1,721,052 research outputs found
The relationship between strigolactones and Striga hermonthica infection in cereals
No full text Cereal production in Africa is under increasing constraint due to the obligate, out-crossing, hemiparasitic weed Striga hermonthica (Del.) Benth, a member of the Scrophulariaceae family. Striga parasitizes roots of cereals like sorghum, pearl millet, maize and upland rice. It has infested about 40% of the African agricultural land, resulting in severe yield losses or even complete crop failure worth US billion per annum. The subsistence farmers or approximately 300 million African people lose about 20-80% of their crop because of this weed. This considerable damage by Striga is due to the fact that existing control measures are often ineffective. Since much of the damage occurs underground during the early stages of parasitism, there is a need to develop control strategies that target the weed prior to attachment and emergence. A crucial step in the lifecycle of Striga is the induction of germination by strigolactones, signalling molecules secreted by the roots of its host. These strigolactones could be an important target to control this weed at the pre attachment phase. Control methods targeted at the germination and attachment phase, based on low strigolactones, might prove to be more effective and result in reduced infestation of this weed in cereal crops. In present thesis we studied the relationship between strigolactones and Striga infection in cereals and explored opportunities for lowering Striga damage at the germination or attachment phase. To this end different aspects like strigolactone biosynthetic inhibitors, genetic variation for strigolactone production, and the effect of fertilizers on strigolactone production were investigated in laboratory studies and – when possible - in the field in Kenya and Mali. The first investigation was on the use of carotenoid inhibitors to see the possibilities of strigolactone reduction in the roots of plants by blocking carotenoid biosynthesis. We postulated in this study that the (mild) inhibition of carotenoid biosynthesis by carotenoid inhibitors, could lead to a reduced production of strigolactones and decreased Striga germination and infection. Very low concentrations of four different carotenoid inhibitors (fluridone, norflurazon, clomazone and amitrole) were applied to rice either through irrigation or through foliar spray. Irrigation application of all carotenoid inhibitors and spray application of amitrole significantly decreased strigolactone production. Application of carotenoid inhibitors caused 61-75% reduction in Striga germination and 65-94% reduction in Striga attachment. The study shows thatthe reducing effect of carotenoid inhibitors (which, in much higher concentrations are widely used as herbicides) on strigolactone secretion and subsequent Striga germination and attachment may be developed into an attractive Striga control technology. Another experiment was aimed at assessing the pre-attachment Striga resistance based on low strigolactone production. We hypothesized that low strigolactones producing crop cultivars might possess pre-attachment Striga resistance due to less germination. For this purpose a set of 18 upland cultivars of NERICA and their parents were screened for strigolactones production and Striga infection parameters like germination, attachment, emergence and Striga dry biomass. NERICA 1 and CG14 produced significantly less strigolactones and showed less Striga infection while NERICAs 7, 8, 11 and 14 produced the highest amounts of strigolactones and showed the most severe Striga infection. This study shows that genetic variation for pre-attachment Striga resistance exists in NERICA rice due to variation in strigolactones. This could be highly relevant for breeding programs aimed at the development of Striga resistant cultivars. In another similar study we hypothesized that variation in strigolactone production in rice might be interconnected with the tillering phenotype and that this link could affect Striga infection. In this study the genetic variation was tested in a series of rice varieties collected from all over the world for strigolactone production, tillering phenotype and Striga infection. Rice cultivars like IAC 165, IAC 1246, Gangweondo and Kinko produced high amounts of the strigolactones, displayed low amounts of tillers and induced high Striga germination, attachment, emergence as well as Striga biomass. In contrast to this, rice cultivars such as Super Basmati, TN 1, Anakila and Agee showed low production of strigolactones and also low Striga germination and infection but high tillering. These results