Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    Chromatin-dependent regulation of secondary metabolite biosynthesis in fungi : is the picture complete?
    Collemare, Jérôme ; Seidl, Michael F. - \ 2019
    FEMS Microbiology Reviews 43 (2019)6. - ISSN 0168-6445 - p. 591 - 607.
    chromatin - fungi - secondary metabolites - transcriptional regulation

    Fungal secondary metabolites are small molecules that exhibit diverse biological activities exploited in medicine, industry and agriculture. Their biosynthesis is governed by co-expressed genes that often co-localize in gene clusters. Most of these secondary metabolite gene clusters are inactive under laboratory conditions, which is due to a tight transcriptional regulation. Modifications of chromatin, the complex of DNA and histone proteins influencing DNA accessibility, play an important role in this regulation. However, tinkering with well-characterised chemical and genetic modifications that affect chromatin alters the expression of only few biosynthetic gene clusters, and thus the regulation of the vast majority of biosynthetic pathways remains enigmatic. In the past, attempts to activate silent gene clusters in fungi mainly focused on histone acetylation and methylation, while in other eukaryotes many other post-translational modifications are involved in transcription regulation. Thus, how chromatin regulates the expression of gene clusters remains a largely unexplored research field. In this review, we argue that focusing on only few well-characterised chromatin modifications is significantly hampering our understanding of the chromatin-based regulation of biosynthetic gene clusters. Research on underexplored chromatin modifications and on the interplay between different modifications is timely to fully explore the largely untapped reservoir of fungal secondary metabolites.

    Astin C Production by the Endophytic Fungus Cyanodermella asteris in Planktonic and Immobilized Culture Conditions
    Vassaux, Antoine ; Tarayre, Cédric ; Arguëlles-Arias, Anthony ; Compère, Philippe ; Delvigne, Frank ; Fickers, Patrick ; Jahn, Linda ; Lang, Alexander ; Leclère, Valérie ; Ludwig-Müller, Jutta ; Ongena, Marc ; Schafhauser, Thomas ; Telek, Samuel ; Théatre, Ariane ; Berkel, Willem J.H. van; Vandenbol, Micheline ; Pée, Karl Heinz van; Willems, Luc ; Wohlleben, Wolfgang ; Jacques, Philippe - \ 2019
    Biotechnology Journal 14 (2019)8. - ISSN 1860-6768
    astin C - biofilms - Cyanodermella asteris - immobilized-cell cultures - secondary metabolites

    The fungal endophyte Cyanodermella asteris (C. asteris) has been recently isolated from the medicinal plant Aster tataricus (A. tataricus). This fungus produces astin C, a cyclic pentapeptide with anticancer and anti-inflammatory properties. The production of this secondary metabolite is compared in immobilized and planktonic conditions. For immobilized cultures, a stainless steel packing immersed in the culture broth is used as a support. In these conditions, the fungus exclusively grows on the packing, which provides a considerable advantage for astin C recovery and purification. C. asteris metabolism is different according to the culture conditions in terms of substrate consumption rate, cell growth, and astin C production. Immobilized-cell cultures yield a 30% increase of astin C production, associated with a 39% increase in biomass. The inoculum type as spores rather than hyphae, and a pre-inoculation washing procedure with sodium hydroxide, turns out to be beneficial both for astin C production and fungus development onto the support. Finally, the influence of culture parameters such as pH and medium composition on astin C production is evaluated. With optimized culture conditions, astin C yield is further improved reaching a five times higher final specific yield compared to the value reported with astin C extraction from A. tataricus (0.89 mg g−1 and 0.16 mg g−1 respectively).

    The plastidial metabolite 2-C-methyl-D-erythritol-2,4-cyclodiphosphate modulates defence responses against aphids
    Onkokesung, Nawaporn ; Reichelt, Michael ; Wright, Louwrance P. ; Phillips, Michael A. ; Gershenzon, Jonathan ; Dicke, Marcel - \ 2019
    Plant, Cell & Environment 42 (2019)7. - ISSN 0140-7791 - p. 2309 - 2323.
    aphid resistance - Arabidopsis - indole glucosinolates - phloem-sucking herbivores - phytohormone signalling - retrograde signalling - secondary metabolites

    Feeding by insect herbivores such as caterpillars and aphids induces plant resistance mechanisms that are mediated by the phytohormones jasmonic acid (JA) and salicylic acid (SA). These phytohormonal pathways often crosstalk. Besides phytohormones, methyl-D-erythriol-2,4-cyclodiphosphate (MEcPP), the penultimate metabolite in the methyl-D-erythritol-4-phosphate pathway, has been speculated to regulate transcription of nuclear genes in response to biotic stressors such as aphids. Here, we show that MEcPP uniquely enhances the SA pathway without attenuating the JA pathway. Arabidopsis mutant plants that accumulate high levels of MEcPP (hds3) are highly resistant to the cabbage aphid (Brevicoryne brassicae), whereas resistance to the large cabbage white caterpillar (Pieris brassicae) remains unaltered. Thus, MEcPP is a distinct signalling molecule that acts beyond phytohormonal crosstalk to induce resistance against the cabbage aphid in Arabidopsis. We dissect the molecular mechanisms of MEcPP mediating plant resistance against the aphid B. brassicae. This shows that MEcPP induces the expression of genes encoding enzymes involved in the biosynthesis of several primary and secondary metabolic pathways contributing to enhanced resistance against this aphid species. A unique ability to regulate multifaceted molecular mechanisms makes MEcPP an attractive target for metabolic engineering in Brassica crop plants to increase resistance to cabbage aphids.

    Comparative Genomics Highlights Symbiotic Capacities and High Metabolic Flexibility of the Marine Genus Pseudovibrio
    Versluis, Dennis ; Nijsse, Bart ; Naim, Mohd Azrul ; Koehorst, Jasper J. ; Wiese, Jutta ; Imhoff, Johannes F. ; Schaap, Peter J. ; Passel, Mark W.J. van; Smidt, Hauke ; Sipkema, Detmer - \ 2018
    Genome Biology and Evolution 10 (2018)1. - ISSN 1759-6653 - p. 125 - 142.
    domainome - microbiota - phylogeny - secondary metabolites - sponge - symbiosis

