Plastid stroma

The accumulation of storage lipids in plants includes de novo biosynthesis of FA in the plastid stroma with their subsequent involvement in the glycerolipid metabolism resulting in TAG formation in the ER [68], The two pathways of TAG biosynthesis were called as acyl-CoA-dependent and acyl-CoA-independent. 

Fig. 15. Vegetative cells of Trebouxia albulescens (A) unaffected vegetative cell, the arrangement of the lamellar system of the chloroplast and the grana formed by a limited number of thylakoids in the plastid stroma are visible (x 9200). (B) vegetative cell from an achloric culture showing an advanced degeneration of the whole cytoplasm. The electron density of the lamellar system was strongly reduced by illumination with around 3000 lux. (x 10,000). (From Laudi et a/., 1969.)...

Fig. 2.17. Bottom left. Chloroplasts (P) in chlorenchyma of pea leaf. Note the large starch grains within the chloroplast (asterisks). IS, intercellular space V, vacuole. X 780. Right. Electron micrograph of a chloroplast of a leaf of spinach Spinacea oleracea). The chloroplast is surrounded by a double membrane (PM) and the internal membrane system is differentiated into grana (asterisks) and stroma lamellae (open arrows). Osmiophilic droplets (small black arrows) occur in the plastid stroma. The structure of the grana is shown in more detail in the inset top left) as are regions of continuity between the grana and stroma lamellae (large solid arrows). Key CM, cell membrane CW, cell wall SG, starch grain ...

Plant cells contain a unique family of organelles, the plastids, of which the chloroplast is the prominent example. Chloroplasts have a double membrane envelope, an inner volume called the stroma, and an internal membrane system rich in thylakoid membranes, which enclose a third compartment, the thylakoid lumen. Chloroplasts are significantly larger than mitochondria. Other plastids are found in specialized structures such as fruits, flower petals, and roots and have specialized roles. 

Starch is stored in plant cells in the form of granules in the stroma of plas-tids (plant cell organelles) of two types chloroplasts, in which photosynthesis takes place, and amyloplasts, plastids that are specialized starch accumulation bodies. When starch is to be mobilized and used by the plant that stored it, it must be broken down into its component monosaccharides. Starch is split into its monosaccharide elements by stepwise phosphorolytic cleavage of glucose units, a reaction catalyzed by starch phosphorylase (Figure 7.23). This is formally an a(1 4)-glucan phosphorylase reaction, and at each step, the prod-... 

Chloroplasts (29-36) are the sites of photosynthesis and their ribosomes can carry out protein synthesis. Chloroplasts that contain chlorophylls and carotenoids, are disc shaped and 4-6 pm in diameter. These plastids are comprised of a ground substance (stroma) and are traversed by thylakoids (flattened membranous sacs). The thylakoids are stacked as grana. In addition, the chloroplasts of green algae and plants contain starch grains, small lipid oil droplets, and DNA. 

Chloroplasts are a typical type of plastid that performs various metabolic reactions as well as photosynthesis. Their envelope consists of two membranes the outer envelope membrane and the inner membrane (Fig. 7). The space between these two membranes is called the intermembrane space, and the space enclosed by the inner envelope membrane is called the stroma. In addition, chloroplasts have another membrane system within the stroma the thylakoid membrane forms the lumen. Therefore, there are six different localization sites and, of course, multiple pathways to each site. Naturally, their sorting mechanisms are very complicated. 

A large part of the biosynthetic capacity of a plant cell is localized in plastids, which are self-replicating organelles surrounded by a doublemembrane envelope. Plastids are present in most cells of photosynthetic eukaryotes. In most angiosperms, however, sperm cells lack plastids, a fact that makes plastid inheritance solely maternal. The envelope is composed of an outer and inner membranes, which differ in their permeability, separated by a 10 to 20-nm gap. The plastids contain DNA that is concentrated in a section of the stroma, which is the background matrix of the plastid. The plastidial ribosomes are smaller that the cytoplasmic ribosomes. 

Cells from the control (embryos not exposed to freezing conditions) displayed features typically found in hydrated and metabolically active cells numerous mitochondria nuclei exhibiting fairly disperse chromatin and plastids with highly dense stroma could be seen (Figure 42.3a and Figure 42.4a and b). Starch, lipids, and proteins were the reserves present in plastids, lipid bodies, and vacuoles, respectively. 

