Thesis (MSc)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: The latest strategies in the protection of crops against microbial pathogens are rooted in harnessing the natural, highly complex defense mechanisms of plants through genetic engineering to ultimately reduce the application of chemical pesticides. This approach relies on an in-depth understanding of plant-pathogen interactions to develop reasonable strategies for plant improvement. Among the highly specialized defense mechanisms in the plant’s arsenal against pathogen attack, is the de novo production of proteinaceous antimicrobial peptides (AMPs) as part of the plant’s innate immunity. These AMPs are small, cysteine-rich peptides such as plant defensins that are known for their broad-spectrum of antifungal activity. These plant defensin peptides have been found to be present in most, if not all plant species and the defensin encoding genes are over-represented in plant genomes. Most of these defensins are generally the products of single genes, allowing the plant to deliver these molecules relatively rapidly and with minimal energetic expense to the plant. These factors contribute to establishing AMPs as excellent candidates for genetic engineering strategies in the pursuit of alternative crop protection mechanisms. The first antimicrobial peptide identified and isolated from grapevine, Vv-AMP1, was found to be developmentally regulated and exclusively expressed in berries from the onset of ripening. Recombinantly produced Vv-AMP1 showed strong antifungal activity against a wide range of plant pathogenic fungi at remarkably low peptide concentrations in vitro, however, no in planta defense phenotype could thus far be linked to this peptide. In this study, the antifungal activity of Vv-AMP1 constitutively overexpressed in its native host (Vitis vinifera) was evaluated against grapevine-specific necrotrophic and biotrophic fungi. Firstly, a hardened-off genetically characterised transgenic V. vinifera (cv. Sultana) population overexpressing Vv-AMP1 was generated and morphologically characterized. In order to evaluate the in planta functionality of Vv-AMP1 overexpressed in grapevine, this confirmed transgenic population was subjected to antifungal assays with the necrotrophic fungus, B. cinerea and the biotrophic powdery mildew fungus, Erysiphe necator. For the purpose of infection assays with a biotrophic fungus, a method for the cultivation and infection with E. necator was optimized to generate a reproducible pathosystem for this fungus on grapevine. Detached leaf assays according to the optimized method with E. necator revealed programmed cell death (PCD) associated resistance linked to overexpression of Vv-AMP1 that can be compared to that of the highly resistant grapevine species, Muscadinia rotundifolia. Contrastingly, whole-plant infection assays with B. cinerea revealed that Vv-AMP1 overexpression does not confer V. vinifera with elevated resistance against this necrotrophic fungus. An in silico analysis of the transcription of defensin-like (DEFL) genes previously identified in grapevine was included in this study. This analysis revealed putative co-expression of these DEFL genes and other genes in the grapevine genome driven by either tissue- or cultivar specific regulation or the plant’s response to biotic and abiotic stress stimuli. In conclusion, this study contributed to our knowledge regarding Vv-AMP1 and revealed an in planta defense phenotype for this defensin in grapevine. In silico analysis of the DEFL genes in grapevine further revealed conditions driving expression of these genes allowing for inferences to be made regarding the possible biological functions of DEFL peptides in grapevine. / AFRIKAANSE OPSOMMING: Die nuutste strategieë wat deel vorm van die beskerming van plant gewasse teen mikrobiese patogene het hul oorsprong in die