show that genetic variation in strigolactone production results in variation in tillering and also in Striga infection. The tillering phenotype could possibly be used as an easy indicator of the strigolactone production in a breeding programme for Striga resistance. Some experiments were also designed with the aim to quantify the relationship between strigolactones and Striga germination and attachment and to explore the mechanism responsible for the reported reduction in Striga parasitism in the field after fertilizer application. Different levels of nitrogen and phosphorous were applied under greenhouse conditions using rice, maize and sorghum. For maize and sorghum, a parallel study was carried out under field conditions in Kenya and Mali to study the translation of greenhouse results to the field. Application of N and P effectively suppressed Striga infection in the greenhouse in all three crop species and the reduction strongly correlated with reduced secretion of strigolactones into the rhizosphere and the Striga germination induced by these exudates. Although the field results with maize in Kenya were less consistent than in the greenhouse, especially with respect to P effect, still there was a trend that fertilizer application reduced Striga infection. These results show that the positive effect of fertilizer against Striga is at least partly due to a reduction in strigolactone production and as a consequence of that lower Striga germination and subsequent attachment. Overall it can be concluded that there is a good correlation between strigolactones andStriga germination, attachment and biomass. We found this using strigolactone biosynthesis inhibitors, genetic variation and using fertilizer application. These technologies can hence be exploited as an important tool to target Striga at a very early phase of its life cycle. The practical field application of these strategies requires further research but could lead to effective Striga control components that can be used in Integrated Striga Management.</p
Regulation of biosynthesis and transport of strigolactones and their effect on plant development
No full text Strigolactonen zijn van carotenoïden afkomstige signaalmoleculen, aanvankelijk geïdentificeerd als de kiemstimulantia voor wortelparasieten van de Orobanchaceae familie en pre-symbiotisch signaal voor arbusculaire mycorrhiza (AM) schimmels. Strigolactonen zijn aangetoond in de wortelextracten en exudaten van diverse plantensoorten. Recent zijn strigolactonen - of hun afgeleiden - geïdentificeerd als plantenhormoon betrokken bij de regulatie van de plantarchitectuur. Het lijkt erop dat strigolactonen vooral in de wortels worden geproduceerd, maar de exacte plaats van strigolacton biosynthese is onbekend. Niettemin is het waarschijnlijk dat strigolactonen vanuit de wortels naar de scheut worden getransporteerd waar zij hun remmende werking op okselknopuitgroei, in samenwerking met auxine en cytokinines, uitoefenen. Tot nu toe zijn strigolactonen in de scheut van planten niet of nauwelijks aangetoond. Ook de biosynthese van strigolactonen is nog niet volledig opgehelderd. Een onbekend carotenoid substraat wordt opeenvolgend geknipt door CAROTENOID CLEAVAGE DIOXYGENASE 7 (CCD7) en CAROTENOID CLEAVAGE DIOXYGENASE 8 (CCD8). Hiernaast, zijn twee enzymen, MORE AXILLARY GROWTH 1 (MAX1) and DWARF 27 (D27), betrokken bij de biosyntheses van strigolactonen, maar hun exacte rol is onbekend. In hoofdstuk 1, worden de wortelparasitaire planten van de Orobanchaceae familie en de problemen welke zij veroorzaken in de landbouw geïntroduceerd. Verder wordt de rol van strigolactonen in de levenscyclus van deze parasieten en in AM symbiose besproken. Daarnaast, wordt de recent ontdekte hormonale functie van strigolactonen in de plantarchitectuur, hun biosynthese, hormonale signalering en de twee gangbare theorieën over hoe strigolactonen een rol spelen bij apicale dominantie beschreven. Van 5-deoxystrigol wordt gedacht dat dit het eerste echte strigolacton afkomstig van de strigolacton biosynthese route is. Aangenomen wordt dat 5-deoxystrigol via een aantal - al dan niet enzymatische - stappen de precursor is voor alle andere bekende strigolactonen. In hoofdstuk 2, wordt de biosynthese van solanacol in de wortel van tomaat (Solanum lycopersicum) beschreven. Solanacol heeft een aromatische A-ring en daardoor is de biosyntheses van dit strigolactone via 5-deoxystrigol niet voor de hand liggend. Op basis van de aanwezigheid van andere strigolactonen in tomaat (orobanchol, orobanchyl acetate, twee 7-hydroxy-orobanchol isomeren, 7-oxo-orobanchol en vier didehydro-orobanchol isomeren) speculeren wij hoe solanacol door een series van