    Pseudovibrio is a marine bacterial genus members of which are predominantly isolated from sessile marine animals, and particularly sponges. It has been hypothesized that Pseudovibrio spp. form mutualistic relationships with their hosts. Here, we studied Pseudovibrio phylogeny and genetic adaptations that may play a role in host colonization by comparative genomics of 31 Pseudovibrio strains, including 25 sponge isolates. All genomes were highly similar in terms of encoded core metabolic pathways, albeit with substantial differences in overall gene content. Based on gene composition, Pseudovibrio spp. clustered by geographic region, indicating geographic speciation. Furthermore, the fact that isolates from the Mediterranean Sea clustered by sponge species suggested host-specific adaptation or colonization. Genome analyses suggest that Pseudovibrio hongkongensis UST20140214-015BT is only distantly related to other Pseudovibrio spp., thereby challenging its status as typical Pseudovibrio member. All Pseudovibrio genomes were found to encode numerous proteins with SEL1 and tetratricopeptide repeats, which have been suggested to play a role in host colonization. For evasion of the host immune system, Pseudovibrio spp. may depend on type III, IV, and VI secretion systems that can inject effector molecules into eukaryotic cells. Furthermore, Pseudovibrio genomes carry on average seven secondary metabolite biosynthesis clusters, reinforcing the role of Pseudovibrio spp. as potential producers of novel bioactive compounds. Tropodithietic acid, bacteriocin, and terpene biosynthesis clusters were highly conserved within the genus, suggesting an essential role in survival, for example through growth inhibition of bacterial competitors. Taken together, these results support the hypothesis that Pseudovibrio spp. have mutualistic relations with sponges.

    Data from: Modification of plant-induced responses by an insect ecosystem engineer influences the colonization behaviour of subsequent shelter-users
    Uesugi, Akane ; Morrell, Kimberly ; Poelman, E.H. ; Raaijmakers, Ciska E. ; Kessler, André - \ 2016
    Wageningen University & Research
    plant-herbivore interaction - induced defense - secondary metabolites - volatile signalling - herbivore community - mutualism - galling insect
    Herbivores that modify plant morphology, such as gall forming insects, can disproportionately impact arthropod community on their host plants by providing novel habitats and shelters from biotic and abiotic stresses. These ecosystem engineers could also modify plant chemical properties, but how such changes in plant quality affect the behaviour of subsequent colonizers has rarely been investigated. We explored how an initial infestation of the tall goldenrod (Solidago altissima) by an ecosystem engineer, the rosette gall-midge (Rhopalomyia solidaginis), affects colonization behaviour of a shelter-using beetle (Microrhopala vittata) through plant-induced responses in the field. Beetles preferentially colonized plants with galls and exhibited a clumped distribution on those plants, which suggested a possible advantage for aggregating on galled plants. Accordingly, we found that beetles remained longer on galled plants with previous beetle damage than those without beetle damage. No such effect of beetle damage was found on plants without a gall. Similar interactions between galler-infestation and beetle damage were found in beetle's feeding choice, leaf diterpene and serine protease inhibitor production, and volatile organic compound (VOC) emission. These plant metabolic induction and herbivore response patterns indicated that the gall-midge can alter how plants respond to the beetle damage, and that gall presence coupled with beetle damage improves leaf palatability for the beetle. Finally, we found reciprocal effects of beetles on gall-midge performance to be neutral to slightly positive, suggesting that the observed field association of the two herbivores could be formed by plant-mediated facilitation. Synthesis: Our study suggests that an ecosystem engineer could have significant impact on herbivore community not only by changing plant morphology, but also by altering host quality and modifying plant induced responses to subsequent herbivory. As such, R. solidaginis also functions as a keystone herbivore that has disproportionate effects on community dynamics and composition meditated by induced plant growth and metabolic responses.
    Mining into interspecific bacterial interactions
    Tyc, Olaf - \ 2016
    Wageningen University. Promotor(en): Wietse de Boer, co-promotor(en): Paolina Garbeva. - Wageningen : Wageningen University - ISBN 9789462578340 - 234
    soil bacteria - secondary metabolites - microbial interactions - antibiotics - nutrients - bodembacteriën - secundaire metabolieten - microbiële interacties - antibiotica - voedingsstoffen

    In terrestrial ecosystems bacteria live in close proximity with many different microbial species and form complex multi-species networks. Within those networks bacteria are constantly interacting with each other and produce a plethora of secondary metabolites like antibiotics, enzymes, volatiles and other compounds from diverse chemical classes. Several independent studies revealed that the production of secondary metabolites by soil bacteria can be influenced by the interaction with other microorganisms in their vicinity.

    In this thesis we show how interspecific interactions between soil bacteria influence the production of soluble and volatile secondary metabolites, gene expression and fitness. To elucidate the effect of interspecific interactions on antimicrobial activity in soil bacteria a high-through-put screening method was developed and applied on a collection of 146 rhizobacterial isolates obtained from similar habitats. In addition we examined if the production of volatile organic compounds is influenced by interspecific interactions. Thus, the identity and antimicrobial activity of volatiles produced by bacteria cultivated in monoculture as well in interaction were examined. Furthermore a sand microcosm approach was applied to investigate how Pseudomonas fluorescens strain Pf0-1 responded to the presence of monocultures and mixtures of a Gram-negative (Pedobacter sp. V48) and a Gram-positive (Bacillus sp. V102) bacterial strain under two nutritional conditions.

    The interaction between a gram-negative Burkholderia and a gram-positive Paenibacillus isolate was subjected to detailed metabolome, volatolome and transcriptome analysis. One distinct volatile and one non-volatile compound produced only during interspecific interaction but not in the monoculture were identified. The activity of the interacting bacteria and the compounds produced during interaction were tested against a range of human and plant pathogens.

    In summary, this thesis extends the knowledge about the effect of interspecific bacterial interactions on secondary metabolites production (soluble and volatiles), gene expression and fitness in bacteria. The exploitation of such bacterial interspecific interactions can be an important “tool” for the discovery of novel antimicrobial and agro-chemical compounds. The obtained knowledge can help in selecting the right players in synthetic communities that fulfil important ecosystem services like disease suppression in agricultural crop systems.