inspanning van die natuurlike, hoogs gekompliseerde verdedigingsmeganismes van die plant deur middel van genetiese enginieurswese ten einde die gebruik van chemiese plaagdoders te verlaag. Hierdie benadering maak staat op ‘n in-diepte begrip van plant-patogeen interaksies om verstandige strategieë vir plantverbetering te kan ontwikkel. Van hierdie hoogs-gespesialiseerde verdedigingsmeganismses in die plant se arsenaal teen patogeen aanvalle sluit die de novo produksie van proteinagtige antimikrobiese peptiede (AMPs) in as deel van die plant se ingebore immuunstelsel. Hierdie AMPs is klein, sisteïen-ryke peptiede soos die plant “defensins” en is bekend vir hul breë-spektrum antifungiese aktiwiteit. Hierdie plant defensinpeptiede word aangetref in meeste, indien nie alle plant spesies nie en die defensin koderende gene word oor-verteenwoordig in plant genome. Meeste van hierdie defensins is gewoonlik die produkte van enkele gene wat die plant in staat stel om hierdie molekules relatief spoedig en met minimale energie verbruik in die plant te vorm. Hierdie faktore dra by tot die vestiging van AMPs as uitstekende kandidate vir genetiese ingenieursstrategieë as deel van die strewe na alternatiewe gewasbeskermingsmeganismes. Die eerste antimikrobiese peptied wat geïdentifiseer en geïsoleer is uit wingerd, Vv-AMP1, word beheer deur die ontwikkelingsstadium en word eksklusief uitgedruk in korrels vanaf die aanvang van rypwording. Rekombinant-geproduseerde Vv-AMP1 het sterk antifungiese aktiwiteit getoon teen ‘n wye reeks plantpatogeniese swamme teen merkwaardige lae peptied konsentrasies in vitro, alhoewel geen in planta verdedigingsfenotipe tot dusver gekoppel kon word aan hierdie peptied nie. In hierdie studie was die antifungiese aktiwiteit van Vv-AMP1 wat ooruitgedruk is in sy natuurlike gasheerplant (Vitis vinifera) ge-evalueer teen wingerd-spesifieke nekrotrofiese- en biotrofiese swamme. Eerstens is ‘n afgeharde geneties-gekarakteriseerde transgeniese V. vinifera (cv. Sultana) populasie wat Vv-AMP1 ooruitdruk gegenereer en morfologies gekarakteriseer. Om die in planta funksionaliteit van Vv-AMP1 ooruitgedruk in wingerd te evalueer is hierdie bevestigde transgeniese populasie blootgestel aan antifungiese toetse met die nekrotrofiese swam, B. cinerea en die biotrofiese swam, Erysiphe necator. Vir die doel om infeksiestudies uit te voer met ‘n biotrofiese swam is ‘n metode geoptimiseer vir die kweek en infeksies met E. necator wat gelei het tot ‘n herhaalbare patosisteem vir hierdie swam op wingerd. Blaarstudies, volgens die pas-verbeterde metode vir E. necator infeksies het ‘n geprogrammeerde seldood-geassosieërde weerstand, gekoppel aan die ooruitdrukking van Vv-AMP1 onthul, wat vergelyk kan word met dié van die hoogs-weerstandige wingerdspesie, Muscadinia rotundifolia. Hierteenoor het heel-plant infeksie studies met B. cinerea onthul dat Vv-AMP1 ooruitdrukking geen verhoogde weerstand teen dié nekrotrofiese swam aan V. vinifera bied nie. ‘n In silico analise van die transkripsie van defensin-agtige (DEFL) gene wat vroeër in wingerd geïdentifiseer is, is by hierdie studie ingesluit. Hierdie analise het vermeende gesamentlike uitdrukking van hierdie DEFL gene en ander gene in die wingerd genoom onthul wat aangedryf word deur weefsel- of kultivar-spesifieke regulering of die plant se reaksie tot biotiese en abiotiese stress stimuli. Ten slotte, hierdie resultate het bygedra tot ons kennis in verband met Vv-AMP1 en het ‘n in planta verdedigingsfenotipe vir hierdie defensin in wingerd onthul. In silico analiese van die DEFL gene in wingerd het verder toestande onthul wat die uitdrukking van hierdie gene aandryf wat ons toelaat om aannames te maak ten opsigte van die moontlike biologiese funksies van DEFL peptiede in wingerd en ondersteun die opstel en toets van hipoteses vir die rol en megansimes van aksie van die wingerd defensin familie.

