ANRV274-PP57-27 ARI 29 March 2006 12:21 Vitamin Synthesis in Plants: Tocopherols and Carotenoids Dean DellaPenna1 and Barry J. Pogson2 1Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing, Michigan 48824; email: [email protected] 2ARC Center of Excellence in Plant Energy Biology, School of Biochemistry and Molecular Biology, Australian National University, Canberra ACT 0200, Australia; email: [email protected] Annu. Rev. Plant Biol. Key Words 2006. 57:711–38 metabolic engineering, vitamin E, provitamin A, Arabidopsis, The Annual Review of Plant Biology is online at chloroplast, photosynthesis plant.annualreviews.org Abstract by UNIVERSITAT BERN on 09/12/09. For personal use only. doi: 10.1146/ annurev.arplant.56.032604.144301 Carotenoids and tocopherols are the two most abundant groups of Copyright c 2006 by lipid-soluble antioxidants in chloroplasts. In addition to their many Annual Reviews. All rights functional roles in photosynthetic organisms, these compounds are Annu. Rev. Plant Biol. 2006.57:711-738. Downloaded from arjournals.annualreviews.org reserved also essential components of animal diets, including humans. During First published online as a the past decade, a near complete set of genes required for the synthe- Review in Advance on February 7, 2006 sis of both classes of compounds in photosynthetic tissues has been identified, primarily as a result of molecular genetic and biochemi- 1543-5008/06/0602- 0711$20.00 cal genomics-based approaches in the model organisms Arabidopsis thaliana and Synechocystis sp. PCC6803. Mutant analysis and trans- genic studies in these and other systems have provided important insight into the regulation, activities, integration, and evolution of individual enzymes and are already providing a knowledge base for breeding and transgenic approaches to modify the types and levels of these important compounds in agricultural crops. 711 ANRV274-PP57-27 ARI 29 March 2006 12:21 man nutrition. Plastidic isoprenoid synthe- Contents sis represents a major source of such com- pounds and includes the two major groups INTRODUCTION: PLASTIDIC of lipid-soluble antioxidants in photosynthetic ISOPRENOID SYNTHESIS . 712 tissues, the tocochromanols and carotenoids. CAROTENOID BIOSYNTHESIS The tocochromanols are a group of eight to- IN PLANTS..................... 714 copherols and tocotrienols that collectively General Considerations and Early constitute vitamin E, an essential nutrient in Steps in Biosynthesis .......... 714 the diet of all mammals. Carotenoids consti- Isomerizations During Carotenoid tute a much larger group of over 700 struc- Desaturation ................. 714 tures (17) that provide fruit and flowers with β-Carotene Derived Xanthophyll distinctive red, orange, and yellow coloring Biosynthesis .................. 717 and are the dietary source of pigmentation Lutein Biosynthesis............... 717 in the tissues of many fish, crustaceans, and Carotenoid Cleavage Products . 718 birds. In some cases specific carotenoids are Carotenoids in Nongreen Plastids . 719 essential components of mammalian diets as Engineering the Carotenoid precursors for vitamin A synthesis. Vitamin A Pathway to Benefit Human deficiency remains a significant global health Health and Agriculture . ..... 720 problem (121, 147a). TOCOCHROMANOL Both carotenoids and tocochromanols BIOSYNTHESIS IN are synthesized in whole or in part from CYANOBACTERIA AND the plastidic isoprenoid biosynthetic path- PLANTS ........................ 721 way. The biosynthesis of isoprenoid pre- General Considerations: cursors is covered in detail elsewhere (67). Structures, Chemistry, and Briefly, two distinct pathways exist for isope- Vitamin E Activities ........... 721 tenylpyrophosphate (IPP) production: the The Tocochromanol Pathway cytosolic mevalonic acid pathway and the plas- Succumbs to Biochemical tidic mevalonate-independent, methylery- Genomics ..................... 722 thritol 4-phosphate (MEP) pathway. The Tocochromanol Aromatic methylerythritol 4-phosphate pathway com- Headgroup Synthesis .......... 723 bines glyceraldehyde-3-phosphate and pyru- Prenylation of Homogentisic Acid. 725 vate to form deoxy-d-xylulose 5-phosphate, An Alternate Route for Phytyl-Tail by UNIVERSITAT BERN on 09/12/09. For personal use only. and a number of steps are then required Synthesis...................... 726 to form IPP and dimethylallylpyrophosphate The Methyltransferases of (DMAPP) (67). IPP is subject to a sequen- Tocochromanol Synthesis ...... 726 tial series of condensation reactions to form Annu. Rev. Plant Biol. 2006.57:711-738. Downloaded from arjournals.annualreviews.org TheTocopherolCyclaseEnzyme . 727 geranylgeranyl diphosphate (GGDP), a key Tocopherol Functions ............ 