enzymatische hydroxylatie-dehydroxylatie reacties, de migratie van een methyl groep en de introductie van dubbele bindingen van 5-deoxystrigol afgeleid kan zijn. In hoofdstuk 3bespreken we de klonering van een tomaat CAROTENOID CLEAVAGE DIOXYGENASE 8(SlCCD8) en demonsteren dat een reductie in CCD8 expressie leid tot een reductie van strigolactonen in de wortel extracten, exudaten en het xyleem sap. Alle lijnen tonen excessieve zijscheutgroei, zijn korter en hebben meer internodes. Ook demonstreren wij dat de sterkte van al deze fenotypes correleert met de concentratie orobanchol in het xyleem sap. Verder tonen we aan dat een milde reductie in strigolacton biosynthese en de gerelateerde secretie naar de rhizosphere voldoende is om parasitisme door Phelipanche ramosa met 90% te verminderen. Dit zonder de apicale dominantie of AM symbiose te sterk nadelig te beïnvloeden. Verder melden we enkele additionele fenotypes in de reproductieve ontwikkeling van tomaat (zoals kleinere bloemen, vruchten en zaden) - normaliter geassocieerd met gereduceerde auxine niveaus. We demonstreren verlaagde niveaus van vrij auxine in deze organen, wat er op wijst dat deze fenotypes mogelijk een consequentie zijn van het ontbreken van het regulerende effect van strigolactonen op het auxine niveau. Het goed beschreven plant hormoon abscissine zuur (ABA) is net als de strigolactonen afkomstig van de carotenoïden. Eerdere experimenten binnen onze groep wezen uit dat een mutatie in 9-cis-epoxycarotenoid dioxygenase (NCED) – een sleutel enzym betrokken bij de ABA biosyntheses – leidt tot een reductie in de strigolacton productie. Dit gaf aanleiding tot onderzoek naar de relatie tussen ABA en strigolactonen. De resultaten van dit onderzoek worden omschreven in hoofdstuk 4. Wij demonstreren dat applicatie van de carotenoid cleavage dioxygenase (CCD) inhibitor D2 het strigolacton niveau verlaagt, maar geen effect heeft op de ABA concentraties. Echter, in planten behandeld met abamineSG – een remmer van de 9-cis-epoxycarotenoid dioxygenases (NCEDs) - en in de ABA deficiënte mutanten notabilis, sitiens en flacca (mutanten in twee verschillende enzymatische stappen van de ABA biosynthese), zijn de niveaus van zowel ABA als strigolactonen gereduceerd. Onze resultaten duiden op een correlatie tussen ABA niveaus en strigolacton biosynthese en wijzen op een mogelijke rol van ABA in de regulatie van strigolacton biosynthese. In hoofdstuk 5wordt de rol van twee GRAS-type transcriptie factoren (NSP1 en NSP2) in de regulatie van strigolacton biosynthese besproken. In peulvruchten hebben deze transcriptie factoren een essentiële rol in de door Rhizobium Nod factor geïnduceerde nodulatie. In dit hoofdstuk tonen we aan dat NSP1 en NSP2 ook vereist zijn voor de strigolactone biosynthese in Medicago truncatula (Medicago) enrijst. Hiervoor is Medicago opgezet als een modelplant voor stigolacton analyse. Ook tonen wij aan dat het strigolactonenmengsel van M. truncatulabestaat uit dedihydro-orobanchol (major) en orobanchol (minor). In dit werk identificeren we voor het eerst transcriptie factoren welke de biosynthese van strigolactonen reguleren. Wij tonen aan dat NSP1 de strigolacton biosynthese reguleert door middel van de regulatie van de expressie van D27. Een in vitro binding studie toont dat MtD27 eenprimair doel van MtNSP1 kan zijn. Verder tonen wij aan dat MtNSP2 essentieel is voor de conversie van orobanchol naar dedihydro-orobanchol. Het feit dat NSP1 en NSP2 vereist zijn voor strigolacton biosynthese in zowel rijst als Medicago - soorten die twee verschillende fylogenetische geslachten vertegenwoordigen welke ongeveer 150 miljoen jaar geleden ontstaan zijn – brengt ons tot de aanname dat de regulatie van de strigolacton biosynthese door NSP1 en NSP2 als een voorouderlijke functie in hogere planten is geconserveerd. In hoofdstuk 6bestuderen we strigolacton biosynthese in Arabidopsis thaliana (Arabidopsis). Strikte controle van de groeicondities en optimalisatie van de analytische protocollen voor strigolacton analyse maakte de detectie van orobanchol, orobanchyl acetaat en 5-deoxystrigol in Arabidopsis mogelijk. In dit hoofdstuk tonen wij aan dat de relatie tussen fosfaat tekort en de inductie van strigolacton biosynthes ook in deze soort aanwezig is. Dat is opmerkelijk omdat Arabidopsis geen AM symbiose aangaat in tegenstelling tot de meeste andere landplanten. Het ontbreken van AM symbiose in Arabidopsis is daarom mogelijk een verloren eigenschap. Wij speculeren dat de exudatie van strigolactonen naar de rhizosfeer – wat Arabidopsis zij het in lage hoeveelheden nog steeds doet - een overblijfsel is van deze voorouderlijke eigenschap welke door Arabidopsis verloren