Thesis (PhD)--University of Stellenbosch, 2004. / ENGLISH ABSTRACT: Plant-pathogen interactions have been intensively investigated in the last decade. This major drive towards understanding the fundamental aspects involved in plant disease resistance is propelled by the obvious agricultural and economical benefits that are intrinsically linked to disease and stress resistant plants. It is, therefore, not surprising that fundamental research in this area is not just restricted to model organisms, such as Arabidopsis and tobacco, but also extends to more traditional crop plants, such as maize, bean, soybean, apples, grapevine etc. In grapevine for instance, several genes involved in disease resistance have been isolated. One of these genes, encoding for a polygalacturonase inhibiting protein (PGIP), has been studied extensively. PGIPs are cell wall bound, contain leucine rich repeats (LRR) and are found in all dicotyledonous plants so far examined. In most cases, pgip genes occur in small multigene families and expression is often tissue specific and developmentally regulated. Up-regulation of PGIP-encoding genes typically occurs upon pathogen infection, treatment with elicitors, salicylic acid (SA), jasmonic acid (JA), cold treatment and wounding. Differential regulation and specificity have been shown to occur between members of the same multigene family. Differential regulation even extends to the utilization of separate pathways to induce pgip genes from the same family in response to a single stress stimulus. PGIPs interact with cell wall macerating polygalacturonases (PGs) that are secreted by pathogenic fungi during the infection process. The antifungal action of PGIPs is thought to depend on a dual action. The physical interaction of PGIP with PGs has an inhibitionary effect, resulting in (i) a slower fungal infection rate and (ii) the prolonged existence of long chain oligogalacturonides (OGs). These oligosaccharides are able to elicit a general plant defense response, enabling the plant to further retard or curb the spread of infection. The main objective of this study was to investigate the regulatory aspects underlying PGIP expression in grapevine. Unlike most characterized PGIP encoding genes from other dicotyledonous plant species, no evidence to support the existence of a V. vinifera PGIP multigene family could be found from either genetic or biochemical analyses. Recently, a genomic DNA fragment from Vitis vinifera cv Pinotage was pathogen interactions with regards to the fundamental processes underlying defense gene regulation. / AFRIKAANSE OPSOMMING: Die ooglopende voordele wat, vanuit 'n landboukundige én ekonomiese oogpunt, uit siekte- en stresbestande plante spruit, het gedurende die laaste dekade aanleiding gegee tot die ontwikkeling van plantpatogeen-interaksies as "n baie belangrike studieveld. Dit was dus ook te verwagte dat fundamentele navorsing in hierdie area nie net beperk gebly het tot modelorganismes soos Arabidopsis en tabak (ook natuurlik van landboukundige belang) nie, maar ook na meer tradisionele landbougewasse soos mielies, boontjies, sojaboontjies, appels, druiwe, ens. oorgevloei het. Verskeie siekteweerstands-verwante gene is byvoorbeeld al vanuit wingerd geïsoleer. Een só "n geen wat vir "n poligalakturonase-inhiberende proteïen (PGIP) kodeer, vorm deel van hierdie groep gene. Die funksie en regulering van PGIP's is baie goed bestudeer. Hierdie proteïene word normaalweg in die selwande van die meeste dikotiele plante aangetref. Leusienryke herhalings is algemeen in PGIP's en hierdie tipe van herhalings is kenmerkend van proteïene betrokke by proteïen-proteïen-interaksies. Verder word pgip-gene gewoonlik in klein multigeenfamilies aangetref, waar in die meeste gevalle die uitdrukking weefselspesifiek en die regulering spesifiek ten opsigte van die ontwikkelingsfase