    Modification of plant-induced responses by an insect ecosystem engineer influences the colonization behaviour of subsequent shelter-users
    Uesugi, Akane ; Morrell, Kimberly ; Poelman, Erik H. ; Raaijmakers, Ciska E. ; Kessler, André - \ 2016
    Journal of Ecology 104 (2016)4. - ISSN 0022-0477 - p. 1096 - 1105.
    herbivore community - induced defence - mutualism - plant–herbivore interactions - secondary metabolites - volatile signalling

    Herbivores that modify plant morphology, such as gall-forming insects, can disproportionately impact arthropod community on their host plants by providing novel habitats and shelters from biotic and abiotic stresses. These ecosystem engineers could also modify plant chemical properties, but how such changes in plant quality affect the behaviour of subsequent colonizers has rarely been investigated. We explored how an initial infestation of the tall goldenrod (Solidago altissima) by an ecosystem engineer, the rosette gall-midge (Rhopalomyia solidaginis), affects colonization behaviour of a shelter-using beetle (Microrhopala vittata) through plant-induced responses in the field. Beetles preferentially colonized plants with galls and exhibited a clumped distribution on those plants, which suggested a possible advantage for aggregating on galled plants. Accordingly, we found that beetles remained longer on galled plants with previous beetle damage than those without beetle damage. No such effect of beetle damage was found on plants without a gall. Similar interactions between galler infestation and beetle damage were found in beetle's feeding choice, leaf diterpene and serine protease inhibitor production, and volatile organic compound (VOC) emission. These plant metabolic induction and herbivore response patterns indicated that the gall-midge can alter how plants respond to the beetle damage and that gall presence coupled with beetle damage improves leaf palatability for the beetle. Finally, we found reciprocal effects of beetles on gall-midge performance to be neutral to slightly positive, suggesting that the observed field association of the two herbivores could be formed by plant-mediated facilitation. Synthesis. Our study suggests that an ecosystem engineer could have significant impact on herbivore community not only by changing plant morphology, but also by altering host quality and modifying plant-induced responses to subsequent herbivory. As such, R. solidaginis also functions as a keystone herbivore that has disproportionate effects on community dynamics and composition meditated by induced plant growth and metabolic responses.

    Stuurlicht beïnvloedt aanmaak inhoudsstoffen en plantenfysiologie : mogelijkheden door bewust gebruik van het lichtspectrum
    Garcia Victoria, Nieves - \ 2015
    horticulture - greenhouse horticulture - ornamental crops - chrysanthemum - phalaenopsis - polygonum - crop growth stage - plant physiology - illumination - red light - far red light - secondary metabolites - flower induction - cuttings

    Licht is meer dan de grote motor achter de fotosynthese. Delen van het lichtspectrum, of juist meer of minder licht, beïnvloeden de ontwikkeling van planten: de kieming, bloei, celdeling en -strekking. Stuurlicht heeft ook invloed op de vorming van voor de mens nuttige inhoudsstoffen. Een nieuw terrein, waarbij nog veel te ontdekken valt.

    The secondary metabolome of the fungal tomato pathogen Cladosporium fulvum
    Griffiths, S.A. - \ 2015
    Wageningen University. Promotor(en): Pierre de Wit; Pedro Crous, co-promotor(en): Jerome Collemare. - Wageningen : Wageningen University - ISBN 9789462575813 - 167
    passalora fulva - secundaire metabolieten - metabolomen - genen - genomica - biologische activiteit - biosynthese - natuurlijke producten - passalora fulva - secondary metabolites - metabolomes - genes - genomics - biological activity - biosynthesis - natural products

    Secondary metabolites (SMs) are biologically active organic compounds that are biosynthesised
    by many plants and microbes. Many SMs that affect the growth, behaviour or survival of other
    organsisms have been re-purposed for use as medicinal drugs, agricultural biocides and animal
    growth promoters. The majority of our anti-infective and anti-cancer drugs are currently derived
    from Streptomyces, bacteria that are free living, filamentous, and ubiquitous in terrestrial habitats.
    Genome sequencing and mature in silico approaches to genome mining has revealed that filamentous
    fungi contain very large numbers of genes related to SM production. Yet these genes are typically
    silent under laboratory conditions. There are now many tools and strategies available to activate
    or clone silent SM genes. This thesis details our efforts to apply various methods to define and
    then manipulate SM genes in Cladosporium fulvum, a biotrophic pathogenic fungus of tomato
    containing many silent SM genes and gene clusters.

    In chapter 1, the relevance of SMs to medicine and agriculture is considered. Filamentous fungi
    are presented as untapped sources of potential useful SMs, as their genomes are often rich in SM
    biosynthetic genes that are silent under most conditions. Methods to activate these silent genes and
    increase the chemical diversity of fungi are detailed. These include the deletion or over-expression
    of genes encoding regulatory proteins, the use of chemical inhibitors, and the manipulation
    of growth conditions. Heterologous expression of silent SM genes in a production host is also
    discussed as a tool for bypassing host regulatory mechanisms altogether. C. fulvum is introduced
    as an organism that has been intensively studied as a biotrophic plant pathogen. Genomic analysis
    showed that this fungus has twenty-three core SM genes, a large catalogue composed of 10
    polyketide synthases (PKSs), 10 non-ribosomal peptide synthases (NPS), one PKS-NPS hybrid
    and one dimethylallyl tryptophan synthase (DMATS). Transcriptional profiling showed that the
    majority was silent during growth on tomato and in vitro. Cladofulvin is introduced as the sole
    detectable SM produced by C. fulvum during growth in vitro. This presented an opportunity to
    apply the aforementioned strategies to induce these silent genes and obtain new compounds. The
    importance of cladofulvin and structurally related anthraquinones are briefly discussed as potential
    medicines. The value of the cladofulvin biosynthetic gene cluster is also emphasised as a potential
    source of novel biosynthetic enzymes.

    In chapter 2 the SM gene catalogue identified during the analysis of the C. fulvum genome was
    analysed in further detail. Each locus containing a core SM gene was inspected for other biosynthetic
    genes linked to SM production, such as those encoding decorating enzymes and regulators. Products
    of these SM genes or gene clusters were speculated, based on their similarity to those characterized
    in other fungi. Six gene clusters were located in the genome of C. fulvum that are conserved in other
    fungal species. Remarkably, two predicted functional gene clusters were linked to the production
    of elsinochrome (PKS1) and cercosporin (PKS7), toxic perylenequinones that generate reactive
    oxygen species (ROS). We profiled the expression of core SM genes during the growth of C. fulvum
    under several in vitro conditions. Expression of each core SM gene was measured by RT-qrtPCR
    and the resulting SM profile was determined by LC-MS and NMR analyses. Confirming previous
    findings, the majority of SM genes remained silent and only cladofulvin was detected. During
    growth on tomato only two core genes, PKS6 and NPS9, were clearly expressed, but both were
    significantly down-regulated during colonization of the mesophyll tissue of tomato leaves. We
    confirmed that cladofulvin does not cause necrosis on solanaceous plants when infiltrated into
    their leaves. In contrast to other biotrophic fungi that have a reduced SM production capacity, our
    studies of C. fulvum suggest that down-regulation of SM biosynthetic pathways might represent
    another mechanism associated with a biotrophic lifestyle.

