Doctoral Degrees (Institute for Wine Biotechnology)

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Browsing Doctoral Degrees (Institute for Wine Biotechnology) by Subject "Carotenoids -- Synthesis"

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    Molecular analyses of candidate carotenoid biosynthetic genes in Vitis vinifera L.
    (Stellenbosch : Stellenbosch University, 2004-03) Young, Philip Richard; Vivier, Melane A.; Pretorius, I. S.; Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology. Institute for Wine Biotechnology.
    ENGLISH ABSTRACT: Plants cannot avoid stress and must therefore be capable of rapidly responding to extreme environmental changes. An inability to control and regulate the photosynthetic process during stress conditions will lead to the formation of highly reactive oxygen species that concomitantly causes photo-oxidative damage to the pigments and proteins of the photosynthetic apparatus. Since light is the primary source of energy for the photosynthetic process, it is clear that plants are continuously required to balance the light energy absorbed for the photochemical reactions against photoprotection in a dynamic way in order to survive. Carotenoids are precursors of abscisic acid, but more importantly structural components of the photosynthetic apparatus. During photosynthesis carotenoids function as accessory light-harvesting pigments, and also fulfil a photoprotective function by quenching the reactive molecules formed during conditions that saturate the photosynthetic process. Due to the importance of carotenoids to plant fitness and human health (as Vitamin A precursors) this study has attempted to isolate and characterise genes that are directly, or indirectly involved in carotenoid biosynthesis in Vitis vinifera. In total eleven full-Iength- and eight partial genes have been isolated, cloned and sequenced. These genes can be grouped into the following pathways: (i) the 1- deoxy-D-xylulose 5-phosphate (DOXP)/2-C-methyl-D-erythritol 4-phosphate (MEP) pathway (i.e. the plastidic isopentenyl diphosphate biosynthetic pathway); (ii) the mevalonate pathway (i.e. the cytosolic/mitochondrial IPP biosynthetic pathway); (iii) the carotenoid biosynthetic pathway; (iv) the abscisic acid biosynthetic pathway (as a degradation product of carotenoids); and general isoprenoid biosynthetic pathways (as precursors of carotenoids). The full-length genes (i.e. from the putative ATG to the STOP codon) of DOXP synthase (DXS), 4-hydroxy-3-methylbut-2-enyl diphosphate reductase (lytB), IPP isomerase (IPI), 3-hydroxy-3-methylglutaryl coenzyme A synthase (HMGS), phytoene synthase (PSY), Iycopene ~-cyclase (LBCY), ~-carotene hydroxylase (BCH), zeaxanthin epoxidase (lEP), 9-cis-epoxy carotenoid dioxygenase (NCED), farnesyl diphosphate synthase (FPS) and geranylgeranyl diphosphate synthase (GGPS) have been isolated from cDNA. In addition, the full-length genomic copy and putative promoters of DXS, PSY, LBCY, BCH, NCED and lEP have also been isolated from genomic DNA by the construction and screening of sub-genomic libraries. Alignments of the genomic copies of these genes to the corresponding cDNA sequences have provided useful information regarding the genomic organisation of these genes, including the intron-exon junction sites in V. vinifera. The copy number of the DXS, PSY, LBCY, BCH, NCED and lEP encoding genes in the Vitis genome have been determined. DXS, PSY, BCH and lEP are single copy genes, whereas LBCY and NCED have two and three copies, respectively. The transcriptional activity of the putative promoters of six of the isolated genes (i.e. DXS, PSY, LBCY, BCH, lEP and NCED) were tested with a transient reporter gene assay. None of the putative promoters tested showed any transcriptional activity of the reporter gene. The transcription of these genes, has however been shown using northern blot analysis and/or RT-PCR. Preliminary expression profiles for PSY, LBCY, BCH, and lEP were determined in different plant organs and the expression of these genes was generally higher in photosynthetically active tissues. The expression of these genes following different treatments (abscisic acid, NaCI and wounding) was also assayed. The functionality of five of the isolated full-length genes (IPI, GGPS, PSY, LBCY and BCH) has been shown in a bacterial colour complementation assay. In silica analysis of the predicted protein sequences of all eleven isolated genes revealed that they are conserved and share a high degree of homology to the corresponding proteins in other plant species. The sequences were further analysed for conserved domains in the protein sequences, and these proteins typically demonstrated similar domain profiles to homologues in other species (plant, bacteria and algae). The predicted protein sequences were further analysed for transit peptides, the presence of which would provide evidence for the sub-cellular localisation of the mature peptides. Since these genes are involved in biosynthetic pathways that are active in discrete organelles, the sub-cellular localisation of most of these proteins is known. The carotenoid biosynthetic genes (PSY, LBCY, BCH and ZEP), the abscisic acid biosynthetic gene, NCED, as well as the DOXP/MEP pathway genes (DXS, lytB and IPI) were all localised to the chloroplast. The mevalonate pathway gene, HMGS, was localised to both the cytosol and the mitochondria, and the general isoprenoid precursor genes, FPS and GGPS, were localised to the cytosol and the chloroplast, respectively. All these results are in agreement with the localisation of the respective pathways. In order to increase our understanding of carotenoid biosynthesis and functions in plants, we constitutively overexpressed one of the isolated genes (BCH) in the model plant, Nicotiana tabacum. Plants expressing the BCH gene in the sense orientation maintained a healthy photosynthetic rate under stress conditions that typically caused photoinhibition and photodamage in the untransformed control plants. This result was inferred using chlorophyll fluorescence and confirmed using CO2 assimilation rates and stomatal conductance. Chlorophyll fluorescence measurements indicated that the photo protective non-photochemical quenching ability of the BCH-expressing plants increased, enabling the plants to maintain photosynthesis under conditions that elicited a stress response in the untransformed control plants. An integral photosynthetic protein component, the D1 protein, was specifically protected by the additional zeaxanthin in the BCH sense plants. Plants expressing an antisense BCH proved the converse, i.e. lower levels of BCH resulted in decreased zeaxanthin levels and made the transgenic plants more susceptible to high-light induced stress. These results have shown the crucial role of carotenoids (specifically the xanthophylls) in the photoprotective mechanism in plants. The increased photoprotection provided by the BCH expressing plants suggests that the scenario in plants is not optimal and can be improved. Any improvement in the photoprotective ability of a plant will affect both the fitness and productivity of the plant as a whole and will therefore find application in a number of crop plants on a global scale. This study has resulted in the successful isolation and characterisation of genes involved in the direct, or indirect, carotenoid biosynthetic pathways. The further study and manipulation of these genes in model plants will provide useful insights into the physiological role of specific carotenoids in photosynthesis and in plants as a whole.