Plastids

Plastid pigments were received by extraction and crystallization. Crystals of these pigments were dissolved in suitable solvent. 

The best separation of plastid pigments were received chloroform -hexane - ethyl alcohol (20 100 1) solvent mix. 

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-... 

The Rieske clusters observed in cytochrome be complexes from traditional sources (i.e., mitochondria, proteobacteria, plastids, and cy-... 

Nieto-Sotelo, J. Ho, T.-H.D. (1987). Absence of heat shock protein synthesis in isolated mitochondria and plastids from maize. Journal of Biological Chemistry, 262, 12288-92. 

Turner speculated that the two families arose from a common ancestor that likely occupied xeric or halophytic habitats in Gondwana in an area that encompassed what were to become southwestern Africa and southeastern South America. The ancestral types do not still exist, but both families share the unusual, betanidin-based floral chemistry, and the equally unique sieve-tube plastids seen in Caryophyllales but are absent elsewhere in the plant kingdom (Behnke and Turner, 1971). 

Behnke, H.-D. and Turner, B. L. 1971. On specific sieve-tube plastids in CaryophyUales. Taxon 20 731-737. 

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). 

Although carotenogenesis in plants takes place in plastids, all of the carotenoid biosynthesis genes are nuclear encoded and their polypeptide products are imported into the plastids. Therefore, they contain a N-terminal transit peptide sequence. For example, the size of the transit peptide of PSY from ripe tomato fruit is approximately 9 kDa, corresponding to about 80 amino acid residues (Misawa et al, 1994). 

Clearly, the control of gene expression at the transcriptional level is a key regulatory mechanism controlling carotenogenesis in vivo. However, post-transcriptional regulation of carotenoid biosynthesis enzymes has been found in chromoplasts of the daffodil. The enzymes phytoene synthase (PSY) and phytoene desaturase (PDS) are inactive in the soluble fraction of the plastid, but are active when membrane-bound (Al-Babili et al, 1996 Schledz et al, 1996). The presence of inactive proteins indicates that a post-translational regulation mechanism is present and is linked to the redox state of the membrane-bound electron acceptors. In addition, substrate specificity of the P- and e-lycopene cyclases may control the proportions of the p, P and P, e carotenoids in plants (Cunningham et al, 1996). 

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

KUNTZ M, ROMER s, suiRE c, HUGUENEY p, WEIL j H, scHANTZ R and CAMARA B (1992) Identification of a cDNA for the plastid-located geranylgeranyl pyrophosphate synthase from Capsicum annuum correlative increase in enzyme activity and transcript level during fruit ripening , Plant J, 2, 25-34. 

The biosynthesis of the tetrapyrrole macrocycle and its branches leading to haem and chlorophylls has been covered in detail in several reviews - - and will be concisely described in this section. Tetrapyrrole biosynthesis occurs entirely in the plastids and is composed of several enzymatic steps starting from 5-aminolevulinic acid (ALA), which is the key precursor of porphyrins and the source of their carbon and nitrogen. 

Thomas, H., Chlorophyll a symptom and a regulator of plastid development. New PhytoL, 136, 163, 1997. 

Beale, S.I., 5-Aminolevulinic acid in plants its biosynthesis, regulation and role in plastid development, Annu. Rev. Plant Physiol, 29, 95, 1978. 

Pterins make no contributions to the colors of plants and microorganisms. One important pterin is the folate produced by plants and microorganisms. Folate and its derivatives are present in plants in various concentrations in mitochondria, cytosols, vacuoles, and plastids. The total amount of fohc acid depends on the plant species, on the developmental stage, and on external factors. Good sources of folates are beans, lentils, spinach, and wheat germ. ... 

For the sake of study, the biosynthesis of carotenoid plant pigments can be divided into parts involving enzymes and their associated activities as listed in Table 5.3.1 and further detailed in Figure 5.3.1 through Figure 5.3.4. Some of the parts have common enzymatic mechanisms and may also be in distinct subcellular compartments such as cytoplasm, endoplasmic reticulum, or plastid thylakoid. 

In plant plastids, GGPP is formed from products of glycolysis and is eight enzymatic steps away from central glucose metabolism. The MEP pathway (reviewed in recent literature - ) operates in plastids in plants and is a preferred source (non-mevalonate) of phosphate-activated prenyl units (IPPs) for plastid iso-prenoid accumulation, such as the phytol tail of chlorophyll, the backbones of carotenoids, and the cores of monoterpenes such as menthol, hnalool, and iridoids, diterpenes such as taxadiene, and the side chains of bioactive prenylated terpenophe-nolics such as humulone, lupulone, and xanthohumol. The mevalonic pathway to IPP that operates in the cytoplasm is the source of the carbon chains in isoprenes such as the polyisoprene, rubber, and the sesquiterpenes such as caryophyllene. 

Recently, a potential cytosolic component of the MEP precursor pathway, xylulose kinase, has been cloned and tested for function in an Escherichia coli complementation system. " The kinase activates exogenous xylulose in the cytoplasm. DXP is the precursor for DXS, which resides in the plastid, suggesting the activated substrate must be transported into the plastid. Another xylulose kinase homologue in Arabidopsis that contains a plastid targeting sequence was not active in the E. coli system, suggesting that it may have some other function in the plastid. Perhaps plant and bacterial tissue cultures may be fed xylulose to condition accumulation of isoprenoid metabolites. 

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." " ... 

Understanding mechanisms controlling metabolon localization in plastids of different membrane architectures Little is known about metabolon structure, assembly, and membrane targeting. The carotenoid biosynthetic pathway exists on plastid membranes. However, plastids have different membrane architectures and therefore tissue- and plastid-specific differences in membrane targeting of the biosynthetic metabolon can be expected. Localization in chloroplasts that harbor both thylakoid and envelope membranes differs from the envelope membranes in endosperm amy-loplasts. In fact, localization on both thylakoid and envelope membranes implies that the carotenoid pathway is really not a single pathway, but a duplicated pathway that may very well have membrane-specific roles with regard to functions in primary and secondary metabolism. 

Mahga, P., Plastid transformation in higher plants, Annu. Rev. Plant Biol. 55, 289,... 

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