the nitrogen fixation in Oscillatoria chalybea. Illumination of cultures with concomitant release of oxygen from the water splitting reaction resulted in strong inhibition of 15N2-uptake.