is. Verskeie faktore kan tot die induksie van pgip-gene lei, soos onder andere patogeen-infeksie, elisitoor-, salisiensuur-, jasmoonsuur- en kouebehandeling, asook verwonding. Differensiële regulering word in baie gevalle tussen lede van dieselfde multigeenfamilie aangetref. Hierdie differensiële regulering kan selfs bemiddel word deur onafhanklike reguleringsweë in reaksie op dieselfde induksiestimulus. PGIP's is in staat om te reageer met poligalakturonases (PGs), wat selwande afbreek en wat gedurende die infeksieproses deur swamme of fungi afgeskei word. Die effek van hierdie interaksie is tweeledig: (i) Die fisiese interaksie tussen PGIP en PG moduleer die aktiwiteit van die PG deur die ensiemaksie te inhibeer, en (ii) PGinhibisie lei tot die verhoogde stabiliteit van langketting-oligogalakturonades, molekules wat daartoe in staat is om die weerstandsrespons van plante te ontlok. Die inhibisie van die patogeen-PG's, tesame met die geïnduseerde weerstandrespons, stel die plant dan in staat om verdere infeksie te vertraag of te verhoed. Die doel van hierdie studie was om die onderliggende aspekte van PGIPregulering in wingerd te bestudeer. In teenstelling met die meeste plantspesies waar pgip-gene in klein multigeenfamilies aangetref word, is daar nie 'n pgip-multigeenfamilie in wingerd nie. Veelvuldige kopieë van In enkele pgip-geen word egter in die wingerdgenoom aangetref. Daar is onlangs in ons laboratorium In genoom-DNAfragment vanaf Vitis vinifera cv Pinotage geïsoleer wat die oopleesraam en 5'-stroomopsekwense van In PGIP-enkoderende geen (Vvpgip1) bevat. In hierdie studie is die uitdrukkingspatroon van Vvpgip1 ten opsigte van weefselspesifisiteit, korrelontwikkelingsfase, asook die effek van verskeie omgewings en patogeenverwante stres-stimuli ontleed. Die regulatoriese meganismes van Vvpgip1 bevat spesifieke in planta-ontwikkelingsfaseseine wat verder deur spesifieke faktore, insluitende omgewings- en patogeenstres, gereguleer word. In lyn hiermee is mRNS-transkripte van Vvpgip1 tot wortel- en korrelweefsels beperk, terwyl die mRNS-vlakke ook tussen verskillende korrelontwikkelingsfases wissel. Kumulatiewe uitdrukking kon waargeneem word in veráison-korrels in reaksie op verwonding en osmotiese stres. Die weefselspesifieke uitdrukkingspatroon tipies van wingerd-PGIP is in blare opgehef in reaksie op Botrytis cinerea-infeksie, verwonding, osmotiese stres, ouksien (indoolasynsuur) en salisiensuur. PGIP-uitdrukking word ook onderdruk deur In staurosporien-sensitiewe proteïenkinase, wat In goeie aanduiding is van die betrokkenheid van proteïenfosforilasie in die seintransduksiekaskade wat tot PGIPuitdrukking aanleiding gee. Die geïnduseerde PGIP-uitdrukkingsprofiel in wingerdblare kan ook nageboots word in tabak wat met die Vvpgip1-geen en -promotor getransformeer is. PG-inhibisie-eksperimente met membraan-geassosieerde proteïenekstrakte van geïnduseerde wingerdblare het ook dieselfde profiel getoon as dié van PGIP wat deur die Vvpgip1-geen geënkodeer is. Die uitdrukkingsprofiel van PGIP in die transgeniese tabakplante het ook bewys dat die promotor van die Vvpgip1-geen vir die geïnduseerde PGIP-uitdrukkingsprofiel in wingerdblare verantwoordelik is. In silica-analise van die promotorarea dui op die teenwoordigheid van verskeie cis-werkende elemente. Die kern promotor en transkripsie-aanvangsgedeelte is gevolglik eksperimenteel bepaal. Verder het uitdrukkingseksperimente met promotorfragmente verskeie dele van die promotor geïdentifiseer wat by stimulis-geassosieerde uitdrukking betrokke is. Posisioneel is hierdie fragmente in goeie konteks met die voorspelde cis-werkende elemente en kan dus die basis vorm vir verdere studies oor Vvpgip-regulering. Met hierdie studie word die eerste data verskaf waar die regulering van PGIP deur omgewingsverwante faktore verbind kan word met onwikkelingspesifieke toestande in die plant. Verder verskaf die resultate verdere bewyse vir die rol van PGIP in plant-patogeen-interaksies en lewer spesifieke bydraes tot die onderliggende prosesse wat by die regulering van siekteweerstandverwante gene betrokke is.