is. Echter, onze resultaten demonstreren dat de verhoging van de strigolacton biosynthese in Arabidopsis onder fosfaat tekort dient om de groei van de zijscheuten te verminderen onder deze suboptimale condities. Wij speculeren dat dit mogelijk een evolutionair voordeel schept welke de nieuwe stuwende kracht is voor het behoud van fosfaat-tekort geïnduceerde verhoging van de strigolacton biosynthese in Arabidopsis, aangezien AM symbiose niet meer aanwezig is.We tonen aan dat orobanchol getransporteerd wordt in het xyleem sap en dat de concentratie hiervan omhoog gaat onder fosfaat tekort. Verder leveren we analytisch bewijs dat MAX1 is vereist voor de biosynthese van orobanchol. Een nieuwe functie van strigolactonen in de regulatie van wortelarchitectuur in tomaat en Arabidopsis is recent ontdekt. In hoofdstuk 7tonen we dat een reductie in strigolacton biosynthese en/of perceptie - als vertoond in de max1,2,4 mutanten - leidt tot een reductie in de lengte van het primaire wortelmeristeem. We demonstreren dat toediening van het synthetische strigolacton GR24 dit fenotype in alle max mutanten - uitgezonderd de strigolacton ongevoelige mutant max2 – kan herstellen.De toediening van GR24 onder toereikende fosfaat condities reduceert het totale aantal laterale wortels per plant - hun ontwikkeling wordt gestopt in fase vijf van de laterale wortel primordium initiatie. Ook tonen wij aan dat hogere concentraties GR24 de initiatie van deze laterale wortel primordia compleet blokkeren wat leidt tot het instorten van het primaire wortelmeristeem.Echter, als GR24 samen met exogeen auxine (NAA) wordt toegediend heeft het een stimulerend effect op de laterale wortelontwikkeling en -uitgroei. Daaraan analoog, onder fosfaatbeperkende condities stimuleert de verhoging van endogene strigolacton niveaus (hoofdstuk 6) in wild type planten een snellere uitgroei van laterale wortel primordia vergeleken met de strigolacton-deficiënte mutanten. Verder demonstreren wij dat – onder toereikende fosfaat condities - GR24 toediening aan het wortelsysteem van Arabidopsis leidt tot een lagere auxineconcentratie in de bladeren. Gecombineerd wijzen deze resultaten erop dat strigolactonen lokale auxine gradiënten modificeren en zo invloed uitoefenen op de wortel architectuur. Daardoor zal het netto resultaat van de strigolactone werking op de ontwikkeling van het wortelstelsel afhangen van de auxine status van een plant. Wij denken dat een uiterst fijn gereguleerde balans tussen auxine en strigolactonen de basis vormt voor het mechanisme dat de wortel-scheut ratio van een plant reguleert, bijvoorbeeld onder fosfaat limiterende condities. Hoofdstuk 8is een samenvatting en discussie van de meest belangrijke resultaten verkregen gedurende het werk gepresenteerd in dit proefschrift. Er wordt getracht de nieuw verkregen kennis te integreren met de huidige kennis van strigolactonen. Zowel op het gebied van hun hormonale eigenschappen als hun rol als rhizosfeer signaal. In dit hoofdstuk word verder vooruit gekeken naar de perspectieven van het onderzoek aan strigolactonen. Speciale aandacht wordt besteed aan de relatie met het probleem van de parasitaire planten. </p
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Elucidation of strigolactone biosynthesis in the host plant rice and the signal perception in the parasitic plant Striga hermonthica
No full textStrigolactones (SLs) are a newly identified class of plant hormones regulating plant architecture, including shoot and root branching. Plants also secrete blends of SLs into the rhizosphere, where they stimulate colonisation of the host roots by arbuscular mycorrhizal (AM) fungi, beneficial organisms for the host. But SLs also induce the seed germination of root parasitic plants, such as Striga, which can have a big negative impact on crop yield. A better insight in how the different SLs are synthesized by the host and how the parasitic plant Striga perceives them could help to develop crops with proper AM colonisation and Striga resistance at the same time. In this thesis, two cytochrome P450 enzymes responsible for the last step in SL formation and SL structural diversification in rice were identified. In addition, the F-Box protein MAX2 of Striga (ShMAX2), a SL signalling component, was characterised, representing the first example from a root parasitic plant species, which is paving the way for furthering our understanding of how SLs are perceived by these parasites. The knowledge gained in this thesis brings us a significant step closer to the possibility to improve crop breeding strategies for parasitic weed resistance.</p
Biosynthesis, regulation and biological role of strigolactones in rice