    In chapter 3 our efforts to activate cryptic pathways in C. fulvum are described, with the aim
    of discovering new compounds. Many Ascomycete-specific global regulators of SM production
    and morphological development in other fungi were identified in C. fulvum. We investigated
    three intensively studied regulators, VeA, LaeA and HdaA. Deleting or over-expressing the genes
    encoding these regulators in C. fulvum yielded no new detectable SMs. Cladofulvin biosynthesis
    was strongly affected by each regulator; HdaA is an activator while VeA and LaeA are repressors of
    cladofulvin production. Attempts were made to stimulate SM production in the mutants and wild
    type strains by growing them on different carbon sources, but only cladofulvin biosynthesis was
    affected. Interestingly, cladofulvin production was stimulated by carbon limitation and strongly
    repressed in the presence of saccharose. Similar to observations made in other fungi, the deletion of
    VeA or LaeA did not affect viability, but maturation and conidiation were affected. Sporulation was
    not overtly affected by the loss of HdaA, but Δhdaa deletion mutants did not produce cladofulvin.
    This suggests that cladofulvin production is not required for asexual reproduction. The main
    finding of this chapter is that global regulator manipulation cannot considered to be a universal
    tool to discover new fungal natural products.

    In chapter 4, anthraquinones and closely related compounds such as anthrones, anthracyclines
    and xanthones are considered. Emodin is perhaps the most well characterised anthraquinone that
    is produced by many fungi and plants. Once synonymous only with constipation, this former
    laxative has since been investigated for its useful anti-cancer, anti-diabetic, anti-infective and antiinflammatory properties. Cladofulvin is a homodimeric anthraquinone composed of nataloe-emodin joined in a remarkably asymmetrical configuration. Dimeric anthraquinones and xanthones are also bioactive, most commonly tested for anti-infective and anti-cancer activities. Despite the ubiquity and medicinal qualities of anthraquinones and related compounds, very few of their biosynthetic pathways are known. No enzymes capable of dimerizing anthraquinones had yet been identified. In this chapter we demonstrated that cladofulvin was very cytotoxic towards human cancer cell-lines, crucially, up-to 300-fold more than its monomeric precursor nataloe-emodin against certain celllines. This became an added incentive to elucidate the cladofulvin pathway and identify the enzyme responsible for dimerizing nataloe-emodin. We confirmed earlier predictions that PKS6/claG is the core gene which starts cladofulvin biosythesis. Deletion of claG abolished cladofulvin production
    and no related metabolites were observed. A route to cladofulvin biosynthesis was proposed, guided
    by the work performed on the monodictyphenone biosynthetic pathway in Aspergillus nidulans.

    We predicted early acting cladofulvin genes and cloned them for heterologous expression in A.
    oryzae strain M-2-3. Using this approach we were able to confirm the first five genes in cladofulvin
    biosynthesis, claBCFGH, which yielded a reduced and dehydrated form of emodin. This is the
    point at which the pathways to cladofulvin and monodictyphenone production diverge. It was
    speculated that this emodin-related intermediate might be converted into nataloe-emodin by claK
    and/or claN. Finally, it was confirmed that the final step in the cladofulvin pathway is encoded by
    claM. Targeted deletion of claM yielded a mutant that accumulated nataloe-emodin and emodin
    but no cladofulvin. We discuss how the sequence of claM and ClaM will accelerate the discovery
    of functionally similar genes and enzymes, providing a template to engineer enzymes capable of
    forming novel dimers from existing monomers.

    In chapter 5 the natural role of cladofulvin was considered. This SM is consistently produced by
    C. fulvum and global regulator mutants in vitro. The respective biosynthetic genes appear most
    active during early and late stages of infection of tomato, but are down-regulated during biotrophic
    growth phase (chapter 2). The Δclag mutants (chapter 3) were not overtly different from the wild
    type during growth in vitro. We inoculated tomato plants with this mutant in order to test whether
    or not cladofulvin was required for normal infection. Simultaneously, we inoculated a C. fulvum
    transformant carrying an extra copy of the cladofulvin pathway-specific relulator, OE.claE, fused
    to the promoter region of the Avr9 effector gene. The strain was expected to produce cladofulvin
    once the fungal hyphae penetrate host stomata and begin to colonise the apoplastic space. In this
    way, we aimed to test the effect of cladofulvin over-production on disease symptom development.
    The growth of each strain on tomato plants was monitored by RT-qrtPCR at 4, 8 and 12 days post
    inoculation (dpi). At each time point the infections were inspected microscopically to detect any
    phenotypic abnormalities. We report that the loss of claG did not result an abnormal infection.
    Both wild type and ΔclaG mutants sporulated without causing necrosis or dessication of host leaves.
    In distinct contrast, brown spots appeared on leaves infected by the OE.claE transformant between
    8 – 12 dpi. This was accompanied by much stronger fungal growth and significant accumulation
    of cladofulvin. The leaves became desiccated and brittle before the fungus conidiated. Possible
    reasons for this phenotype are discussed. A small suite of in vitro experiments was performed on the
    Δclag and wild type strains in order to test the role of cladofulvin in survival. Consistent with the
    absence of a photoprotective pigment, Δclag spores were considerably more sensitive to ultraviolet
    (UV) radiation. Suggesting a role in protection against low temperatures, Δclag spores were less
    resistant to repeated cycles of freezing and thawing. Cladofulvin biosynthesis was stimulated and
    repressed by cold and heat shocking mature C. fulvum colonies, respectively. Altogether, these
    results suggested that cladofulvin confers resistance to abiotic stress.

    In chapter 6 the results obtained in this thesis are discussed in a broader context. Particularly,
    the discovery of the cytochrome P450 that is involved in dimerization of anthraquinones might
    enable discovery of homologous genes encoding enzymes with different specificities. Combining
    bioinformatic and functional analyses should prove to be a powerful strategy for discovering
    compounds with new biological activities, or enzymes relevant to metabolic engineering.

