The plasma membranes of oat normally resemble those of other eukaryotes in containing mainly phospholipids and sterols. We here report the novel ¢nding that the galactolipid digalactosyldiacylglycerol (DGDG) can constitute a substantial proportion of oat plasma membrane lipids, in both shoots and roots. When oat was cultivated under severe phosphate limitation, up to 70% of the plasma membrane phosphoglycerolipids were replaced by DGDG. Our ¢nding not only re£ects a far more developed potential for plasticity in plasma membrane lipid composition than often assumed, but also merits interest in the context of the limited phosphate availability in many soils. ß

Here we present the first demonstration of the physical association between membranes involved in MCSs: by using optical imaging and manipulation, strong attracting forces between ER and chloroplasts are revealed. We used Arabidopsis thaliana expressing green fluorescent protein in the ER lumen and observed leaf protoplasts by confocal microscopy. The ER network was evident, with ER branch end points apparently localized at chloroplast surfaces. After rupture of a protoplast using a laser scalpel, the cell content was released. ER fragments remained attached to the released chloroplasts and could be stretched out by optical tweezers. The applied force, 400 pN, could not drag a chloroplast free from its attached ER, which could reflect protein-protein interactions at the ER-chloroplast MCSs. As chloroplasts rely on import of ER-synthesized lipids, we propose that lipid transfer occurs at these MCSs. We suggest that lipid transfer at the MCSs also occurs in the opposite direction, for example to channel plastid-synthesized acyl groups to supply substrates for ER-localized synthesis of membrane and storage lipids.

Leaf discs of four dicotyledonous species, when incubated at temperatures of 4 to 180C (optimum at 120C) for 30 or 60 minutes, responded by accumulations of membranes in the chloroplast stroma in the space between the inner membrane of the envelope and the thylakoids. The accumulated membranes, here referred to as the low temperature compartment, were frequently continuous with the envelope membrane and exhibited kinetics of formation consistent with a derivation from the envelope. Results were similar for expanding leaves of garden pea (Pisum sativum), soybean (Glycine max), spinach (Spinacia oleracea), and tobacco (Nicotiana tabacum). We suggest that the stromal low temperature compartment may be analogous to the compartment induced to form between the transitional endoplasmic reticulum and the Golgi apparatus at low temperatures. The findings provide evidence for the possibflity of a vesicular transfer of membrane constituents between the inner membrane of the chloroplast envelope and the thylakoids of mature chloroplasts in expanding leaves.

It is well established that phosphate deficiency induces the replacement of membrane phospholipid with nonphosphorous lipids in extra-plastidial membranes (e.g. plasma membrane, tonoplast, mitochondria). The predominant replacement lipid is digalactosyl diacylglycerol (DGDG). This paper reports that the phospholipid-to-DGDG replacement is reversible, and that when oat seedlings are re-supplied with radio-labelled phosphate, it is initially recovered primarily in phosphatidylcholine (PC). Within 2 d, the shoot contains more than half of the lipidassociated radiolabel, reflecting phosphate translocation. Oat was also cultivated in different concentrations of phosphate and the DGDG/PC ratio in roots and phospholipase activities in isolated plasma membranes was assayed after different times of cultivation. The DGDG/PC ratio in root tissue correlated more closely with plasma membranelocalized phospholipase D, yielding phosphatidic acid (PA), than with plasma membrane-localized PA phosphatase, the activity that results in a decreased proportion of phospolipids. The lipid degradation data did not reflect a significant involvement of phospholipase C, although a putative phospholipase C analogue, non-specific phospholipase C4 (NPC4), was present in oat roots. The correlation between increased phospholipase D activity and DGDG/PC ratio is consistent with a model where phospholipid-to-DGDG replacement involves formation of PA that readily is removed from the plasma membrane for further degradation elsewhere.

The aim of the present investigation was to find factors critical for the co-existence of prolamellar bodies and prothylakoids in etioplasts of wheat (Triticum aestivum L. cv Starke II). The lipid composition of the prolamellar body and prothylakoid fractions was qualitatively similar. However, the molar ratio of m ogalactosyl diacy4gycerol to digaactosyl diacylglycerol was higher in the prolamellar body fraction (1.6 ± 0.1), as was the lipid content on a protein basis. Protochlorophyllide was present in both fractions. The system with the planar bilayer membranes of PT (13,20). It is therefore reasonable to assume that several membrane components are common between PLBs and PTs. We previously showed that glycolipid composition of PLB and PT fractions was qualitatively similar (22). Ryberg and Sundqvist (21) and Ikeuchi and Murakami (10) showed that PT fractions contained several polypeptides while the PLB fraction was dominated by one polypeptide. The methods used in these studies (10, 21, 22) for separating PLBs and PTs resulted in pure PLB fractions whereas the PT fractions probably contained PLB fragments and/or envelope membranes.To study which factors could be important for the existence of the continuous system of prothylakoid membranes and branched prolamellar body membranes, a PT fraction representing prothylakoids that in situ had been connected with PLBs was needed. A method for isolating such a PT fraction, as well as a PLB fraction, has been previously described (23). In the present investigation, this method was used for isolating a PLB and a PT fraction. These fractions were compared with respect to lipid, pigment, and polypeptide composition with the aim of finding which compositional differences could explain the structural differences in membrane organization between PLBs and PTs. The generally accepted model for the organization of lipids and proteins in a membrane is the fluid mosaic membrane model, proposed by Singer and Nicolson (26). According to this model, the lipids form a planar bilayer in which the proteins are more or less embedded. Membrane lipids and proteins can also form branched bilayer structures in the cells. In plants, the most conspicuous ones are the crystalline PLBs2, found in plastids. Based on ultrastructural studies, the structure of the PLB is considered to be a branched lattice constructed of four-or sixarmed units (5,8,30). The PLBs are found in developing plastids (33), in etioplasts (32), and in nonirradiated chloroplasts (9, 12). It has been proposed that the PLBs store material to be utilized in the assembly of membranes at later stages of plastid development (20,34 MATERIAIS AND METHODSIsolation of Membrane Fractions. Etioplasts were isolated from dark-grown wheat (Triticum aestivum L. cv Starke II Weibull, Sweden) and purified on a Percoll gradient. After osmotical and mechanical rupture of the plastids, a fraction rich in envelope membranes ('env'), a PLB fraction, and a PT fraction were isolated according to Figure 1. After centrifugation of the ru...