No full textIn her thesis Catarina Cardoso studied strigolactone biosynthesis in rice. Strigolactones are multifunctional compounds produced by plants. They are plant hormones that regulate plant architecture, but in addition plants release strigolactones into the soil to communicate and initiate beneficial symbiosis with arbuscular mycorrhizal (AM) fungi. Parasitic plants of the genera Striga, Orobanche and Phelipanche take advantage of this communication to also recognize their hosts and infest them. These parasites infect crops and cause significant economic losses in Mediterranean regions and especially in Sub Saharan Africa where they put food security at risk. Catarina found there is large variation in strigolactone biosynthesis between the two major rice groups (indica and japonica) and located the genes responsible for this. She also showed that the different strigolactones produced by rice have a differential impact on AM fungi and seeds of parasitic plants. These findings suggest that it is possible to select crop varieties that can interact with AM fungi, without inducing parasitism. The knowledge generated in this study can contribute to the urgent need to control the worldwide parasitic weed problems. At the same time strigolactones also control plant development and the results of this study may resuylt in tools to develop better yielding and sustainable crops.</p
Biosynthesis of germination stimulants of parasitic weeds Striga and Orobanche
Full text linkMy research focused on the biosynthetic origin of germination stimulants of the root parasitic plants, Striga spp. and Orobanche spp., which have an increasing impact on cereal and other economically important crops in many regions of the world. The traditional control methods are not sufficient and therefore efficient and feasible control methods are urgently required. Control methods that target the early steps in the parasitic weed life cycle are of particular interest as they could prevent parasitic weed infection in an early stage. The first chemical signal exchange between the host and the parasitic plant, the secretion by the host of germination stimulants, is obviously interesting as a target for control methods. Recent research on the germination stimulants in our group has shown that the most important class of germination stimulants, the strigolactones, are derived from the carotenoid pathway. To explain this carotenoid origin, carotenoid cleavage enzymes were predicted to be involved in the strigolactone formation in maize. Therefore my research efforts were directed at cloning carotenoid cleavage enzymes from maize. The first and only characterised carotenoid cleavage enzyme in maize so far is VP14 that belongs to a small family of related genes. In this thesis I have shown that knocking out the NCED (VP14) gene family in maize using RNAi technology suppressed germination stimulant formation leading to reduce Striga seed germination (Chapter 3). This result confirms the previous results with maize vp14 that induced 40% lower Striga seed germination and the tomato mutant, notabilis, with a mutation in tomato NCED1 (VP14 ortholog) that also induced about 40% lower germination of O. ramosa. These results indicate that the NCEDs are directly involved in the strigolactone formation in maize or indirectly through the reduced formation of abscisic acid (ABA) through an as yet unknown mechanism. In my thesis work I cloned a second, as yet unknown maize carotenoid cleavage dioxygenase, ZmCCD1 (Chapter 2). I showed that ZmCCD1 is not involved in strigolactone formation but catalyses the formation of “yellow pigment” in mycorrhizal maize roots. Interestingly, mycorrhizal maize roots show a clear reduction in germination stimulant formation (Chapter 2). The possible explanation is that ZmCCD1 competes with other carotenoid cleavage enzymes for a common carotenoid precursor. A higher activity of ZmCCD1 therefore leads to a decreased germination stimulant production in AM root. These results suggest that the reduction of Striga infestation of maize by AM-fungi may be caused by a reduction in the formation of strigolactones. Using model plants may have a lot of advantages to dissect the germination stimulant biosynthetic pathway, because they take advantage of the extensive genetic, biochemical and physiological information and can be genetically engineered more easily and rapidly. However, the first important question of course is if these model plants produce germination stimulants, what kind of germination stimulants and from which pathway. Two model plants were used to study the germination stimulant biosynthesis, the dicotyledonous model plant Arabidopsis and the monocotyledonous