Plants plastids

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. 

ADP-Glucose Is the Substrate for Starch Synthesis in Plant Plastids and for Glycogen Synthesis in Bacteria... 

Until 1993, all terpenes were considered to be derived from the classical acetate/mevalonate pathway involving the condensation of three units of acetyl CoA to 3-hydroxy-3-methylglutaryl CoA, reduction of this intermediate to mevalonic acid and the conversion of the latter to the essential, biological isoprenoid unit, isopentenyl diphosphate (IPP) [17,18,15]. Recently, a totally different IPP biosynthesis was found to operate in certain eubacteria, green algae and higher plants. In this new pathway glyceradehyde-3-phosphate (GAP) and pyruvate are precursurs of isopentenyl diphosphate, but not acetyl-CoA and mevalonate [19,20]. So, an isoprene unit is derived from isopentenyl diphosphate, and can be formed via two alternative pathways, the mevalonate pathway (in eukaryotes) and the deoxyxylulose pathway in prokaryotes and plant plastids [16,19]. 

Incorporation of H-labeled deoxyxylulose and methylerythritol into terpenoids from bacteria and from plant plastids... 

Since the first successful transformation of tobacco chloroplasts [83], expression systems based on the transformation of plant plastids has attracted the attention of plant biotechnologists. The features that make this technology so attractive are its potential for high protein yield, along with inherent biosafety features such as limited plastid transfer via pollen (due to maternal inheritance of plastid-encoded genes) [84] and the relatively low probability of transgene movement from the chloroplast to the nucleus [85, 86]. 

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... 

Fig. 5.4 Precursors of carotenoid biosynthesis. Abbreviations for inLcrmcdiatcs arc MEP, methyl cry thritol phosphate IPP, isopcnlcnyl pyrophosphate GGPP, geranylgeranyl pyrophosphate DMAPP, dimethallyl pyrophosphate. Enzymes are shown to the right of the steps catalyzed in plant plastids. DXS (D-1-dcoxyxylulosc 5-phosphatc synthase, DXP synthase) DXR (DXP reductoisomerase) IPPI (IPP isomerase) GGPPS (GGPP synthase).

Using plastid transformation, high levels of expression can be obtained without the use of modified or synthetic genes. It has been observed that bacterial genes are well expressed in plant plastids without any optimization of the codon usage. 

Neuhaus, H.E. Wagner, R. (2000). Solute pores, ion channels, and metabohte transporters in the outer and iimer envelope membranes of higher plant plastids. Biochimica et Biophysica Acta -Biomembranes, 1465, 307 323. 

Kates, M. Hydrolysis of lecithin by plant plastid enzymes. Canad. J. Biochem. 33, 575 (1955). 

Carotenoids are synthesized from the basic C5 terpenoid precursor isopentenyl pyrophosphate (IPP) and dimethylallyl diphosphate (DMAPP). These precursors can be obtained from two distinct pathways the mevalonate pathway (MVA) and the non-MVA pathway also known as 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway [84, 85]. All eukaryotes use the MVA pathway, whereas plant plastids and most bacteria use the MEP pathway [86,87]. Some bacteria also use the MVA pathway [84]. In the MEP pathway, the first step in IPP biosynthesis is the formation of l-deoxy-D-xylulose-5-phosphate (DXP) from pyruvate and glyceraldehyde-3-phosphate catalyzed by DXP synthase (Figure 10.7). DXP is then reduced to MEP by DXP reductase. Additional MEP pathway enzymes are then used in subsequent reactions for converting MEP to IPP, which is isomer-ized to DMAPP by the enzyme IPP isomerase. The MVA pathway begins with the conversion of three molecules of acetyl-CoA to MVA through acetoacetyl-CoA... 

This paper reports briefly other methods for enhancing the endogenous DAG content of pea (18 3 plant) plastids and the effects these have on UDPgalT activity in isolated envelope membrane fractions. 

IPP and DMAPP (Ershov etal. 2000). This enzyme, whilst a key feature of the mevalonate pathway, is not vital to the 1-DXP pathway because the final step in the pathway (the reductive dehydration of ( )-4-hydroxy-3-methylbut-2-enyl pyrophosphate or HMBPP, 15) was found to produce IPP and DMAPP in an approximately 6 1 ratio in green plant plastids (Adam et al. 2002). Despite this, IDl is present in plastids. When viral infection was used to silence expression of this enzyme in the tobacco Nicotiana henthamiana the result was an 80% reduction of essential chlorophyll and carotenoid levels in infected leaves. This suggests that IDI, whilst not strictly necessary for viable plants, is highly beneficial to green tissue growth (Page etal. 2004). 

Hitz WD, Carlson TJ, Booth R, Kinney AJ, Stecca KL, Yadav NS. Cloning of a higher plant plastid co-6 fatty acid desaturase cDNA and its expression in a cyanobacterium. Plant Physiol 1994 105 635-641. 

Ernes MJ, Tobin AK. Control of metabolism and development in higher plant plastids. Int Rev Cytol 1993 145 149-216. 

Figure 6.6 T/ie methylerythritol 4-phosphate route to isopentenylpyrophosphate and terpenes in plant plastids and bacteria. The intermediate in brackets is immediately reduced on the same enzyme which catalyzes the previous rearrangement...

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