Structural and functional changes associated with heat-induced phase-separations of non-bilayer lipids in chloroplast thylakoid membranes

AbstractThe effect of hearing isolated bean chloroplasts on the structure of their thylakoid membranes has been examined by freeze-fracture electron microscopy. A normal morphology of the membranes in which stacked grana can be observed is preserved up to 35°C. Incubation at 35–45°C causes complete destacking of the grana but no alteration in the distribution of the membrane-associated particles between the exoplasmic and protoplasmic fracture faces. Heating above 45°C causes phase-separation of non-bilayer lipids into aggregates of cylindrical inverted micelles. Bleaching experiments show that destacking is associated with disruption of chlorophyll-protein complexes of both photosystems I and II. The rate of electron transport through the photosystems is also perturbed. These results are discussed in terms of the role of non-bilayer lipids in packaging the membrane proteins

Reinterpretation of Azolla primaeva (Azollaceae, Eocene, Canada) using electron microscopy and X-ray tomographic microscopy

Azolla primaeva (Penhallow) Arnold fertile whole plants from the lower Eocene of Driftwood Creek, Canada have been examined using LM, SEM, TEM and SRXTM methods on hand specimens and sieved residues. The new data have resulted in an emended diagnosis. The megaspore is partly covered by filosum and the wall lacks excrescences. The exoperine is highly variable with nodular, to clavate, baculate and tabulate elements, which fuse to form a foveolate to reticulate megaspore surface, which is complex and contorted. Glochidia on the microspore massulae are sometimes septate and the flukes on their anchor-shaped tips are variable, some being long, narrow and wavy, others shorter and broad and a few are recurved. Hairs are lacking on the massula surface and glochidia. The organization of the float zone of the megaspore apparatus has always been controversial with 0, 1, 3 or 9 floats proposed. The new results show that there are no pseudovacuolate floats. By contrast, the ‘float’ zone consists of bundles of modified granular and filamentous endoperine elements surrounded by bundles of broader suprafilosum hairs. The absence of pseudovacuolate floats in A. primaeva is suggested to be an important character for phylogenetic analyses. The high variability in glochidia tips, glochidia septa and exoperine organisation is unusual within the genus Azolla. The new data on A. primaeva are compared with the other two whole plant Azolla species from Canada (Azolla stanleyi and Azolla schopfii) and with dispersed Azolla reproductive structures from Canada including the Arctic. This confirms that all are distinct species

Photoprotective Energy Dissipation Involves the Reorganization of Photosystem II Light-Harvesting Complexes in the Grana Membranes of Spinach Chloroplasts[W]

The rapidly reversible macrostructural changes in higher-plant chloroplast thylakoid membrane organization accompanying photoprotective energy dissipation (qE) are studied using freeze-fracture electron and laser confocal microscopy. qE is shown to involve the aggregation of light-harvesting complexes and their segregation from photosystem II