model plant rice. Arabidopsis is a host of several Orobanche spp. and rice can be infected by several Striga spp. Interestingly, Arabidopsis is not a host of mycorrhizal fungi, whereas rice is. Arabidopsis is a host of Orobanche aegyptica, Orobanche ramosa and Orobanche minor. The germination stimulants of Arabidopsis have so far not been identified but could be supposed not to be strigolactones because Arabidopsis is not a host of arbuscular mycorrhizal fungi. Nevertheless, my research has shown that orobanchol (a strigolactone) is present in the Arabidopsis root exudates and in a hairy root culture under phosphate starvation (Chapter 4) and this suggests that Arabidopsis is therefore a suitable model for studying the early stage of host-parasite interaction. However, the concentration of orobanchol found is much lower than in other plant species (compare Chapter 5) and, in addition to orobanchol, I also found several root exudates fractions containing unknown, most likely non-strigolactone, compounds in the Arabidopsis exudates, which have germination stimulating activity on O. ramosa (Chapter 4). This could make it difficult to use fast screens to look for mutants. The biosynthetic origin of these other germination stimulants remains unknown with the exception perhaps of one of these that I showed is probably derived from the early plastidic pathway (Chapter 4). The question remains why Arabidopsis produces strigolactones considering it is not a host of arbuscular mycorrhizal fungi. Is it because strigolactones are involved in other symbiotic associations? Also it is possible that the strigolactones have another physiological significance that we are currently not aware of. For the host plants of Striga, strigolactones have been found in the root exudates of cereals in tropical areas such as maize, sorghum, and millet. The cereal model rice could be good candidate for our research. Indeed we were able to detect orobanchol and another strigolactone, possibly an epimer of 5-deoxystrigol, in the root exudates of rice grown under phosphate starvation. The production of these compounds was completely inhibited by fluridone. These results are confirmed by the germination bioassay, in which root exudates from fluridone treated rice induced much lower Striga seed germination than that of non-treated plants grown under phosphate starvation. In addition, in a mycorrhizal branching assay fluridone treated root exudates induced much lower branching of mycorrhizal hyphae than the exudates of non-treated, phosphate-starved, plants. These results indicate that strigolactones are probably the major germination stimulants in rice root exudates. Thus, compared with Arabidopsis, rice has a great advantage to study the biosynthetic pathway of strigolactones. My work has contributed new insight in the chemical regulation of the host-parasitic plant lifecycle and as such can contribute to the development of new control methods. Once the strigolactone biosynthetic pathway has been further elucidated, it would also become feasible to make low-stimulant producing plants through the inactivation of one or more steps in the pathway. For the time being, as suitable targets enzymes of the primary carotenoid pathway could be used but better targets would be the dedicated pathway enzymes, i.e. the postulated enzymes involved in cleavage and further conversion of the cleavage product to the strigolactones. For example, we have designed RNAi constructs to block the carotenoid cleavage dioxygenases (VP14 cluster enzymes) using both root specific and universal promoters and these constructs were transformed to maize and the transgenic plants induced lower germination of Striga seeds than wildtype maize plants (Chapter 3). As an alternative to knocking out enzymes from the germination stimulant pathway, overexpression of key enzymes of competing pathways to channel away substrate can also be considered as a strategy to reduce germination stimulant formation. Possible candidates are the cleavage enzymes that are responsible for apocarotenoid formation upon mycorrhizal colonization, ZmCCD1 (Chapter 2). Also the suppressive effect of AM fungi on Striga infection of sorghum and maize may be developed into a Striga control strategy. The mechanism of this reduction was so far unknown, and therefore the possibilities to optimize and exploit this phenomenon for practical use were limited. However, I have shown that this reduction is - in any case partly - due to a decrease in germination stimulant formation after mycorrhizal colonisation (Chapter 2). A possible explanation is that due to the formation of mycorrhiza specific apocarotenoids or abscisic acid the formation