Plastid envelopes

The carotenoids are located in photosynthetic pigment-protein complexes (PPCs) in the thylakoid membranes (Young, 1993), with minor amounts in the chloroplast envelope (Joyard et al, 1991) and the envelope of amyloplasts (Fishwick and Wright, 1980). In all plastid envelope membranes, violaxanthin is the major carotenoid. Carotenes are also found in plastoglobuli (Lichtenthaler and Peveling, 1966). 

JOYARD J, BLOCK M A and DOUCE R (1991) Moleculat aspects of plastid envelope biochemistry , Eur J Biochem, 199, 489-509. 

GGPPS functions as part of a complex metabolon. In the plastid, as shown in Capsicum chromoplasts," GGPPS is a homodimer and associated but not integral to the plastid envelope. GGPPS is also associated with the next enzyme in the pathway as part of a holoenzyme complex." " ... 

Many of the metabolite uptake studies cited above rely on combined uptake and incorporation into starch. In order to separate uptake from incorporation, Schott et al.226 extracted amyloplast membrane proteins from potato tubers and reconstituted them into liposomes. These reconstituted liposomes transported Pi, triose phosphates and G6P in a counter-exchange mode. The liposomes were ineffective in the transfer of G1P uptake of ADP-Glc was not tested. Mohlmann et al.236 have used a proteoliposomic system to reconstitute plastid envelope proteins. In this system, ADP-Glc is transported in exchange for AMP. Thus the more widely studied plastid ATP/ADP transporter was not responsible for ADP-Glc uptake. More recently, Bowsher et al.237 reported that wheat endosperm amyloplasts membrane proteins reconstituted into proteoliposomes took up ADP-Glc in exchange for AMP and ADP. In addition, they showed that under conditions of ADP-Glc dependent starch biosynthesis, the efflux of ADP from intact amyloplasts was equal to that of ADP-Glc utilization by starch synthesis. The amyloplast membrane ADP-Glc/ADP transporter was a 38 000 molecular weight integral membrane protein.237... 

Two distinct membrane-bound systems have been described for Jerusalem artichoke plastids in tissue cultures under EM an electron-dense sac-like central system (corpo opaco) and a peripheral system (Gerola and Dassu, 1960). The central system is morphologically variable and likely to store proteins. The peripheral system consists of irregular tubules and cistemae, with plastids occurring in clusters, especially near the nucleus (Tulett et al., 1969). The peripheral system may be involved in the transport of materials through the plastid envelope (Yeoman and Street, 1973). 

HELLIWELL, C.A., SULLIVAN, JA. MOULD, R.M., GRAY, J.C., PEACOCK W.J., DENNIS, E.S., A plastid envelope location of Arabidopsis enf-kaurene oxidase links the plastid and endoplasmic reticulum steps of the gibberellin biosynthesis pathway., Plant J., 2001,28,201-208... 

The fatty acids synthesized in the plastid are exported to the cytoplasmic compartment as acyl- CoAs generated by enzymic activity of the outer membrane of the plastid envelope. These acyl- CoAs are utilized in the synthesis of phospholipids in the mitochondria and the endoplasmic reticulum. The fatty acid specificity in the synthesis of phosphatidyl choline (PC) is such that palmitate (16 0) is never found at the sn-2 position. Thus the predominant molecular species are sn1-l8, sn2-l8 and sn1-l6, sn2-l8. These molecular species of PC supply the diaoylglycerol (DAG) moiety for synthesis of MGDG, DGDG, and SQDG in the plastid. 

In contrast to pigments, the sterol accumulation (defined as increase over dark control) is completely blocked by the lower mevinolin concentration (Table IV). With primary leaves of Wheat, incubated under comparable conditions but supplied with [ C]-acetate and [ H]-mevalonate, precursors able to enter the isoprenoid pathway before or after the HM6-CoA reductase step, respectively, it was shown (75, 76.) that mevinolin could completely prevent acetate incroporation into phytosterols. Incorporation of tritium from labeled MVA was unaffected. This elimination of [ C]-aeetate incorporation into phytosterols was observed at a mevinolin concentration Which had no effect on chlorophyll and carotenoid accumulation in controls virtually identical accumulation in bands of TLC-plates identifed as pheophytins or B-carotene was found for controls and mevinolin treatment (Bach Nes, unpublished). The limited ability of mevinolin to prevent pigment accumulation in chloroplasts favors the assumption that plastids contain their own independent enzyme system for MVA production. The plastidic envelope is apparently not at all or only poorly permeable to mevinolin. The ability of plastids to synthesize MVA has been questioned (70, 71). Our observations, together with in vitro measurements of enzyme activity (16, ), support the view that plastids possess their own HMG-CoA reductase. 

