Large parts of the endoplasmic reticulum of the yeast, Saccharomyces cerevisiae, are located close to intracellular organelles, i.e. mitochondria and the plasma membrane, as shown by fluorescence and electron microscopy. Here we report the isolation and characterization of the subfraction of the endoplasmic reticulum that is closely associated with the plasma membrane. This plasma membrane associated membrane (PAM) is characterized by its high capacity to synthesize phosphatidylserine and phosphatidylinositol. As such, PAM is reminiscent of MAM, a mitochondria associated membrane fraction of the yeast [Gaigg, B., Simbeni, R., Hrastnik, C., Paltauf, F. & Daum, G. (1995) Biochim.Biophys. Acta 1234, 214±220], although the specific activity of phosphatidylserine synthase and phosphatidylinositol synthase in PAM exceeds several-fold the activity in MAM and also in the bulk endoplasmic reticulum. In addition, several enzymes involved in ergosterol biosynthesis, namely squalene synthase (Erg9p), squalene epoxidase (Erg1p) and sterolD 24 -methyltransferase (Erg6p), are highly enriched in PAM. A possible role of PAM in the supply of lipids to the plasma membrane is discussed.

Membrane association between mitochondria and the endoplasmic reticulum of the yeast Saccharomyces cerevisiae is probably a prerequisite for phospholipid translocation between these two organelles. This association was visualized by fluorescence microscopy and computer-aided three-dimensional reconstruction of electron micrographs from serial ultrathin sections of yeast cells. A mitochondria-associated membrane (MAM), which is a subfraction of the endoplasmic reticulum, was isolated and re-associated with mitochondria in vitro. In the reconstituted system, phosphatidylserine synthesized in MAM was imported into mitochondria independently of cytosolic factors, bivalent cations, ATP, and ongoing synthesis of phosphatidylserine. Proteolysis of mitochondrial surface proteins by treatment with proteinase K reduced the capacity to import phosphatidylserine. Phosphatidylethanolamine formed in mitochondria by decarboxylation of phosphatidylserine is exported to the endoplasmic reticulum where part of it is converted into phosphatidylcholine. In contrast with previous observations with permeabilized yeast cells [Achleitner, G., Zweytick, D., Trotter, P., Voelker, D. & Daum, G. (1995) J. Biol. Chem. 270, 29836±29842], export of phosphatidylethanolamine from mitochondria to the endoplasmic reticulum was shown to be energy-independent in the reconstituted yeast system.

This study was aimed at a better understanding of organelle organization in the yeast Saccharomyces cerevisiae with special emphasis on the interaction and physical association of organelles. For this purpose, a computer aided method was employed to generate three-dimensional ultrastructural reconstructions of chemically and cryofixed yeast cells. This approach showed at a high level of resolution that yeast cells were densely packed with organelles that had a strong tendency to associate at a distance of <30 nm. The methods employed here also allowed us to measure the total surface area and volume of organelles, the number of associations between organelles, and the ratio of associations between organelles per surface area. In general, the degree of organelle associations was found to be much higher in chemically fixed cells than in cryofixed cells, with endoplasmic reticulum/plasma membrane, endoplasmic reticulum/mitochondria and lipid particles/nuclei being the most prominent pairs of associated fractions. In cryofixed cells, similar preferences for organelle association were seen, although at lower frequency. The occurrence of specific organelle associations is believed to be important for intracellular translocation and communication. Membrane contact as a possible means of interorganelle transport of cellular components, especially of lipids, is discussed.

Selected cell organelles were investigated at a high level of resolution with the transmission electron microscope, using ultrathin serial sections to create three-dimensional reconstructions. On the basis of these reconstructions, morphological data of chloroplast fine structures, mitochondria, and peroxisomes from control and drought-stressed spinach leaves were evaluated and compared. Mesophyll cell chloroplasts of control plants contained 60% stroma, 23% thylakoids, and 16% starch. In drought-stressed plants, the volume of both the stroma and the thylakoids increased to 68% and 32%, respectively. The amount of plastoglobuli was about 0.3% in both samples. Chloroplasts of stressed plants differed from control plants not only in the thylakoid and stroma values but also in the lack of starch grains. Mitochondria occurred in variable forms in control and stressed samples. In stressed plants, mitochondria had only 65% of the volume compared with control plants. Peroxisomes were inconspicuous.

The three‐dimensional (3D) morphology of the stomata in leaves of Pinus canariensis is described with respect to the spatial arrangement of guard cells, subsidiary cells, polar and lateral cells. Serial semi‐thin sections of the stomatal apparatus were digitally reconstructed and analysed with regard to the position, shape and size of the cell types involved. The stomatal complex consists of 16 cells with uniquely shaped polar and lateral cells. The polar cells form a kind of roof above the epistomatal chamber thereby reducing the surface aperture. The structural features of the stomatal complex differ from other Pinus species and are presumed to be an adaptation to extreme environmental conditions.