of the Striga germination stimulant is reduced. Apocarotenoid formation may be competing for substrate with germination stimulant formation. Alternatively, mycorrhizal colonisation may be down-regulating the strigolactone production pathway directly. Therefore, research could now be aimed to optimize the use of AM fungi for controlling parasitic plants including Orobanche through reduced germination, for example through the selection of suitable AM fungus – host (variety) combinations. In rice, we found that irrigation but also spraying with a low concentration of fluridone significantly reduced the number of germinated/attached Striga seeds even with very low concentrations (Chapter 5). In the spraying experiment, the concentration of fluridone used was so low that bleaching of the leaves did not occur. This effect of fluridone suggests that the rice germination stimulants are strigolactones and indeed we have confirmed that they are (Chapter 5). These experiments show that herbicides that inhibit carotenoid biosynthesis can be used to significantly reduce the germination of parasitic seeds and that spraying or irrigating plants with such herbicides at one or more time intervals may be an effective and cheap method to reduce parasitic-weed induced yield losses of crop plants. Our research has shown that under increased phosphate levels, plants (maize, rice, Arabidopsis) produce less germination stimulants. Therefore increasing the phosphate level in deficient soils would probably inhibit Striga infection. However, strigolactones may not be the only germination stimulants of Striga and they are also the signal for mycorrhizal symbionts. Inhibition of production and exudation of strigolactones may also negatively effect the colonization by mycorrhizal fungi. Thus the effective control method should depend on the specific situation in the soil. <br/
Study of drimane sesquiterpenoids from the Persicaria genus and zigiberene from Callitropsis noorkatensis and their effect on the feeding behaviour of Myzus persicae and Bemisia tabaci
Full text linkSummary Whitefly is an insect pest that has systematically spread into colder latitudes for the past two decades and it poses a serious threat to crops, mainly due to the viruses for which it acts as a vector. As the application of synthetic pesticides is often less effective due to development of resistance or restricted by crop- and country-specific regulations, Integrated Pest Management (IPM) strategies to combat insect pests become attractive. In this thesis, I discuss the potential of the use of secondary metabolites, particularly sesquiterpenoids, of plant origin, both as sprayed repellents or antifeedants and as part of host plant resistance against whitefly. In Chapter 1, I give a detailed description of the silverleaf whitefly, Bemisia tabaci, its ecology and its effect on crop yield, taking both direct and indirect damage (caused by the viruses the insect transmits) into account. This chapter also provides a detailed account of pest management strategies, both traditional and emergent, with their advantages and disadvantages. The chapter introduces the reader to plant secondary metabolites, and specifically terpenes, discussing their role in plant ecology and their potential as pest management tools with a low environmental impact. Finally, a short overview of the following chapters is given. Chapter 2 focusses on the antifeedant activity of the drimane sesquiterpene polygodial. This dialdehyde had been described before as antifeedant to a number of insects, such as the Colorado potato beetle, Leptinotarsa decemlineata, the African cotton leafworm, Spodoptera littoralis, and the green peach aphid, Myzus persicae. In this chapter, the effect of polygodial on the feeding preference of whitefly is reported for the first time. The effect of polygodial was benchmarked against that of the more widely used natural pyrethrins, and both were also tested against M. persicae. From the results, we conclude that pyrethrins were effective against whiteflies at 18-fold lower concentrations than polygodial (ED50 of 1.4 and 25 μg gFW-1 respectively), while in the case of aphids this difference in efficacy was only two-fold (ED50 of 28 and 54 μg gFW-1, respectively). To adopt polygodial as a more persistent and easy to implement pest management strategy, we set out to isolate the genes responsible for its biosynthesis which could then be used to transform crops. As a basis for the selection of the right species and plant tissue to achieve this objective, Chapter 3 describes the chemical composition of one of the sources of polygodial – Persicaria hydropiper (water pepper), as well as of two other