Early signs of injury appeared in the outer chloroplast or etioplast envelope and in the tonoplast. This information supports the proposed mechanism of action of DPE herbicides. Carotenoids are known to be present in the outer plastid envelope... 

Protox is thought to be a membrane-bound enzyme probably in or on the plastid envelope (Fig. 6). Fluorescence microscopy of achlorophyllous tissues treated with acifluorfen in darkness, results in strong porphyrin fluorescence in both plastids and cytoplasm, whereas, fluorescence was localized almost exclusively in plastids of untreated cells (fi8). PPIX concentrations were almost 200-fold greater in treated than untreated tissues. These data suggest that PPIX leaks from plastids or plastid envelopes of treated tissues and that this leakage is independent of membrane damage due to lipid peroxidation. Thus, as in our previous model (2S)> the cellular site of action of these herbicides may be the plastid envelope. 

Accumulation of Flavonoids in the Endoplasmic Reticulum The ER is well developed in glands producing flavonoids or other lipophilic compounds. During the secretory process continuity of the plastid envelope with peripheric... 

Later, when the secretion from the plastid envelope accumulates, two different groups of flavonoids are successively found first, ring B-hydroxylated aglyoons and their ring A 0-methylated derivatives and, second ring B- and ring A-methylated aglycons. 

The detection of chalcopinostrobine in gland tissues argues that 0-methylation also occurs before cycliza-tion of the chalcone to flavanone, in contrast to what has been found in grapefruit (Eef. 32). However, in the plastid envelope 7 0-methylation probably occurs after aglycon formation in the endoplasmic reticulum when this material enters the plastid. -... 

These results shown that the xanthophyll cycle is light dependent in etioplasts as it is in thylakoids. Therefore the de-epoxidation must take place in other stroma membranes like prothylakoids or plastid envelope which have no photosynthetic activity. This mechanism would be triggered in response to pH changes, suggesting the idea that the main role of the xanthophyll cycle may be the photoprotection of the chlorophyll by dissipating the energy that this pigment cannot process. 

Space between the membranes of the plastid envelope, plastic vacuoles, and in the endoplasmic reticulum (Neumann, 1975). 

Experiments with gene expression inhibitors have demonstrated that the secondary metabolic enzymes, which are present in chloroplasts are synthesized in the cytoplasm (A 2.1). Hence those enzymes must penetrate the plastid envelope, which shows maximum permeability at early stages of greening. During this period the precursors of secondary products, such as oxalacetate, succinate or mevalonate, and even the CoA-esters of cinnamic acid derivatives may also be transported across the plastid envelope. 

Fatty acyl-CoA synthetase. Cytosolic processing of fatty acid and lipid formation is a highly integrated process. An active acyl-CoA pool exists and is dependent upon a supply of fatty acid from the plastid. The composition of this pool can be analyzed by capillary electrophoresis (MacKenzie and Taylor, 1990). Formation of acyl-CoAs involves fatty acyl-CoA synthetase (EC 6.2.1.3) and appears to occur on the inner face of the plastidic envelope. While a broad specificity for fatty acids exists in various oilseeds, a preference is shown for medium-chain fatty acids in Cuphea and for very-long-chain fatty acids in Crambe (Battey and... 

The membranes recovered in the four bands obtained from the sucrose gradient were also examined by electron microscopy after tannic acid postfixation or impregnation with zinc iodide osmium tetroxide mixtures. A secretory cell impregnated with the zinc-osmium mixture is shown on Figure 2. The staining concerns mainly the nuclear envelope, ER membranes, Golgi apparatus and mitochondria. On the contrary, the plasma membrane, the tonoplast and the plastid envelopes are not contrasted. On the basis of membrane thickness, it was concluded that... 

The introduction of partition in aqueous two-phase polymer systems by Albertsson (1971) contributed, in the recent past, to the isolation of plant membranes and organelles. This method, based upon the membrane surface properties in particular, resulted in the separation of plasma membranes from E. R., Golgi membrane or plastid envelopes. In the present study, we adapted this procedure in order to isolate and characterize a plasma membrane fraction from fungal material. 

After non-esterified fatty acids are attached to CoA in the plastid envelope they can, apparently, move rapidly to the endoplasmic reticulum. It is not known how this transport takes place, although lipid-binding... 

---