INVOLVEMENT OF THYLAKOID MEMBRANE-DEPENDENT PHOTOSENSITIZATION IN PHOTOINHIBITION OF THE CALVIN CYCLE ACTIVITY IN SPINACH CHLOROPLASTS

Singlet Oxygen in Plants—Its Significance and Possible Detection with Double (Fluorescent and Spin) Indicator Reagents

Direct detection of reactive oxygen species (ROS), especially singlet oxygen, in plants under stress conditions is of special importance, not only to identify primary events of oxidative damage, but also in studies exploring the potential role of ROS as signal molecules. Due to short life-times and diffusion distances of ROS, these tasks require highly reactive and selective indicator reagents, localized at the presumed site of production. In the present study, we compared four double sensors: ROS indicator reagents in which partial fluorescence quenching of a dansyl moiety occurs as a result of nitroxide radical formation from a sterically hindered amine constituent. Our experiments support the idea that shorter donor-acceptor distances within these molecules result in higher reactivity to ROS. The presence of a diethylaminoethyl side chain resulted in better selectivity to singlet oxygen: reagents lacking such substituent had an additional reactivity to superoxide anions, probably as a result of the formation of zwitterionic structures. Fluorescence localization studies of the indicator reagents in tobacco leaves and in Chlamydomonas cells show promising perspectives of their applications to plant stress studies.

Photoinhibition of photosynthesis in chilled potato leaves is not correlated with a loss of Photosystem-II activity : Preferential inactivation of Photosystem I

When 23 °C-grown potato leaves (Solanum tuberosum L.) were irradiated at 23 °C with a strong white light, photosynthetic electron transport and Photosystem-II (PS II) activity were inhibited in parallel. When the light treatment was given at a low temperature of 3 °C, the photoinhibition of photosynthesis was considerably enhanced, as expected. Surprisingly, no such stimulation of photoinhibition was observed with respect to the PS II function. A detailed functional analysis of the photosynthetic apparatus, using in-vivo fluorescence, absorbance, oxygen and photoacoustic measurements, and artificial electron donors/acceptors, showed a pronounced alteration of PS I activity during light stress at low temperature. More precisely, it was observed that both the pool of photooxidizeable reaction center pigment (P700) of PS I and the efficiency of PS I to oxidize P700 were dramatically reduced. Loss of P700 activity was shown to be essentially dependent on atmospheric O2 and to require a continued flow of electrons from PS II, suggesting the involvement of the superoxide anion radical which is produced by the interaction of O2 and the photosynthetic electron-transfer chain through the Mehler reaction. Mass spectrometric measurements of O2 exchange by potato leaves under strong illumination did not reveal, however, any stimulation of the Mehler reaction at low temperature, thus leading to the conclusion that O2 toxicity mainly resulted from a chilling-induced inhibition of the scavenging system for O2-radicals. Support for this interpretation was provided by the light response of potato leaves infiltrated with an inhibitor (diethyldithiocarbamate) of the chloroplastic Cu-Zn superoxide dismutase. It was indeed possible to simulate the differential inhibition of the PS II photochemical activity and the linear electron transport observed during light stress at low temperature by illuminating at 23 °C diethyldithiocarbamate-poisoned leaves. The experimental data presented here suggests that (i) the previously reported resistance of PS I to photoinhibition damage in-vivo is not an intrinsic property of PS I but results from efficient protective systems against O2 toxicity, (ii) PS I is photoinhibited in chilled potato leaf due to the inactivation of this PS I defence system and (iii) PS I is more sensitive to superoxide anion radicals than PS II.

Iron-sulfur centers as endogenous blue light sensitizers in cells: a study with an artificial non-heme iron protein

The possible involvement of Fe-S clusters in photodynamic reactions as endogenous sensitizing chromophores in cells has been investigated, by using an artificial non-heme iron protein (ANHIP) derived from bovine serum albumin and ferredoxins isolated from spinach and a red marine algae. Ferredoxins and ANHIP, when exposed to visible light, generate singlet oxygen, as measured by the imidazole plus RNO method. Irradiation with intense blue light of the ANHIP-entrapped liposomes caused severe membrane-damage such as liposomal lysis and lipid peroxidation. In the presence of ANHIP, isocitrate dehydrogenase and fructose-1,6-diphosphatase were photoinactivated by blue light. However, all of these photosensitized reactions were significantly suppressed by a singlet oxygen (1O2) quencher, azide, but enhanced by a medium containing deuterium oxide. Further, the Fe-S proteins with the prosthetic groups destroyed did not initiate the blue light-induced reactions. In addition, the action spectrum for 1O2 generation from ANHIP was very similar to the visible absorption spectrum of Fe-S centers. The results obtained in this investigation appear consistent with the suggestion that Fe-S centers are involved in photosensitization in cells via a singlet oxygen mechanism.