    Nicotinate O-Glucosylation Is an Evolutionarily Metabolic Trait Important for Seed Germination under Stress Conditions in Arabidopsis thaliana
    Wei, L. ; Zhang, F. ; Chang, Y. ; Zhao, T. ; Schranz, M.E. ; Wang, M. - \ 2015
    The Plant Cell 27 (2015)7. - ISSN 1040-4651 - p. 1907 - 1924.
    salicylic-acid glucosyltransferase - ugt74f2 glucosyltransferase - poly(adp-ribose) polymerase - secondary metabolites - glandular trichomes - nad biosynthesis - salvage pathway - plants - glycosyltransferases - trigonelline
    The glycosylation of nicotinate (NA), a key intermediate of the NAD salvage pathway, occurs widely in land plants. However, the physiological function of NA glycosylation is not well understood in planta, and no gene encoding NA glycosyltransferase has been reported to date. NA glycosylation in Arabidopsis thaliana occurs at either the N- or the O-position of the NA molecule, and O-glucosylation appears to be unique to the Brassicaceae. Using gene-enzyme correlations focused on Family 1 glycosyltransferases (GTs; EC 2.4), we identified and characterized three Arabidopsis GTs, which are likely involved in NA glycosylation. These include one NAOGT (UGT74F2; previously identified as a salicylic acid glycosyltransferases) and two NANGTs (UGT76C4 and UGT76C5). Arabidopsis mutants of UGT74F2 accumulate higher levels of free NA, but not salicylic acid, than that of the wild type, and this inversely correlated with seed germination rates under various abiotic stresses. The germination defect of the ugt74f2-1 mutant could be fully complemented by overexpression of UGT74F2. These observations, together with comprehensive chemical analysis, suggest that NA glycosylation may function to protect plant cells from the toxicity of NA overaccumulation during seed germination. Combined with phylogenetic analysis, our results suggest that NAOGTs arose recently in the Brassicaceae family and may provide a fitness benefit. The multifunctionality of UGT74F2 in Arabidopsis is also investigated and discussed.
    Testing the Generalist-Specialist Dilemma: The Role of Pyrrolizidine Alkaloids in Resistance to Invertebrate Herbivores in Jacobaea Species
    Wei, X. ; Vrieling, K. ; Mulder, P.P.J. ; Klinkhamer, P.G.L. - \ 2015
    Journal of Chemical Ecology 41 (2015). - ISSN 0098-0331 - p. 159 - 167.
    longitarsus flea beetles - senecio-jacobaea - secondary metabolites - natural-selection - chemical defense - ipomopsis-aggregata - plant defense - scarlet-gilia - host plants - hybridization
    Plants produce a diversity of secondary metabolites (SMs) to protect them from generalist herbivores. On the other hand, specialist herbivores use SMs for host plant recognition, feeding and oviposition cues, and even sequester SMs for their own defense. Therefore, plants are assumed to face an evolutionary dilemma stemming from the contrasting effects of generalist and specialist herbivores on SMs. To test this hypothesis, bioassays were performed with F2 hybrids from Jacobaea species segregating for their pyrrolizidine alkaloids (PAs), using a specialist flea beetle (Longitarsus jacobaeae) and a generalist slug (Deroceras invadens). Our study demonstrated that while slug feeding damage was negatively correlated with the concentration of total PAs and that of senecionine-like PAs, flea beetle feeding damage was not affected by PAs. It was positively correlated though, with leaf fresh weight. The generalist slug was deterred by senecionine-like PAs but the specialist flea beetle was adapted to PAs in its host plant. Testing other herbivores in the same plant system, it was observed that the egg number of the specialist cinnabar moth was positively correlated with jacobine-like PAs, while the silver damage of generalist thrips was negatively correlated with senecionine- and jacobine-like PAs, and the pupae number of generalist leaf miner was negatively correlated with otosenine-like PAs. Therefore, while the specialist herbivores showed no correlation whatsoever with PA concentration, the generalist herbivores all showed a negative correlation with at least one type of PA. We concluded that the generalist herbivores were deterred by different structural groups of PAs while the specialist herbivores were attracted or adapted to PAs in its host plants.
    Ozone affects growth and development of Pieris brassicae on the wild host plant Brassica nigra
    Khaling, E. ; Papazian, S. ; Poelman, E.H. ; Holopainen, J.K. ; Albrectsen, B.R. ; Blande, J.D. - \ 2015
    Environmental Pollution 199 (2015). - ISSN 0269-7491 - p. 119 - 129.
    elevated atmospheric co2 - beetle epilachna-varivestis - betula-pendula roth - glucosinolate concentrations - secondary metabolites - feeding preference - oviposition preference - specialist herbivores - plutella-xylostella - leaf beetle
    When plants are exposed to ozone they exhibit changes in both primary and secondary metabolism, which may affect their interactions with herbivorous insects. Here we investigated the performance and preferences of the specialist herbivore Pieris brassicae on the wild plant Brassica nigra under elevated ozone conditions. The direct and indirect effects of ozone on the plant-herbivore system were studied. In both cases ozone exposure had a negative effect on P. brassicae development. However, in dual-choice tests larvae preferentially consumed plant material previously fumigated with the highest concentration tested, showing a lack of correlation between larval preference and performance on ozone exposed plants. Metabolomic analysis of leaf material subjected to combinations of ozone and herbivore-feeding, and focussing on known defence metabolites, indicated that P. brassicae behaviour and performance were associated with ozone-induced alterations to glucosinolate and phenolic pools.
    The Freshwater Sponge Ephydatia fluviatilis Harbours Diverse Pseudomonas Species (Gammaproteobacteria, Pseudomonadales) with Broad-Spectrum Antimicrobial Activity
    Keller-Costa, T. ; Jousset, A. ; Overbeek, L.S. van; Elsas, J.D. ; Costa, R. - \ 2014
    PLoS ONE 9 (2014)2. - ISSN 1932-6203
    fluorescent pseudomonads - secondary metabolites - phenotypic variation - bacterial symbiont - biological-control - biofilm formation - small rnas - sp nov. - rhizosphere - community
