570 Biowissenschaften; Biologie
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Plasticity in metabolism underpins local responses to nitrogen in Arabidopsis thaliana populations
(2019)
Nitrogen (N) is central for plant growth, and metabolic plasticity can provide a strategy to respond to changing N availability. We showed that two local A. thaliana populations exhibited differential plasticity in the compounds of photorespiratory and starch degradation pathways in response to three N conditions. Association of metabolite levels with growth-related and fitness traits indicated that controlled plasticity in these pathways could contribute to local adaptation and play a role in plant evolution.
Plants encounter biotic and abiotic stresses many times during their life cycle and this limits their productivity. Moderate heat stress (HS) primes a plant to survive higher temperatures that are lethal in the naïve state. Once temperature stress subsides, the memory of the priming event is actively retained for several days preparing the plant to better cope with recurring HS. Recently, chromatin regulation at different levels has been implicated in HS memory. Here, we report that the chromatin protein BRUSHY1 (BRU1)/TONSOKU/MGOUN3 plays a role in the HS memory in Arabidopsis thaliana. BRU1 is also involved in transcriptional gene silencing and DNA damage repair. This corresponds with the functions of its mammalian orthologue TONSOKU‐LIKE/NFΚBIL2. During HS memory, BRU1 is required to maintain sustained induction of HS memory‐associated genes, whereas it is dispensable for the acquisition of thermotolerance. In summary, we report that BRU1 is required for HS memory in A. thaliana, and propose a model where BRU1 mediates the epigenetic inheritance of chromatin states across DNA replication and cell division.
The AtNFXL1 gene encodes a NF-X1 type zinc finger protein required for growth under salt stress
(2006)
The human NF-X1 protein and homologous proteins in eukaryotes represent a class of transcription factors which are characterised. by NF-X1 type zinc finger motifs. The Arabidopsis genome encodes two NF-X1 homologs, which we termed AtNFXL1 and AtNFXL2. Growth and survival was impaired in atnfxl1 knock-out mutants and AtNFXL1-antisense plants under salt stress in comparison to wild-type plants. In contrast, 35S: :AtNFXL1 plants showed higher survival rates. The AtNFXL2 protein potentially plays an antagonistic role. The Arabidopsis NF-X1 type zinc finger proteins likely are part of regulatory mechanisms, which protect major processes such as photosynthesis.
Für alle Organismen ist die Aufrechterhaltung ihres energetischen Gleichgewichts unter fluktuierenden Umweltbedingungen lebensnotwendig. In Eukaryoten steuern evolutionär konservierte Proteinkinasen, die in Pflanzen als SNF1-RELATED PROTEIN KINASE1 (SnRK1) bezeichnet werden, die Adaption an Stresssignale aus der Umwelt und an die Limitierung von Nährstoffen und zellulärer Energie. Die Aktivierung von SnRK1 bedingt eine umfangreiche transkriptionelle Umprogrammierung, die allgemein zu einer Repression energiekonsumierender Prozesse wie beispielsweise Zellteilung und Proteinbiosynthese und zu einer Induktion energieerzeugender, katabolischer Stoffwechselwege führt. Wie unterschiedliche Signale zu einer generellen sowie teilweise gewebe- und stressspezifischen SnRK1-vermittelten Antwort führen ist bisher noch nicht ausreichend geklärt, auch weil bislang nur wenige Komponenten der SnRK1-Signaltransduktion identifiziert wurden. In dieser Arbeit konnte ein Protein-Protein-Interaktionsnetzwerk um die SnRK1αUntereinheiten aus Arabidopsis AKIN10/AKIN11 etabliert werden. Dadurch wurden zunächst Mitglieder der pflanzenspezifischen DUF581-Proteinfamilie als Interaktionspartner der SnRK1α-Untereinheiten identifiziert. Diese Proteine sind über ihre konservierte DUF581Domäne, in der ein Zinkfinger-Motiv lokalisiert ist, fähig mit AKIN10/AKIN11 zu interagieren. In planta Ko-Expressionsanalysen zeigten, dass die DUF581-Proteine eine Verschiebung der nucleo-cytoplasmatischen Lokalisierung von AKIN10 hin zu einer nahezu ausschließlichen zellkernspezifischen Lokalisierung begünstigen sowie die Ko-Lokalisierung von AKIN10 und DUF581-Proteinen im Nucleus. In Bimolekularen Fluoreszenzkomplementations-Analysen konnte die zellkernspezifische Interaktion von DUF581-Proteinen mit SnRK1α-Untereinheiten in planta bestätigt werden. Außerhalb der DUF581-Domäne weisen die Proteine einander keine große Sequenzähnlichkeit auf. Aufgrund ihrer Fähigkeit mit SnRK1 zu interagieren, dem Fehlen von SnRK1Phosphorylierungsmotiven sowie ihrer untereinander sehr variabler gewebs-, entwicklungs- und