The vesicle-inducing protein in plastids (VIPP1) was suggested to play a role in thylakoid membrane formation via membrane vesicles. As this functional assignment is under debate, we investigated the function of VIPP1 in Chlamydomonas reinhardtii. Using immunofluorescence, we localized VIPP1 to distinct spots within the chloroplast. In VIPP1-RNA interference/artificial microRNA cells, we consistently observed aberrant, prolamellar body-like structures at the origin of multiple thylakoid membrane layers, which appear to coincide with the immunofluorescent VIPP1 spots and suggest a defect in thylakoid membrane biogenesis. Accordingly, using quantitative shotgun proteomics, we found that unstressed vipp1 mutant cells accumulate 14 to 20% less photosystems, cytochrome b 6 f complex, and ATP synthase but 30% more light-harvesting complex II than control cells, while complex assembly, thylakoid membrane ultrastructure, and bulk lipid composition appeared unaltered. Photosystems in vipp1 mutants are sensitive to high light, which coincides with a lowered midpoint potential of the Q A /Q A 2 redox couple and increased thermosensitivity of photosystem II (PSII), suggesting structural defects in PSII. Moreover, swollen thylakoids, despite reduced membrane energization, in vipp1 mutants grown on ammonium suggest defects in the supermolecular organization of thylakoid membrane complexes. Overall, our data suggest a role of VIPP1 in the biogenesis/assembly of thylakoid membrane core complexes, most likely by supplying structural lipids.

High irradiances may lead to photooxidative stress in plants, and non-photochemical quenching (NPQ) contributes to protection against excess excitation. One of the NPQ mechanisms, qE, involves thermal dissipation of the light energy captured. Importantly, plants need to tune down qE under light-limiting conditions for efficient utilization of the available quanta. Considering the possible redox control of responses to excess light implying enzymes, such as thioredoxins, we have studied the role of the NADPH thioredoxin reductase C (NTRC). Whereas Arabidopsis thaliana plants lacking NTRC tolerate high light intensities, these plants display drastically elevated qE, have larger trans-thylakoid ΔpH and have 10-fold higher zeaxanthin levels under low and medium light intensities, leading to extremely low linear electron transport rates. To test the impact of the high qE on plant growth, we generated an ntrc-psbs double-knockout mutant, which is devoid of qE. This double mutant grows faster than the ntrc mutant and has a higher chlorophyll content. The photosystem II activity is partially restored in the ntrc-psbs mutant, and linear electron transport rates under low and medium light intensities are twice as high as compared with plants lacking ntrc alone. These data uncover a new role for NTRC in the control of photosynthetic yield.

Myriophyllum spicatum (Haloragaceae) is a highly competitive freshwater macrophyte that produces and releases algicidal and cyanobactericidal polyphenols. Among them, ␤-1,2,3-tri-O-galloyl-4,6-(S)-hexahydroxydiphenoyl-d-glucose (tellimagrandin II) is the major active substance and is an effective inhibitor of microalgal exoenzymes. However, this mode of action does not fully explain the strong allelopathic activity observed in bioassays. Lipophilic extracts of M. spicatum inhibit photosynthetic oxygen evolution of intact cyanobacteria and other photoautotrophs. Fractionation of the extract provided evidence for tellimagrandin II as the active compound. Separate measurements of photosystem I and II activity with spinach (Spinacia oleracea) thylakoid membranes indicated that the site of inhibition is located at photosystem II (PSII). In thermoluminescence measurements with thylakoid membranes and PSII-enriched membrane fragments M. spicatum extracts shifted the maximum temperature of the B-band (S 2 Q B Ϫ recombination) to higher temperatures. Purified tellimagrandin II in concentrations as low as 3 m caused a comparable shift of the B-band. This demonstrates that the target site of this inhibitor is different from the Q B -binding site, a common target of commercial herbicides like 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Measurements with electron paramagnetic resonance spectroscopy suggest a higher redox midpoint potential for the non-heme iron, located between the primary and the secondary quinone electron acceptors, Q A and Q B . Thus, tellimagrandin II has at least two modes of action, inhibition of exoenzymes and inhibition of PSII. Multiple target sites are a common characteristic of many potent allelochemicals. Allelopathy (sensu;Molisch, 1937) is considered an effective trait of submerged aquatic angiosperms (Wium-Andersen, 1987; Gross, 1999, and refs. therein) to counteract the usually strong competition for light and carbon with other primary producers, especially phytoplankton and epiphytes (SandJensen and Søndergaard, 1981). Allelopathically active compounds known from terrestrial plants generally act as natural herbicides and often have multiple effects on the metabolism of target organisms (Einhellig, 2001). Some have even been explored as natural substitutes for commercial herbicides (Duke et al., 2000). Few studies have, however, targeted the mode of action of allelochemicals from aquatic angiosperms.Several allelochemicals isolated from cyanobacteria or higher plants specifically inhibit photosynthesis of target organisms (Einhellig et al., 1993;Gonzalez et al., 1997;Smith and Doan, 1999). This results in a lower primary production and might consequently cause slower growth of competing photoautotrophs. To prove effects on primary production, the radiocarbon assay (fixation of radioactive labeled 14 C) has been frequently used, e.g. with extracts from Zostera marina (Harrison and Durance, 1985) and Chara globularis (Wium-Andersen et al., 1983). More detailed studies involved separate measureme...

Summary• Cadmium (Cd 2+ ) is an environmental pollutant that causes increased reactive oxygen species (ROS) production. To determine the site of ROS production, the effect of Cd 2+ on ROS production was studied in isolated soybean (Glycine max) plasma membranes, potato (Solanum tuberosum) tuber mitochondria and roots of intact seedlings of soybean or cucumber (Cucumis sativus).• The effects of Cd 2+ on the kinetics of superoxide ( ), hydrogen peroxide (H 2 O 2 ) and hydroxyl radical ( • OH) generation were followed using absorption, fluorescence and spin-trapping electron paramagnetic resonance spectroscopy.• In isolated plasma membranes, Cd 2+ inhibited production. This inhibition was reversed by calcium (Ca 2+ ) and magnesium (Mg 2+ ). In isolated mitochondria, Cd 2+ increased and H 2 O 2 production. In intact roots, Cd 2+ stimulated H 2 O 2 production whereas it inhibited and • OH production in a Ca 2+ -reversible manner.• Cd 2+ can be used to distinguish between ROS originating from mitochondria and from the plasma membrane. This is achieved by measuring different ROS individually. The immediate (≤ 1 h) consequence of exposure to Cd 2+ in vivo is stimulation of ROS production in the mitochondrial electron transfer chain and inhibition of NADPH oxidase activity in the plasma membrane.