    Bacteria are believed to play an important role in the fitness and biochemistry of sponges (Porifera). Pseudomonas species (Gammaproteobacteria, Pseudomonadales) are capable of colonizing a broad range of eukaryotic hosts, but knowledge of their diversity and function in freshwater invertebrates is rudimentary. We assessed the diversity, structure and antimicrobial activities of Pseudomonas spp. in the freshwater sponge Ephydatia fluviatilis. Polymerase Chain Reaction - Denaturing Gradient Gel Electrophoresis (PCR-DGGE) fingerprints of the global regulator gene gacA revealed distinct structures between sponge-associated and free-living Pseudomonas communities, unveiling previously unsuspected diversity of these assemblages in freshwater. Community structures varied across E. fluviatilis specimens, yet specific gacA phylotypes could be detected by PCR-DGGE in almost all sponge individuals sampled over two consecutive years. By means of whole-genome fingerprinting, 39 distinct genotypes were found within 90 fluorescent Pseudomonas isolates retrieved from E. fluviatilis. High frequency of in vitro antibacterial (49%), antiprotozoan (35%) and anti-oomycetal (32%) activities was found among these isolates, contrasting less-pronounced basidiomycetal (17%) and ascomycetal (8%) antagonism. Culture extracts of highly predation-resistant isolates rapidly caused complete immobility or lysis of cells of the protozoan Colpoda steinii. Isolates tentatively identified as P. jessenii, P. protegens and P. oryzihabitans showed conspicuous inhibitory traits and correspondence with dominant sponge-associated phylotypes registered by cultivation-independent analysis. Our findings suggest that E. fluviatilis hosts both transient and persistent Pseudomonas symbionts displaying antimicrobial activities of potential ecological and biotechnological value.
    Variation in plant defences among populations of a range-expanding plant: consequences for trophic interactions
    Fortuna, T.M. ; Eckert, S. ; Harvey, J.A. ; Vet, L.E.M. ; Müller, C. ; Gols, R. - \ 2014
    New Phytologist 204 (2014)4. - ISSN 0028-646X - p. 989 - 999.
    increased competitive ability - bunias orientalis l - invasive plant - insect herbivores - secondary metabolites - specialist herbivore - alliaria-petiolata - chemical defense - native insect - host
    Although plant-herbivore-enemy interactions have been studied extensively in cross-continental plant invasions, little is known about intra-continental range expanders, despite their rapid spread globally. Using an ecological and metabolomics approach, we compared the insect performance of a generalist and specialist herbivore and a parasitoid, as well as plant defence traits, among native, exotic invasive and exotic non-invasive populations of the Turkish rocket, Bunias orientalis, a range-expanding species across parts of Eurasia. In the glasshouse, the generalist herbivore, Mamestra brassicae, and its parasitoid, Microplitis mediator, performed better on non-native than on native plant populations. Insect performance did not differ between the two non-native origins. By contrast, the specialist herbivore, Pieris brassicae, developed poorly on all populations. Differences in trichome densities and in the metabolome, particularly in the family-specific secondary metabolites (i.e. glucosinolates), may explain population-related variation in the performance of the generalist herbivore and its parasitoid. Total glucosinolate concentrations were significantly induced by herbivory, particularly in native populations. Native populations of B. orientalis are generally better defended than non-native populations. The role of insect herbivores and dietary specialization as a selection force on defence traits in the range-expanding B. orientalis is discussed.
    Production of guaianolides in Agrobacterium rhizogenes - transformedchicory regenerants flowering in vitro
    Bogdanovic, M.D. ; Todorovic, S.I. ; Banjanac, T. ; Dragicevic, M.B. ; Verstappen, F.W.A. ; Bouwmeester, H.J. ; Simonovic, A.D. - \ 2014
    Industrial Crops and Products 60 (2014). - ISSN 0926-6690 - p. 52 - 59.
    hairy root cultures - spontaneous plant-regeneration - cichorium-intybus l - sesquiterpene lactones - mediated transformation - rol genes - antiproliferative activity - secondary metabolites - bioactive substances - witloof chicory
    Chicory (Cichorium intybus L.) is rich in bitter sesquiterpene lactones, mainly guaianolides: lactucin,8-deoxylactucin, lactupicrin and their 11(S),13-dihydroderivatives—compounds recognized for theirantimicrobial and anti-cancer effects. In vitro plant tissue culture, and particularly Agrobacteriumrhizogenes—generated hairy root (HR) cultures, have many advantages as systems for production of valu-able secondary metabolites. Although chicory HRs grow better than control culture, having nearly 60times greater fresh weight gain, they do not contain a higher content of guaianolides than wild type(wt) roots. Thus we have established in vitro system comprised of wt root and HR cultures, and wt andtransformed regenerated plants of the same age, in rosette and flowering stage, in order to study theeffects of transformation, organogenesis and flowering on guaianolides production. Both regenerationand flowering in vitro were spontaneous, so the results were not influenced by exogenous growth regu-lators. Some of the transformed clones grew better, but all flowered earlier in comparison to wt plants.Floral transition increased guaianolides content in both roots and leaves of transformed, but not of wtplants. Expression of RolC oncogene correlated with floral transition and with guaianolides accumula-tion. We propose A. rhizogenes—transformed plants at the flowering stage as an alternative source of freeguaianolides, where, in contrast to HRs, entire plants can be used for the extraction.
    Weerbaarheid van de plant wordt langzamerhand ontrafeld : Op zoek naar de juiste balans tussen weerbaarheid en productie (interview met Jantineke Hofland-Zijlstra)
    Kierkels, T. ; Hofland-Zijlstra, J.D. - \ 2014
    Onder Glas 11 (2014)5. - p. 42 - 43.
    glastuinbouw - gewasproductie - sierplanten - stressfactoren - verdedigingsmechanismen - secundaire metabolieten - optimalisatiemethoden - landbouwkundig onderzoek - gewasbescherming - plantgezondheid - greenhouse horticulture - crop production - ornamental plants - stress factors - defence mechanisms - secondary metabolites - optimization methods - agricultural research - plant protection - plant health
    Met een goede klimaatbeheersing kan de teler Botrytis in de hand houden. Maar eigenlijk is de weerbaarheid van de plant bij deze schimmel veel belangrijker. Bij een hogere weerbaarheid hoef je veel minder scherp op het klimaat te letten en bespaar je op het middelengebruik. Diverse onderzoekinstellingen werken aan meer inzicht.
    Breeding for pepper fruit quality: a genitical metabolomics approach
    Wahyuni, Y. - \ 2014
    Wageningen University. Promotor(en): Raoul Bino, co-promotor(en): Arnaud Bovy; E. Sudarmonowati; A.R. Ballester. - Wageningen : Wageningen University - ISBN 9789461739582 - 193
    capsicum annuum - capsicum frutescens - capsicum chinense - capsicum baccatum - plantenveredeling - metabolomica - gewaskwaliteit - secundaire metabolieten - gezondheid - genetische kartering - rijp worden - capsicum annuum - capsicum frutescens - capsicum chinense - capsicum baccatum - plant breeding - metabolomics - crop quality - secondary metabolites - health - genetic mapping - ripening