Thesis (MScAgric)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: Plant carotenoid cleavage dioxygenases (CCD) are a family of enzymes that catalyse the oxidative cleavage of carotenoids and/or apocarotenoids. Carotenoids are synthesised in plastids (primarily chloroplasts and chromoplasts), where they are involved in light-harvesting and protecting the photosynthetic apparatus from photo-oxidation. The carotenoid-derived apocarotenoids fulfil a number of roles in plants such as phytohormones, pollinator attractants and flavour and aroma compounds. Due to the floral and fruity characteristics that apocarotenoids contribute to wine, these C13 compounds have received interest in grapevine (Vitis vinifera L.). The CCD gene family in Arabidopsis consists of nine members, all encoding for enzymes that catalyse the cleavage of carotenoids. The enzymes in this family include 9-cis-epoxydioxygenases (NCEDs) and four classes of CCD. NCEDs and CCD7 and CCD8 are involved with plant hormone synthesis, e.g. abscisic acid (ABA) through cleavage by NCED and strigolactone (SL) through the sequential cleavage of carotenoids by CCD7 and CCD8, respectively. SLs are a fairly new class of plant hormone which are involved in several aspects of plant growth and development. The most extensively characterised role of SLs is their involvement in the inhibition of shoot-branching. CCD1 and CCD4 cleave a variety of carotenoids to form pigments and aroma compounds. For example, CCD1 forms β-ionone and β-damascenone, which are important varietal flavours of wine, and CCD4 is involved in synthesis of the pigment and aroma compounds of saffron and annatto. CCD1 enzymes symmetrically cleave the 9,10 (9’,10’) double bonds of multiple carotenoids to produce a C14 dialdehyde and two C13 products. Additional CCD1 cleavage activity at 5,6 (5’,6’) double bonds of lycopene has been reported. Previous studies have shown that CCD1 isolated from V. vinifera (VvCCD1) was able to cleave multiple carotenoid substrates in vitro, namely zeaxanthin, lutein and β-carotene at 9,10 (9’,10’) double bonds and both the 5,6 (5’,6’) and 9,10 (9’,10’) double bonds of lycopene. None of the other VvCCDs, except VvCCD4a have been isolated (but no functionality was illustrated) and characterised yet. CCD4 enzymes also cleave carotenoids at the 9,10 (9’,10’) double bond positions. The presence of plastid-target peptides implies that the CCD4 enzymes have continuous access to carotenoids. Therefore it is suggested that CCD4s are responsible for carotenoid maintenance, where CCD1s contribute towards volatile production. To test this hypothesis VvCCD1, VvCCD4a and VvCCD4b were isolated from V. vinifera (cv Pinotage) cDNA and cloned into a pTWIN1 protein expression vector. Substrate specificity of each VvCCD was tested by co-transforming a carotenoid accumulating E. coli strain with a CCD expression vector. Carotenoids synthesized by the bacteria were identified and quantified by UPLC-analysis, while the concentration of the apocarotenoids, were measured in the headspace of the bacterial cultures using HS-SPME-GC-MS. Several optimisations were done to minimize the natural degradation of the carotenoids; to ensure that the apocarotenoid formation is predominantly due to the enzymatic cleavage by the VvCCDs and not due to oxidation or other non-enzymatic degradation. The HS-SPME-GC-MS analysis indicated that all isoforms cleaved phytoene, lycopene and ε-carotene. Additionally VvCCD1 cleaved a carotenoid involved in photosynthesis, namely β-carotene, while VvCCD4a cleaves neurosporene and VvCCD4b cleaves neurosporene and ζ-carotene, carotenoids not involved