congeners (Persicaria minor and Persicaria maculosa). For all three species, GC-MS analysis of extracts of leaves and flowers was performed, which gave insight into the interspecific differences and similarities as well as into the differences between the two tissues. P. hydropiper was the species with the biggest variety and the greatest abundance of secondary metabolites, while P. maculosa had the fewest. The flowers of all species were richer in secondary metabolites compared with the leaves of the same species. Furthermore, the accumulation pattern of the identified compounds throughout the development of flowers and leaves is described. Finally, in this chapter, the possible ecological role of polygodial is also briefly addressed. Chapter 4 focuses on the discovery and characterization of genes involved in the biosynthesis of drimane sesquiterpenoids. Based on the findings of Chapter 3, we used 454 sequencing of a cDNA library constructed from young flowers of P. hydropiper and P. maculosa, for comparison, to identify a drimenol synthase (PhDS) and a drimenol oxidase (PhDOX1) which can convert farnesyl diphosphate (FPP) into drimenol and an array of other sesquiterpenoids when working in concert. Of the compounds produced in the heterologous systems used (Saccharomyces cerevisiae and Nicotiana benthamiana), two were identified as drimendiol and cinnamolide. The latter was purified and tested against whiteflies and aphids as described for polygodial in Chapter 2. Cinnamolide also displayed antifeedant activity against both insects, although with at slightly lower efficacy than polygodial. In the heterologous hosts used, no polygodial was detected amongst the products of the enzymatic activity of the two genes studied in this chapter. The potential reasons for this are explored in the discussion section of this chapter. Chapter 5 focuses on the volatile zingiberene, member of the bisabolane family of sesquiterpenes. This compound and its enantiomer, 7-epi-zingiberene have both been previously identified as repellent to whiteflies. In this chapter we used Alaska yellow cedar (Callitropsis nootkatensis) as a possible source of a zingiberene synthase, which we wanted to use a primary line of defence against whitefly next to polygodial, which would be a secondary line of defence. Alaska yellow cedar produces curcumene in some of its tissues, and this metabolite is known to be a dehydrogenation product of zingiberene. An EST from a C. nootkatensis cDNA library with homology to sesquiterpene synthases was cloned and expressed in E. coli. The resulting protein converted FPP to zingiberene as the sole product. This enzyme was therefore named zingiberene synthase (CnZIS). Although no strict correlation was found between the expression levels of CnZIS in Alaska yellow cedar tissues and the accumulation of curcumene in the same tissues, those with high CnZIS expression such as leaves also produced high amounts of curcumene, while heartwood neither expressed the gene nor had detectable levels of curcumene. Subsequently, we tested the effect of transiently expressed CnZIS in tobacco (Nicotiana tabacum) on whiteflies. Except for one time point, the effect of zingiberene on whitefly feeding was negligible, likely due to the fact that only trace amounts of zingiberene were emitted. When genes upstream of CnZIS in the biosynthetic pathway – 3-hydroxy-3-methyl-glutaryl-CoA reductase (HMGR) and farnesyl diphosphate synthase (FPS) – were co-expressed in tobacco, the antifeedant effect on whiteflies became very strong; however, there was still no detectable level of zingiberene emitted. Instead, the emission of the tobacco sesquiterpene, 5-epi-aristolochene, was almost 100-fold higher than in the control, not expressing HMGR and FPS. We discuss potential explanations of this phenomenon, as well as the uncommon outcome of having an endogenous sesquiterpene boosted by the co-expression and redirection to the mitochondria of the three genes of the zingiberene biosynthetic pathway. Chapter 6 brings all the findings together, discussing their place within a wider scientific perspective and their potential in the frame of IPM. Advantages as well as drawbacks of the use of GM crops are addressed. Finally, the emerging new agricultural paradigm, of a sustainable way of growing crops with less environmental impact compared with the current intensive industrial approach, is brought forth as a broad spectrum solution to most agricultural problems which arose in parallel to the intensification of agriculture that stemmed from the Green revolution.</p
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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