    A diverse collection of 32 pepper accessions was analysed for variation in health-related metabolites, such as carotenoids, capsaicinoids, flavonoids and vitamins C and E. For each of the metabolites analysed, there was a lot of variation among the accessions and it was possible to identify accessions with high amounts of specific metabolites. While all accessions contained high levels of vitamin C, accession C. chinenseAC2212 was found to be an excellent source of vitamin E, whereas C. annuumLong Sweet accumulated high levels of the flavonoid quercetin. A genetical metabolomics approach was used to study the genetic basis of metabolic traits in a segregating F2 population based on a cross between two contrasting pepper genotypes. This led to the identification of several metabolites QTL hotspots. The genetic basis for the accumulation of several flavonoids in pepper fruit was further investigated, by combining metabolic, gene expression and candidate gene-based marker data. This provided valuable insight into the key genes important for flavonoid accumulation in pepper fruit. The results of this study will help breeders to assist future breeding programs aimed at optimizing the levels of nutritional compounds in pepper fruit.

    Discovery and reconstitution of the secoiridoid pathway of Catharanthus roseus
    Dong, L. - \ 2014
    Wageningen University. Promotor(en): Richard Immink, co-promotor(en): Sander van der Krol. - Wageningen : Wageningen University - ISBN 9789461738462 - 201
    catharanthus roseus - secoïridoïden - secundaire metabolieten - medicinale eigenschappen - genen - biosynthese - catharanthus roseus - secoiridoids - secondary metabolites - medicinal properties - genes - biosynthesis

    Terpene indole alkaloids (TIAs) are important plant-produced secondary metabolites for

    humans, because of their anti-cancer properties. The production of TIAs still fully relies on

    extraction from medicinal plants like Catharanthus roseus, which only contains extreme low

    amounts of these compounds and new ways need to be found to efficiently produce these

    anticancer drugs at low cost. The common precursor for TIAs is strictosidine and in my PhD

    project I tried to produce strictosidine in fast-growing tobacco by transferring the genes of the

    whole biosynthesis pathway into tobacco. At the onset of my project 6 out of the presumed

    12 genes of the pathway in C. roseus had not been discovered yet. My thesis tells the story of

    discovery and characterization of the missing genes and reconstruction of the full strictosidine

    pathway in tobacco.

    1

    Genetics of insect resistance to plant defence
    Vermeer, K.M.C.A. - \ 2014
    Wageningen University. Promotor(en): Marcel Dicke, co-promotor(en): Peter de Jong. - Wageningen : Wageningen University - ISBN 9789461738363 - 199
    insect-plant relaties - co-evolutie - planten - plaagresistentie - verdedigingsmechanismen - insectenplagen - secundaire metabolieten - phyllotreta nemorum - barbarea vulgaris - genetisch bepaalde resistentie - genetische analyse - insect plant relations - coevolution - plants - pest resistance - defence mechanisms - insect pests - secondary metabolites - phyllotreta nemorum - barbarea vulgaris - genetic resistance - genetic analysis

    Plants are chemically defended against insect herbivory in various ways. They produce a broad range of secondary metabolites that may be toxic or deterrent to insects. Specialist insects, however, are often capable of overcoming these defences. The yellow striped flea beetle (Phyllotreta nemorumL.) is a specialist that feeds on crucifers (Brassicaceae) such as Sinapis arvensisand Barbarea vulgaris. In Denmark, two types of Barbarea vulgarisvar. arcuataare distinguished: one with pubescent leaves (P-type) and one with glabrous leaves (G-type). All individuals of P. nemorumcan feed on B. vulgarisP-type. Barbarea vulgarisG-type, on the other hand, is chemically defended against most P. nemorumindividuals during the flea beetle reproductive season. The defence compounds are hypothesized to be saponins, a class of compounds with various biological effects and insecticidal properties. Despite high levels of these saponins during summer, some flea beetles can and do feed on B. vulgaris G-type. The ability of P. nemorumto feed on B. vulgarisG-type is heritable; resistance against the defence of B. vulgarisG-type is controlled by dominant major resistance genes (R-genes). One dominant R-allele of an R-gene is enough to convert a susceptible beetle into a resistant one. Despite knowledge of the inheritance patterns of resistance in the flea beetles, which have been demonstrated to be variable, the underlying mechanism of flea beetle resistance has, so far, remained unclear. This prompted me to investigate, as an initial part of my thesis, the genetic basis of the flea beetle adaptation to the defence of B. vulgarisG-type.

    The interaction between B. vulgarisand the flea beetle is a unique natural model system to study chemical defences in plants and counter-adaptations in insects. Plant and insect are both polymorphic with respect to the trait involved in resistance and hereby provide an excellent opportunity to study the geographic aspects of the evolution of the resistance trait in both interacting species. In this thesis, I focus on the resistance of the flea beetle, and take the presence of different genotypes of the plant as a given. Phyllotreta nemorumis a major pest, for example in oil seed rape. Understanding how resistance evolves in P. nemorumwill not only benefit flea beetle control, but also control of other pest insects. Understanding insect resistance includes knowledge of seasonal, geographic and genetic variation in both plant defense and herbivore adaptation.The R-gene has a remarkable distribution. Flea beetle populations living on B. vulgarisG-type consist solely of resistant individuals, but on host plant patches nearby B. vulgarisG-type lower frequencies of resistant beetles are found than one would expect with the amount of gene flow found at the neutral level between these subpopulations.

    The aim of this thesis was to find the gene that is held responsible for the resistance of P. nemorumto the defences of B. vulgaris, investigate the distribution of this resistance trait and explain the distribution of this trait in natural populations. The following questions were addressed: (1) what is the genetic basis of the adaptation under study? (2) how is the resistance distributed across flea beetle populations in Denmark? and (3) which factors underlie this distribution?

    In order to answer these questions, I used an integrated approach. I have combined a candidate gene approach (CHAPTER3) with an empirical approach via the study of variation in resistance in flea beetle populations (CHAPTER4), and a population genomics approach by using molecular markers to gain insight in the genomic make-up of the population and its connection with the resistance trait (CHAPTERS5 and 6). The population genomics approach is a recent advance in methods to detect the involvement of selection in the distribution of alleles at presumably adaptive loci. Using this approach one can distinguish locus-specific effects, like directional selection, from genome-wide effects, on the distribution of alleles at loci of interest.

    The population genomics approach is introduced in CHAPTER2 together with the Geographic Mosaic Theory of Coevolution. I illustrate how processes underlying this theory of coevolution can be investigated with the population genomics approach. According to the geographic mosaic theory of coevolution, reciprocal selection between interacting species only happens in so-called hot-spots. Hot spots can be identified using population genomics and genetic variation found at specific loci can be attributed to locus-specific processes such as directional selection. For the B. vulgaris- flea beetle system this means that with a population genomics approach we can examine whether the distribution of resistant flea beetles on alternative host plants is only influenced by migration, or also by selection (CHAPTER5). Another valuable utility of the population genomics approach is to investigate whether a candidate gene for the R-gene is under selection, by looking whether a candidate gene is experiencing locus-specific effects beside genome-wide effects when comparing flea beetle populations living on B. vulgarisG-type with populations living on alternative host plants (CHAPTER6).