in photosynthesis. This study has illustrated that VvCCD1 cleave carotenoids necessary for photosynthesis and VvCCD4s cleave carotenoids which were not present in berry tissue, suggesting their role in carotenoid maintenance. Therefore in planta substrates for CCD1 could possibly be C27 apocarotenoids generated from enzymatic cleavage through CCD4 (role in carotenoid maintenance), CCD7 and/or photo-oxidation, which are then transported from the plastid to the cytosol or possibly C40 carotenoids that are released during senescence or when the plastid membrane is damaged, thus releasing important aroma compounds. Thus the identification of the in vivo substrates has contributed to the understanding the in planta functions of these enzymes / AFRIKAANSE OPSOMMING: Die plant ensiemfamilie van karotenoïedsplitsingdioksigenases (CCDs) kataliseer die oksidatiewe splitsing van karotenoïede en/of apokarotenoïede. Karotenoïede word in plastiede (primêr chloroplaste en chromoplaste) sintetiseer en is betrokke by lig-absorpsie en die beskerming van die fotosintetiese apparaat teen foto-oksidasie. Die apokarotenïede afkomstig van karotenoïede dien onder meer as planthormone, geur- en aromakomponente en om bestuiwers aan te lok. Aangesien apokarotenoïede bydra tot die vrug- en blomgeure van wyn is die C13-verbindings binne wingerd (Vitis vinifera L.) van belang. Al nege lede van die CCD geenfamilie in Arabidopsis kodeer karotenoïedsplitsingsensieme. Die ensiemfamilie sluit 9-sis-epoksidioksigenases (NCEDs), en vier klasse CCD in. NCEDs en CCD7 en 8 is betrokke by die sintese van planthormone, naamlik absissiensuur (ABA) deur NCED en strigolaktone (SL) deur die opeenvolgende aksie van onderskeidelik CCD7 en CCD8. SLe is redelik onlangs as planthormone indentifiseer en is betrokke by ‘n verskeie aspekte van die groei en ontwikkeling van plante. Die rol van SL in inhibisie van vertakking is die beste gekarakteriseerde van hierdie aspekte. CCD1 en CCD4 splits ‘n verskeidenheid karotenoïede om pigmente en aromakomponente te vorm. CCD1 vorm byvoorbeeld β-jonoon en β-damasenoon, beide belangrike kultivar-spesifieke wyngeure. CCD4 vorm weer die pigment en aromakomponente van saffraan en annatto. Die CCD1 ensieme splits die 9,10 (9’,10’) dubbelbindingsetels van verskeie karotenoïede simmetries en vorm een C14-dialdehied en twee C13-produkte. Daar is voorheen melding gemaak van verdere splitsing deur CCD1 by die 5,6 (5’,6’) dubbelbindingsetels van likopeen. Vroeër is getoon dat die CCD1 isovorm wat uit V. vinifera geïsoleer is, naamlik VvCCD1, in vitro seaxantin, luteïen en β-karoteen by die 9,10 (9’,10’) dubbelbindingsetels kon splits, en likopeen by beide die 9,10 (9’,10’) en 5,6 (5’,6’) dubbelbindingsetels. Geen ander VvCCDs is al isoleer en funksioneel gekarakteriseer. VvCCD4a is isoleer, maar geen funksie is bepaal nie. CCD4 ensieme splits ook die 9,10 (9’,10’) dubbelbindingsetels van karotenoïede. Aangesien CCD4 ensieme ‘n plastied-bestemmingspeptied besit behoort dié ensieme konstant toegang tot karotenoïede te hê, wat dui op hul rol in die handhawing van die karotenoïedbalans, terwyl CCD1-ensieme bydra tot die sintese van vlugtige verbindings. Om hierdie hipotese te toets is VvCCD1, VvCCD4a en VvCCD4b uit V. vinifera (kv Pinotage) kDNS isoleer in binne ‘n pTWIN1 proteïenuitdrukkingsvektor kloneer. Die substraatspesifisiteit van elke VvCCD is getoets deur ‘n karotenoïedakkumulerende E. coil stam te transvormeer met ‘n CCD-uitdrukkingsvektor. UPLC-analise is gebruik om karotenoïede wat deur die bakterium sintetiseer is te kwantifiseer en identifiseer, terwyl die apokarotenoïedinhoud en -konsentrasie van die boruimte van die bakteriële kultuur met HS-SPME-GC-MS bepaal is. Verskeie aspekte van die proses is optimaliseer om natuurlike afbreking van karotenoïede te minimeer. Daardeur is verseker dat die apokarotenoïedvorming primêr vanweë die