Plastids, starch formed

The amyloplasts (starch forming plastids) (Fig. 2.7), chromo-plasts and chloroplasts arise from structures, proplastids (Fig. 2.8), similar in size to mitochondria but not readily stained with Janus Green B and containing internally an array of granules or vesicles (100-250 A diam.) rather than defined cristae. In the cells of higher plants these proplastids are transmitted from cell to cell, increasing... 

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-... 

During primary wall formation the plastids contain starch and other materials which stain heavily with uranyl acetate and lead citrate. When the tracheid starts to form the Si layer, the plastid becomes surrounded by an endoplasmic reticulum. While the fate of these compounds is unknown, it can be envisaged that they are used for generation of energy and/or a source of cell wall materials. 

Starch, another of the most abundant polymers of glucose, is stored by most green plants in a semicrystalline form in numerous small granules. These granules, which are usually formed within colorless membrane-bounded plastids, have characteristic shapes and appearances (Fig. 4-6) that vary from plant to plant. One component of starch, amylose, is a linear polymer of many a-D-glucopyranose units in 1,4 linkage (Fig. 4-7) as in maltose. Starch granules always contain a second kind of molecule known as amylopectin.58... 

When not indicated otherwise, our observations refer to cells in the sub apical area between 300 and 600 pm from the root tip. In the actively growing root, this area is the site of active cell division along with the first stages of cell differentiation, depending on the tissue. Root cells from 2 h-imbibed seeds contained numerous protein bodies19,24 of spheroidal shape, about 1.5-3 pm in diameter and nearly completely filled with highly omiophilic protein material they also contained abundant lipid reserves in the form of minute droplets, mainly concentrated at the cell periphery. The nucleus had spheroid or ellipsoidal shape and showed a distinct nucleolus. The cytoplasm contained numerous mitochondria with a dense matrix as well as relatively small and scarcely differentiated plastids with no or very little starch (Fig. 15.3a,b). 

In green algae and in leaf cells of higher plants, ADP-Glc PPase has been demonstrated to reside in the chloroplast (82). More recently, using plastids isolated from maize and barley endosperm (83-85), the existence of two ADP-Glc PPases, a plastidial form, and a major cytosolic form were found. Subsequently, cytosolic forms of ADP-glucose pyrophosphorylase have been found in wheat (86, 87) and rice (88). Because starch synthesis occurs in plastids, it was proposed that in cereal endosperms, synthesis of ADP-Glc in the cytosol requires the involvement of an ADP-Glc carrier in the amyloplast envelope (85). Subsequently, characterization of the ADP-Glc transporter has been reported for maize endosperm (89, 90), barley endosperm (91), and wheat endosperm (92). 

Starch is the most economically important reserve polysaccharide in the plant kingdom and is in addition the major source of carbohydrates in human nutrition. In contrast to non-starch reserve polysaccharides, which are outside the cell and the plasmalemma, starch is located in the so-called plastids or in vacuoles within the plant cells [1f In seeds, the highest starch content can be found in the endosperm, whereas its content in the embryo and the pericarp is very low. In general, the starch content of seeds or fruits varies with the degree of maturation121. Starch occurs in semicrystalline form in granules. The size and the shape of the granules is dependent on the plant species and may reach about 175 mm. 

Both potato tuher and potato leaf ADP-Glc PPases are plastidic the leaf enzyme in the chloroplast and the tuher enzyme in the amyloplast. The ferredoxin—thioredoxin system is located in the chloroplast and thus, with photosynthesis, reduced thioredoxin is formed and would activate the leaf ADP-Glc PPase. At night, oxidized thioredoxin is formed and would oxidize and inactivate the ADP-Glc PPase. This activation/ inactivation process during the light/dark cycle allows a fine tuning and dynamic regulation of starch synthesis in the chloroplasts. Thioredoxin isoforms is present in many different suhcellular locations of plant tissues cytosol, mitochondria, chloroplasts, even nuclei as well as in amyloplasts. ... 

It appears, then, that starch synthetase can exist in different forms, with rather different specificities tx)wards the possible glucosyl donors. The balance between these forms can be developmentally regulated and appears to differ with the tissue under study. Indeed, starch may be synthesised in a variety of organelles, varying from chloroplasts, though chlorophyll-free plastids and amyloplasts to large starch grains tenuously bounded by membrane. All are related, but are not identical. 

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