Aquaporin-deficient mutant of Synechocystis is sensitive to salt and high-light stress.

Cyanobacterial aquaporins play an important role in the regulation of various physiological functions: cell volume control, osmotic stress responses, gas exchange. We employed the AqpZ-deficient mutant of Synechocystis to study the role of aquaporins in responses to salt (NaCl) and high light stress. Electron microscopy and paramagnetic resonance revealed that AqpZ-deficient cells are unable to efficiently regulate the cytoplasmic volume under salt stress. Both photosystems (PSII and, especially, PSI) of these cells are more sensitive to NaCl and to high light. Thus, AqpZ of Synechocystis participates in regulation of the photosynthetic activity of PSI and PSII under salt and high-light stress. Our results demonstrate that AqpZ might be necessary for the repair of PSII and PSI after photodamage.

The three-dimensional network of the thylakoid membranes in plants: quasihelical model of the granum-stroma assembly.

The three-dimensional (3-D) network of the granum-stroma thylakoid assembly of vascular plant chloroplasts exhibits complex structural/functional heterogeneity. A complete understanding of the ultrastructure of this assembly is critical for our understanding of thylakoid function. The prevailing historical model of thylakoid structure, based on information derived from serial section analyses of electron microscopy (EM) images, suggests a helical arrangement of stroma membranes wound around the granum stacks. More recently, electron tomography has emerged as the leading method for the study of thylakoid ultrastructure, as it provides for higher resolution in the depth dimension. The first detailed 3D topological model derived from electron tomography was in disagreement with the helical model, whereas a more recent electron tomography study, conducted under somewhat different experimental conditions, suggested that basic features of the helical model are still valid. Here, we review the conventional EM data and present a critical discussion of the two electron tomography data sets in an attempt to establish a consensus model that accommodates all the information presently available.

Structurally flexible macro-organization of the pigment-protein complexes of the diatom Phaeodactylum tricornutum.

By means of circular dichroism (CD) spectroscopy, we have characterized the organization of the photosynthetic complexes of the diatom Phaeodactylum tricornutum at different levels of structural complexity: in intact cells, isolated thylakoid membranes and purified fucoxanthin chlorophyll protein (FCP) complexes. We found that the CD spectrum of whole cells was dominated by a large band at (+)698 nm, accompanied by a long tail from differential scattering, features typical for psi-type (polymerization or salt-induced) CD. The CD spectrum additionally contained intense (-)679 nm, (+)445 nm and (-)470 nm bands, which were also present in isolated thylakoid membranes and FCPs. While the latter two bands were evidently produced by excitonic interactions, the nature of the (-)679 nm band remained unclear. Electrochromic absorbance changes also revealed the existence of a CD-silent long-wavelength ( approximately 545 nm) absorbing fucoxanthin molecule with very high sensitivity to the transmembrane electrical field. In intact cells the main CD band at (+)698 nm appeared to be associated with the multilamellar organization of the thylakoid membranes. It was sensitive to the osmotic pressure and was selectively diminished at elevated temperatures and was capable of undergoing light-induced reversible changes. In isolated thylakoid membranes, the psi-type CD band, which was lost during the isolation procedure, could be partially restored by addition of Mg-ions, along with the maximum quantum yield and the non-photochemical quenching of singlet excited chlorophyll a, measured by fluorescence transients.

Protective effect of supplemental low intensity white light on ultraviolet-B exposure-induced impairment in cyanobacterium Spirulina platensis: formation of air vacuoles as a possible protective measure.

Intact trichomes of Spirulina platensis were exposed to 1-5 h of low (0.2 mW cm(-2)) or high (0.6 mW cm(-2)) intensity UV-B (280-320 nm) radiation, alone or with photosynthetically active radiation (PAR) of supplemental 50 muE m(-2) s(-1) white light (WL). The mitigating effect of supplemental WL on UV-B induced alterations in Spirulina were investigated by monitoring time-dependent change in photosystem (PS) II mediated O(2) evolution, absorption, circular dichroism (CD) spectra, and ultrastructure. At low intensity, UV-B induced loss in PS II-catalyzed O(2) evolution, but caused no change in the absorption spectrum. At high intensity, UV-B caused a decrease in absorption by phycobilisomes (PBsomes), which was only partly prevented by the presence of low-intensity supplemental WL. The CD spectral analysis revealed that UV-B exposure caused time-dependent enhancement of the negative psi-type bands at 452 and 689 nm, reflecting alterations in the macroaggregation of chlorophyll-protein complexes. This enhancement of negative PS II-type bands was substantially arrested by the presence of supplemental WL exposure, even when UV-B exposure was continued for 5 h. These changes in UV-B-induced CD spectrum suggest alterations in the antenna structure of Spirulina involving both PBsomes and Chlorophyll a. Thus, supplemental low intensity WL arrests, to large extent, the macroaggregation of pigment-protein complexes. Furthermore, the electron micrographs of Spirulina revealed that UV-B exposure caused disorganization of the cellular ultrastructure, while the inclusion of supplemental WL enhanced the formation of air vacuoles in Spirulina. We suggest that the formation of vacuoles by supplemental WL is a protective feature against UV-B.