ensiematiese splitsing deur VvCCDs plaasvind en nie deur oksidasie of ander nie-ensiematiese afbreking. Die HS-SPME-GC-MS metings het aangedui dat al drie isovorme fitoëen, likopeen en ε-karoteen kan splits. VvCCD1 kan daarby β-karoteen splits, terwyl VvCCD4a neurosporeen, en VvCCD4b neurosporeen en ζ-karoteen kan splits, beide karotene wat nie betrokke is by fotosintese nie. Dié studie toon dat VvCCD1 die karotenoïede splits wat benodig word vir fotosintese, terwyl beide VvCCD4 isovorme karotenoïede splits wat nie in druiwekorrels gevind word nie. Dit dui op hulle rol in die handhawing van karotenoïedpoele. Die in planta substrate vir CCD1 mag dus die C27-apokarotenoïede wees wat deur CCD4 (as deel van karotenoïedhandhawing), CCD7 en/of foto-oksidasie gevorm word en na die sitosol vervoer word, of moontlik die C40-karotenoïede wat tydens veroudering óf wanner die plastiedmembraan beskadig is in die sitosol vrygestel word. Die identifisering van die in vivo substrate het dus bygedra to die begrip van die in planta funksies van die ensieme.

Thesis (MSc (Wine Biotechnology))--University of Stellenbosch, 2006. / Plants have evolved photosynthetic systems to efficiently harvest sunlight energy for the production of carbohydrates, but these systems also are extremely susceptible to an excess of light. To combat the potential damaging effects of light, plants have developed various mechanisms to control and cope with light stress. These mechanisms include the movement of either leaves, cells (negative phototaxis) or chloroplasts to adjust the light-capturing potential, the adjustment of the light-harvesting antenna size through gene expression or protein degradation, the removal of excess excitation energy either through an alternative electron transport pathway or as heat. However, the latter mechanism based on thermal dissipation, remains the most effective to rid the plant of damaging excess light energy. This process involves several carotenoid pathway pigments, specifically the de-epoxidised xanthophyll cycle pigments. The process and extent of thermal dissipation in plants can be measured and quantified as non-photochemical quenching (NPQ) of chlorophyll fluorescence by using well-established methodologies. Several Arabidopsis and Chlamydomonas mutants affected in the xanthophyll cycle have been isolated. These mutants have provided evidence for the correlation between the de-epoxidised xanthophyll cycle pigments and NPQ as well as better understanding of the operation of the xanthophyll cycle and the related carotenoid biosynthetic enzymes. This key photoprotective role of the xanthophyll cycle is therefore a promising target for genetic engineering to enhance environmental stress tolerance in plants. Several genes from the carotenoid biosynthetic pathway of grapevine (Vitis vinifera L.) were isolated previously in our laboratory. The main aim of this study was to over-express two xanthophyll cycle genes from grapevine in Arabidopsis and to analyse the transgenic population with regards to pigment content and levels as well as certain photosynthetic parameters. The transgenic lines were compared with wild type Arabidopsis (untransformed) plants and two xanthophyll cycle mutants under non-limiting conditions as well as a stress condition, specifically a high light treatment to induce possible photodamage and photoinhibition. Transgenic Arabidopsis lines over-expressing the two V. vinifera xanthophyll cycle genes, β-carotene hydroxylase (VvBCH) and zeaxanthin epoxidase (VvZEP), were established following Agrobacterium transformation. In addition to the untransformed wild type, two NPQ mutants, npq1 (lacking violaxanthin de-epoxidase) and npq2 (lacking zeaxanthin epoxidase), were used as controls throughout this study. The transgenic lines were propagated to a homozygous T3-generation, where stable integration and expression