Chromoplast

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). 

A further consideration in the choice of gene relates to those that are members of a gene family. Only one member of such a family may be involved in carotenogenesis in a particular tissue. A good example of this is Psy-1 and-2 of tomato. PSY-1 is responsible for phytoene synthesis in ripening fruit (Fraser et al, 1999) whereas PSY-2 is not functional in chromoplasts, even if the protein is produced. 

AL-BABiLi s, VON LiNTiG J, HAUBRUCK H and BEYER p (1996) A novel, soluble form of phytoene desaturase from Narcissus pseudonarcissus chromoplasts is Hsp70-complexed and competent for flavinylation, membrane association and enzymatic activation . Plant J,9, 601-12. 

MORSTADT L, GRABER P, DE PASCALIS L, KLEINIG H, SPETH V and BEYER P (2002) Chemiosmotic ATP synthesis in photo synthetically inactive chromoplasts from Narcissus pseudonarcissus L. linked to a redox pathway potentially also involved in carotene desaturation , Planta, 215, 132-40. 

RONEN G, CARMEL-GOREN L, ZAMIR D and HIRSCHBERG J (2000) An alternative pathway to 13-carotene formation in plant chromoplast discovered by map-based cloning of beta and old-gold color mutations in tomato , Proc Natl Acad Sci, 97, 11102-7. 

SCHLEDZ M, AL-BABILI S, VON LINTIG J, HAUBRUCK H, RABBANI S, KLEINIG H and BEYER P (1996) Phytoene synthase fcom Narcissus pseudonarcissus functional expression, galactolipid requirement, topological distribution in chromoplasts and induction during flowering . Plant J, 10, 781-92. 

Subsequent cyclizations, dehydrogenations, oxidations, etc., lead to the individual naturally occurring carotenoids, but little is known about the biochemistry of the many interesting final structural modifications that give rise to the hundreds of diverse natural carotenoids. The carotenoids are isoprenoid compounds and are biosynthesised by a branch of the great isoprenoid pathway from the basic C5-terpenoid precursor, isopentenyl diphosphate (IPP). The entire biosynthesis takes place in the chloroplasts (in green tissues) or chromoplasts (in yellow to red tissues). 

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." " ... 

Beyer, R, Weiss, G., and Kleinig, H., Solubilization and reconstitution of the membrane-bound carotenogenic enzymes from daffodil chromoplasts, Eur. J. Biochem. 153, 341, 1985. 

Bonk, M. et al.. Purification and characterization of chaperonin 60 and heat-shock protein 70 from chromoplast of Narcissus pseudonarcissus involvement of heat-shock protein 70 in a soluble protein complex containing phytoene desaturase. Plant Physiol. Ill, 931, 1996. 

Beyer, R, Croncke, U., and Nievelstein, V., Biochemical aspects of carotene desaturation and cyclization in chromoplasts membranes from Narcissus pseudonarcissus. Pure Appl. Chem. 66, 1047, 1994. 

Bouvier, F. et ah, Xanthophyll biosynthesis in chromoplasts isolation and molecular cloning of an enzyme catalyzing the conversion of 5,6-epoxycarotenoid into ketocar-otenoid. Plant J. 6, 45, 1994. 

Bouvier, F. et al.. Oxidative remodeling of chromoplast carotenoids Identification of the carotenoid dioxygenase CsCCD and CsZCD genes involved in Crocus secondary metabolite biogenesis. Plant Cell 15, 47, 2003. 

Hugueney, P. et ah. Metabolism of cyclic carotenoids a model for the alteration of this biosynthetic pathway in Capsicum annuum chromoplasts. Plant J. 8, 417, 1995. 

Another requirement for oral delivery is the ability to express foreign proteins in plastids that are present in non-green tissues. One such example is the expression of an antibiotic resistance gene (aadA) in tomato chromoplasts [28]. More recently, stable and highly efficient plastid transformation has been achieved in the non-green... 

Chromoplasts are pigmented plastids that contain carotenoids but lack chlorophyll. These plastids are attractants to insects and animals. 

Yellow, orange, and red color in mature pepper fruits is a result of carotenoid metabolism and accumulation in the chromoplasts of the pericarp (Fig. 8.1). 

Homero-Mendez D, Minguez-Mosquera Ml (2000) Xanthophyll esterification accompanying carotenoid overaccumulation in chromoplasts of Capsicum annuum ripening fruits is a constitutive process and useful for ripeness index. J Agric Food Chem 48 1617-1622... 

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