    However, before using a population genomics approach to compare the resistance trait or a candidate gene with parts of the genome that only experience genome-wide effects, I have tried to identify the genetic basis of the flea beetle adaptation to the defence ofB. vulgarisG-type. In CHAPTER3, I have addressed this question by using a candidate gene approach to examine the involvement of a possible detoxifying enzyme in P. nemorum. Genes coding for β-glucosidase were a candidate for genes underlying the difference between resistant and susceptible beetles, because β-glucosidase is used as detoxifying enzyme by other organisms resistant to saponin defence. Three different β-glucosidase cDNA sequences were cloned from Danish flea beetle lines. We named them β-glucosidase A, B and C. β-glucosidase C was only found in resistant lines and not in the susceptible line. We then tested if recombinant β-glucosidase C breaks down the most abundant and most effective defence compound in B. vulgarisG-type, hederagenin cellobioside. β-glucosidase C was able to deglycosylate one glucose unit of hederagenin cellobioside, when expressed in an insect cell line. This suggests that expressed β-glucosidase C can deglycosylate antifeedant saponins and may play a role in the resistant flea beetle’s ability to overcome the defence of B. vulgaris. Next, a segregating family was created in which offspring differed in resistance genotype. Again β-glucosidase cDNA sequences were cloned to find a difference in the presence of these β-glucosidases between resistant and susceptible individuals. This time cDNA sequences of β-glucosidases A, B and C were present in both resistant and susceptible individuals although significantly fewer β-glucosidase C cDNA sequence variants were found in susceptible individuals than in resistant individuals. Thus, the genetic basis of flea beetle resistance remains unclear. Further investigation is needed to explore if the β-glucosidase C protein is also capable of inactivating hederagenin cellobioside by hydrolysizing the second glucose unit from the saponin and if there is a difference in enzyme activity of β-glucosidase C between resistant and susceptible beetles.

    Subsequently, in CHAPTER4 I have investigated whether the frequency of resistant beetles decreased in populations living on other host plant patches than B. vulgarisG-type and whether the change in frequency was significant within the flea beetle season. I found that the frequency of resistant beetles varied significantly among years, but there was no evidence for a decrease in the frequency of resistant beetles, the latter being expected if selection acts against the resistance on other host plants than B. vulgarisG-type. Furthermore, I found that the frequency of resistant beetles varied significantly within a flea beetle season. This study demonstrates that relative frequencies of different resistance phenotypes of P. nemorumon other host plants than B. vulgarisG-type are highly dynamic, both within and across years. It is, therefore, important to sample season-wide when one wants to monitor the changes in frequencies of insect resistance in natural systems.

    In CHAPTERS5 and 6 I took a population genomics approach to investigate if the observed geographicaldistribution of resistance of P. nemorumto chemically defendedB. vulgarisin flea beetle populations could be explained by factors that are solely associated with genome-wide effects, such as migration, or also by locus-specific factors like selection at the resistance locus. First, neutral microsatellites were used to reveal the genetic differentiation at parts of the genome that are only influenced by genome-wide processes. Next, the level of neutral genetic differentiation was compared with the genetic differentiation found for the resistance trait. The resistance trait was an outlier in pairwise comparisons between flea beetle populations on B. vulgarisand S. arvensis, meaning that the level of genetic differentiation was significantly higher than expected if the resistance trait experiences only genome-wide effects. The resistance trait was also an outlier in the pairwise comparison between populations on S. arvensis, which suggests that the resistance trait is also under directional selection on other host plants than B. vulgarisG-type.

    Additionally, I examined in CHAPTER6 if the homologous β-glucosidases B and C sequences found in CHAPTER3 correspond to two alleles of the major resistance gene, because of their similarity and their presence in flea beetle lines. The sequence of β-glucosidases C had so far only been found in resistant individuals, so we hypothesized it to be the dominant resistance allele and the sequence of β-glucosidases B would then correspond to the susceptible allele. In order to find out if this hypothesized PneR-gene (Phyllotreta nemorum R-gene) is the resistance gene, we first directly compared resistance phenotypes of beetles collected from populations on B. vulgarisG-type and S. arvensiswith genotypes derived with primers developed for β-glucosidase B and C. The phenotype of the flea beetles did not match the genotype derived with the β-glucosidase primers. Additionally, the frequency of heterozygotes and homozygotes of the PneR-gene genotype was not significantly deviating from Hardy-Weinberg Equilibrium which implies that there are no locus-specific effects involved when both sequences are seen as one gene with two alleles. A population approach was taken like in CHAPTER5, this time including the genetic differentiation estimated for the candidate gene as well. The candidate gene behaved similar to the neutral loci while the resistance trait was an outlier in most pairwise comparisons between flea beetle populations. If both sequences are alleles of the same gene, then the candidate gene is not directly responsible for the flea beetle resistance to B. vulgarisG-type defence.

    The results presented in this thesis show the complexity of genetic processes (either genome-wide or locus specific) affecting local adaptation and the distribution of a resistance trait in insects in natural populations. Furthermore, the present study shows that when studying coevolution between insect and host plant by means of adaptive traits, also geographical and seasonal variation in allele frequencies should be considered. A multidisciplinary approach to study adaptation in plant-insect interactions such as used in this thesis, will benefit research on plant-insect interactions, including applied research such as studying the potential of host plants as dead-end traps for pest insects and preventing/diminishing the development of resistance by pest insects to crop defences.

    Bidirectional Secretions from Glandular Trichomes (AQ1) of Pyrethrum (Tanacetum cinerariifolium) Enable Immunization of Seedlings
    Ramirez, A.M. ; Stoopen, G.M. ; Menzel, T.R. ; Gols, R. ; Bouwmeester, H.J. ; Dicke, M. ; Jongsma, M.A. - \ 2012
    The Plant Cell 24 (2012)10. - ISSN 1040-4651 - p. 4252 - 4265.
    artemisia-annua l - sesquiterpene lactones - chrysanthemum-cinerariaefolium - tanacetum-cinerariifolium - isoprenoid biosynthesis - secondary metabolites - diphosphate synthase - natural pyrethrins - cells - expression
    Glandular trichomes are currently known only to store mono- and sesquiterpene compounds in the subcuticular cavity just above the apical cells of trichomes or emit them into the headspace. We demonstrate that basipetal secretions can also occur, by addressing the organization of the biosynthesis and storage of pyrethrins in pyrethrum (Tanacetum cinerariifolium) flowers. Pyrethrum produces a diverse array of pyrethrins and sesquiterpene lactones for plant defense. The highest concentrations accumulate in the flower achenes, which are densely covered by glandular trichomes. The trichomes of mature achenes contain sesquiterpene lactones and other secondary metabolites, but no pyrethrins. However, during achene maturation, the key pyrethrin biosynthetic pathway enzyme chrysanthemyl diphosphate synthase is expressed only in glandular trichomes. We show evidence that chrysanthemic acid is translocated from trichomes to pericarp, where it is esterified into pyrethrins that accumulate in intercellular spaces. During seed maturation, pyrethrins are then absorbed by the embryo, and during seed germination, the embryo-stored pyrethrins are recruited by seedling tissues, which, for lack of trichomes, cannot produce pyrethrins themselves. The findings demonstrate that plant glandular trichomes can selectively secrete in a basipetal direction monoterpenoids, which can reach distant tissues, participate in chemical conversions, and immunize seedlings against insects and fungi.
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