728 intermediate in the synthesis of carotenoids, tocochromanols, and many other plastidic iso- prenoids (Figure 1). The tocochromanol and carotenoid INTRODUCTION: PLASTIDIC biosynthetic pathways are typical of many ISOPRENOID SYNTHESIS plant compounds in that the enzymes from Plastids contain sophisticated biochemical plant sources have historically proven ex- machinery producing an enormous array of tremely difficult to purify and analyze. This compounds that perform vital plastidic and is a result of a combination of properties, cellular functions. Many of these compounds including membrane association, low specific are also important for agriculture and hu- activity and poor stability of the enzymes 712 DellaPenna · Pogson ANRV274-PP57-27 ARI 29 March 2006 12:21 Figure 1 Overview of carotenoid and tocopherol biosynthesis in plants. The 2-C-methyl-d-erythritol- 4-phosphate (MEP) pathway provides isopetenylpyrophosphate (IPP) for synthesis of the central intermediate geranylgeranyl diphosphate (GGDP). GGDP can be used for synthesis of phytoene, chlorophylls, and tocotrienols or reduced to phytyl-diphosphate (PDP) used for phylloquinone, chlorophyll, and tocopherol synthesis. Phytol released from chlorophyll degradation is also used for tocopherol synthesis (not shown, see text). The pathway shown by orange arrows provides the by UNIVERSITAT BERN on 09/12/09. For personal use only. carotenoids found in leaves of most species. Other carotenoids and carotenoid cleavage/modification products are produced in certain species and/or particular tissues, and when known, the primary substrate for cleavage is given in parentheses. The pathway shown by green arrows is the synthesis of tocopherols from homogentisate, a product of the shikimate pathway. For clarity, only tocopherols are Annu. Rev. Plant Biol. 2006.57:711-738. Downloaded from arjournals.annualreviews.org shown, but when GGDP is condensed with homogentisate, the corresponding tocotrienols are produced by the same pathway. ABA, abscisic acid; MPBQ, methyl-6-phytyl-1,4-benzoquinone. during isolation as well as limitations in syn- approaches. This review focuses on progress thesizing or obtaining commercially available since the last reviews on these subjects in substrates for assays. Thus, much of our this series (30, 45). However, several recent understanding of the genes and enzymes of reviews on aspects of the biosynthesis, func- tocochromanol and carotenoid biosynthesis tion, and catabolism of these compounds are has resulted from the increasing ease of available, and readers are directed to these integrating complementary comparative ge- sources for additional information (29, 33, nomics, biochemical genetics, and molecular 38, 49, 74, 82–84, 104, 140, 144). www.annualreviews.org • Vite and Vita Synthesis in Plants 713 ANRV274-PP57-27 ARI 29 March 2006 12:21 CAROTENOID BIOSYNTHESIS form lycopene. Desaturation requires a plastid IN PLANTS terminal oxidase and plastoquinone in pho- tosynthetic tissues (8, 21, 85). Bacterial de- General Considerations and Early saturation differs from plants in that a single Steps in Biosynthesis enzyme, crtI (phytoene desaturase), intro- Carotenoids comprise a large isoprenoid fam- duces four double bonds into phytoene to ily and most are C40 tetraterpenoids de- yield all-trans-lycopene (28). rived from phytoene. The carotenoid back- bone is either linear or contains one or more cyclic β-ionone or ε-ionone rings or, less fre- Isomerizations During Carotenoid quently, the unusual cyclopentane ring of cap- Desaturation santhin and capsorubin that imparts the dis- Until recently, the higher plant desaturases tinct red color to peppers. Nonoxygenated were assumed sufficient for the production carotenoids are referred to as carotenes, of all-trans-lycopene. This conclusion was whereas their oxygenated derivatives are des- reached despite the accumulation of tetra- ignated as xanthophylls. The most com- cis-lycopene in tangerine tomato and algal monly occurring carotenes are β-carotene in mutants (27, 135) and biochemical evidence chloroplasts and lycopene in chromoplasts to the contrary from daffodil (9). Recently, of some flowers and fruits, e.g., tomatoes. the carotenoid isomerase gene, CRTISO, was The most abundant xanthophylls in photo- identified in Arabidopsis and tomato (55, synthetic plant tissues (lutein, violaxanthin, 89). Intriguingly, the protein shows 20%– and neoxanthin) are key components of the 30% identity to the bacterial carotenoid light-harvesting complexes. Carotenoids are desaturases; however it has no desaturase ac- involved in photosystem assembly, light har- tivity (89). Rather, the pathway to all-trans- vesting and photoprotection, photomorpho- lycopene proceeds via cis intermediates (16, genesis, nonphotochemical quenching,