of the transgenes were confirmed in only 16% and 12% for VvBCH and VvZEP lines, respectively. No phenotypical differences could be observed for the transgenic lines compared to the wild type, but the npq2 mutant showed a stunted and ‘wilty’ phenotype, as was previously described. To evaluate the pigment composition of the transgenic lines a reliable and reproducible method was needed to analyse carotenoids from leafy material. To this end a new high-performance liquid chromatography (HPLC) method was developed for the quantitative profiling of eight major carotenoids and chlorophyll a and b. Emphasis was placed on baseline separation of the xanthophyll pigments, lutein and zeaxanthin as well as the cis- and trans-forms of violaxanthin and neoxanthin. The method effectively distinguished Arabidopsis wild type plantlets from the two NPQ mutant lines (npq1 and 2) and could possibly find application for green leafy tissue samples in general. The carotenoid content of the NPQ mutants were in accordance with previous reports. The lack of zeaxanthin epoxidase activity in the npq2 mutant resulted in the accumulation of zeaxanthin under both low and high light conditions. This high level zeaxanthin was found to cause an initial rapid induction of NPQ at low to moderate light intensities, but this difference disappeared at high light, where zeaxanthin formation induced considerable NPQ in the wild type. Similarly, the npq1 mutant was unable to de-epoxidise violaxanthin to zeaxanthin under high light conditions, which resulted in severe inhibition of NPQ induction. Furthermore, these mutant plantlets were shown to be more susceptible to photoinhibition compared to that of the wild type. The over-expression of VvBCH resulted in a marked increase in the xanthophyll cycle pool pigments (violaxanthin, antheraxanthin and zeaxanthin) and reduced β-carotene levels under both low and high light conditions compared to that the wild type, indicating elevated β-carotene hydroxylase activity possibly due to over-expression of the VvBCH gene. Similar to the induction of NPQ in the npq2 mutant, the increased levels of zeaxanthin in the VvBCH lines did not offer any additional photoprotection. This would suggest that the heightened zeaxanthin levels observed for the VvBCH lines do not necessarily enhance photoprotection, however may protect the thylakoid membrane against lipid peroxidation as has been shown previously. The VvZEP lines however, showed reduce levels of zeaxanthin in high light conditions to that of the wild type, probably due to the competing epoxidation and de-epoxidation reactions of the xanthophyll cycle. This reduction in zeaxanthin synthesis in the VvZEP lines resulted in significant reduced NPQ induction compared that of the wild type, a phenomenon also observed for the npq1 mutant. Similar to the npq1 mutant, these lines displayed significantly increased photoinhibition, which may be due to photodamage of the reaction centers if one considers the lowered photosystem II photochemistry efficiency and reaction center openness of these lines compared to the wild type. This may suggest that even small reductions in zeaxanthin amounts can result in an increase in photoinhibition, under high light conditions. This study and its results provide fundamental information regarding two grapevine-derived carotenoid pathway genes and their possible physiological roles. Moreover, studies like these provide information that is essential when possible biotechnological approaches are planned with this central plant metabolic pathway in mind. The results highlighted the complex regulation of this pathway, necessitating attention to flux control, simultaneous manipulation of several pathway genes, and the measurement of other compounds derived from this pathway when evaluating the possible applications of the carotenoid pathway of plants.