Enhanced formation of flowers in salt-stressed Arabidopsis after genetic engineering of the synthesis of glycine betaine.

Previously, we showed that transformation with the codA gene for choline oxidase allows plants to synthesize glycine betaine (GB) and enhances their ability to tolerate various kinds of stress during germination and vegetative growth. In this study, we examined the tolerance of transformed plants to salt stress at the reproductive stage, which is the stage at which plants are most sensitive to environmental stress. Salt-shock treatment of wild-type plants for 3 days resulted in the abortion of flower buds and decreased the number of seeds per silique. These deleterious effects were clearly visible 6 days after the termination of salt-shock treatment. Microscopic examination of floral structures revealed that salt stress inhibited the development of anthers, pistils, and petals. In particular, the production of pollen grains and ovules was dramatically inhibited. These effects of salt stress were significantly reduced by transformation with the codA gene, and our observations suggested that the enhanced tolerance of the transgenic plants was a result of the accumulation of GB in the reproductive organs. Indeed, levels of GB in flowers, siliques, and inflorescence apices were about five times higher than in leaves.

Glucosylglycerol, a compatible solute, sustains cell division under salt stress.

The cyanobacterium Synechocystis sp. PCC 6803 accumulates the compatible solute glucosylglycerol (GG) and sucrose under salt stress. Although the molecular mechanisms for GG synthesis including regulation of the GG-phosphate synthase (ggpS) gene, which encodes GgpS, has been intensively investigated, the role of GG in protection against salt stress remains poorly understood. In our study of the role of GG in the tolerance to salt stress, we found that salt stress due to 450 mM NaCl inhibited cell division and significantly increased cell size in DeltaggpS mutant cells, whereas the inhibition of cell division and increase in cell size were observed in wild-type cells at high concentrations of NaCl, such as 800 mM. Electron microscopy revealed that, in DeltaggpS cells, separation of daughter cells was incomplete, and aborted division could be recognized by the presence of a structure that resembled a division ring. The addition of GG to the culture medium protected DeltaggpS cells against salt stress and reversed the adverse effects of NaCl on cell division and cell size. These observations suggest that GG is important for salt tolerance and thus for the proper division of cells under salt stress conditions.

Granum revisited. A three-dimensional model--where things fall into place.

The complex structure of higher plant chloroplasts has fascinated researchers for many years. Although the spatial relationship between granum and stroma thylakoids has been known for more than 20 years, most textbooks and research papers continue to include erroneous 3D models and simplified schemes. Here we present a simple computer model, based on electron micrographs from serial section of granum-stroma assemblies, showing the striking 3D structure of the stroma membrane wound around the granum. This model also provides an insight into some previously unknown functions of this intriguing multilamellar membrane system. However, many areas, such as self-assembly, structural flexibility and evolutionary niche, still remain to be explored.

Light-induced trimer to monomer transition in the main light-harvesting antenna complex of plants: thermo-optic mechanism.

The main chlorophyll a/b light-harvesting complex of photosystem II, LHCIIb, has earlier been shown to be capable of undergoing light-induced reversible structural changes and chlorophyll a fluorescence quenching in a way resembling those observed in granal thylakoids when exposed to excess light [Barzda, V., et al. (1996) Biochemistry 35, 8981-8985]. The nature and mechanism of this unexpected structural flexibility has not been elucidated. In this work, by using density gradient centrifugation and nondenaturing green gel electrophoresis, as well as absorbance and circular dichroic spectroscopy, we show that light induces a significant degree of monomerization, which is in contrast with the preferentially trimeric organization of the isolated complexes in the dark. Monomerization is accompanied by a reversible release of Mg ions, most likely from the outer loop of the complexes. These data, as well as the built-in thermal and light instability of the trimeric organization, are explained in terms of a simple theoretical model of thermo-optic mechanism, effect of fast thermal transients (local T-jumps) due to dissipated photon energies in the vicinity of the cation binding sites, which lead to thermally assisted elementary structural transitions. Disruption of trimers to monomers by excess light is not confined to isolated trimers and lamellar aggregates of LHCII but occurs in photosystem II-enriched grana membranes, intact thylakoid membranes, and whole plants. As indicated by differences in the quenching capability of trimers and monomers, the appearance of monomers could facilitate the nonphotochemical quenching of the singlet excited state of chlorophyll a. The light-induced formation of monomers may also be important in regulated proteolytic degradation of the complexes. Structural changes driven by thermo-optic mechanisms may therefore provide plants with a novel mechanism for regulation of light harvesting in excess light.