According to its solubility these enzymes can be classified into two non evolutively related groups the soluble acyl carrier protein (AGP) desaturases and the membrane-bound desaturases, which includes the acyl-lipid desaturases and the acyl-CoA desaturases. The soluble AGP desaturases introduce double bonds into fatty acids esterified to AGP, and are found in the stroma of plant plastids (Shanklin and Gaboon, 1998) and some bacteria, as Mycobacterium and Streptomyces (Phetsuksiri et al., 2003). The acyl-lipid desaturases, that... 

In confirmation of earlier indieations that enf-kaurene synthesis is localised in plastids [60-62], Aaeh et al. [44] demonstrated clearly the presence of CPS/EKS activity in the stroma of wheat and pea etioplasts and leucoplasts from C. maxima endosperm. They showed, furthermore, that mature chloroplasts contain no CPS/EKS activity and that the activity is associated with dividing cells in the meristem [45]. These findings, together with the demonstration that CPS is targeted to plastids, provide firm evidence that the first part of GA biosynthesis takes place in plastids. 

In plants, FA are synthesized in the stroma of the plastids (Lynen, 1961 Ohlrogge Browse, 1995 Voelker Kinney, 2001 Sasaki Nagano, 2004), where 2-carbon units from malonyl-acyl carrier protein (ACP) are incorporated into the growing acyl-ACP chain. Malonyl-CoA is synthesized from acetyl-CoA and CO by acetyl-CoA carboxylase. Malonyl-CoA ACP acyltransacylase produces malonyl-ACP from ACP and malonyl-CoA (Ohlrogge Browse, 1995) (Fig. 7.5). 

The three main products formed during the FA synthesis in the stroma of the plastid are hydrolyzed by two possible acyl-ACP thioesterases (FAT) FAT A and FAT B. FAT A has preference for oleoyl-ACP but also hydrolyzes stearoyl- and palmitoyl-ACP. FAT B, which has preference for saturated FA-ACP and especially palmitoyl-ACP, can also hydrolyze oleoyl-ACP (Voelker Kinney, 2001). 

Most studies of fatty acid synthesis by isolated chloroplasts are made under photosynthetic conditions. Illumination of the chloroplasts generates ATP and reductant necessary for the incorporation of acetate into the fatty acids. Other effects of illumination may influence fatty acid synthesis. For example the pH and the magnesium ion concentration of the stroma both rise when the chloroplast is illuminated. It should be noted that non-photosynthetlc plastids are also assumed to be the sole site of fatty acid synthesis and they must have sources... 

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. 

It was further shown that in the stroma-lamellae pLHCP was "free" and thus able to move to the grana thylakoids where it was found in the monomeric and trimeric forms of the pigmented LHCIIb (15). Based on these finding the following model is suggested, for the path taken by pLHCP after it is translocated into the plastid (Fig. 4). 

Lupine alkaloids are formed in the green, aerial parts of Lupinus polyphyllus that incorporate labeled cadaverine into the lupanine skeleton, consistent with the fact that the enzymes of alkaloid biosynthesis, in this case, are located in the chloroplast stroma (Hartmann, 1985). Roots of the intact plants or in vitro cultured roots do not. A similar situation obtains for coniine in Conium maculatum, where the en-Z5nnes occur in both the chloroplasts and mitochondria. However, alkaloids are rarely formed in plastids (Hartmann, 1985), but are usually formed in the cytoplasm. Chloroplasts are not only the site of photosynthesis, but also of lipid, amino acid, and terpenoid biosynthesis (Schultz et al., 1985 Wink, 1987). 

Synthesis of fatty acids occurs in the stroma but involves mitochondria and the cytosol (Lawlor, 1993). Dihydroxyace-tone phosphate (DHAP) moves from the stroma to the cytosol. Pyruvate is formed from the triosephosphate DHAP and enters the mitochondria where acetyl-CoA and acetate are produced. This reaction also produces NADH, which provides part of the reducing power needed for later reactions in the sequence. Acetate returns to the chloroplasts where acetyl-CoA again is synthesized (Lawlor, 1993). Because acetyl-CoA does not cross membranes, this molecule must be resynthesized in plastids to provide the carbon precursors for fatty acid biosynthesis (Ohlrogge et al., 1993). 

The synthesis of malonyl-CoA (14) from acetyl-CoA (15), which also occurs in the stroma, with acetyl-CoA carboxylase (ACCase) is the first committed step of fatty acid biosynthesis (Fig. 2.3). Malonyl-CoA produced by plastids primarily is used for fatty acid biosynthesis (Ohlrogge et al., 1993). 

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