stimulusspezifischer Expression wurde für DUF581-Proteine eine Funktion als Adaptoren postuliert, die unter bestimmten physiologischen Bedingungen spezifische Substratproteine in den SnRK1-Komplex rekrutieren. Auf diese Weise könnten DUF581Proteine die Interaktion von SnRK1 mit deren Zielproteinen modifizieren und eine Feinjustierung der SnRK1-Signalweiterleitung ermöglichen. Durch weiterführende Interaktionsstudien konnten DUF581-interagierende Proteine darunter Transkriptionsfaktoren, Proteinkinasen sowie regulatorische Proteine gefunden werden, die teilweise ebenfalls Wechselwirkungen mit SnRK1α-Untereinheiten aufzeigten. Im Rahmen dieser Arbeit wurde eines dieser Proteine für das eine Beteiligung an der SnRK1Signalweiterleitung als Transkriptionsregulator vermutet wurde näher charakterisiert. STKR1 (STOREKEEPER RELATED 1), ein spezifischer Interaktionspartner von DUF581-18, gehört zu einer pflanzenspezifischen Leucin-Zipper-Transkriptionsfaktorfamilie und interagiert in Hefe sowie in planta mit SnRK1. Die zellkernspezifische Interaktion von STKR1 und AKIN10 in Pflanzen unterstützt die Vermutung der kooperativen Regulation von Zielgenen. Weiterhin stabilisierte die Anwesenheit von AKIN10 die Proteingehalte von STKR1, das wahrscheinlich über das 26S Proteasom abgebaut wird. Da es sich bei STKR1 um ein Phosphoprotein mit SnRK1-Phosphorylierungsmotiv handelt, stellt es sehr wahrscheinlich ein SnRK1-Substrat dar. Allerdings konnte eine SnRK1-vermittelte Phosphorylierung von STKR1 in dieser Arbeit nicht gezeigt werden. Der Verlust von einer Phosphorylierungsstelle beeinflusste die Homo- und Heterodimerisierungsfähigkeit von STKR1 in Hefeinteraktionsstudien, wodurch eine erhöhte Spezifität der Zielgenregulation ermöglicht werden könnte. Außerdem wurden Arabidopsis-Pflanzen mit einer veränderten STKR1-Expression phänotypisch, physiologisch und molekularbiologisch charakterisiert. Während der Verlust der STKR1-Expression zu Pflanzen führte, die sich kaum von Wildtyp-Pflanzen unterschieden, bedingte die konstitutive Überexpression von STKR1 ein stark vermindertes Pflanzenwachstum sowie Entwicklungsverzögerungen hinsichtlich der Blühinduktion und Seneszenz ähnlich wie sie auch bei SnRK1α-Überexpression beschrieben wurden. Pflanzen dieser Linien waren nicht in der Lage Anthocyane zu akkumulieren und enthielten geringere Gehalte an Chlorophyll und Carotinoiden. Neben einem erhöhten nächtlichen Stärkeumsatz waren die Pflanzen durch geringere Saccharosegehalte im Vergleich zum Wildtyp gekennzeichnet. Eine Transkriptomanalyse ergab, dass in den STKR1-überexprimierenden Pflanzen unter Energiemangelbedingungen, hervorgerufen durch eine verlängerte Dunkelphase, eine größere Anzahl an Genen im Vergleich zum Wildtyp differentiell reguliert war als während der Lichtphase. Dies spricht für eine Beteiligung von STKR1 an Prozessen, die während der verlängerten Dunkelphase aktiv sind. Ein solcher ist beispielsweise die SnRK1-Signaltransduktion, die unter energetischem Stress aktiviert wird. Die STKR1Überexpression führte zudem zu einer verstärkten transkriptionellen Induktion von Abwehrassoziierten Genen sowie NAC- und WRKY-Transkriptionsfaktoren nach verlängerter Dunkelphase. Die Transkriptomdaten deuteten auf eine stimulusunabhängige Induktion von Abwehrprozessen hin und konnten eine Erklärung für die phänotypischen und physiologischen Auffälligkeiten der STKR1-Überexprimierer liefern.
The GABI Primary Database, GabiPD (http:// www.gabipd.org/), was established in the frame of the German initiative for Genome Analysis of the Plant Biological System (GABI). The goal of GabiPD is to collect, integrate, analyze and visualize primary information from GABI projects. GabiPD constitutes a repository and analysis platform for a wide array of heterogeneous data from high-throughput experiments in several plant species. Data from different ‘omics’ fronts are incorporated (i.e. genomics, transcriptomics, proteomics and metabolomics), originating from 14 different model or crop species. We have developed the concept of GreenCards for textbased retrieval of all data types in GabiPD (e.g. clones, genes, mutant lines). All data types point to a central Gene GreenCard, where gene information is integrated from genome projects or NCBI UniGene sets. The centralized Gene GreenCard allows visualizing ESTs aligned to annotated transcripts as well as displaying identified protein domains and gene structure. Moreover, GabiPD makes available interactive genetic maps from potato and barley, and protein 2DE gels from Arabidopsis thaliana and Brassica napus. Gene expression and metabolic-profiling data can be visualized through MapManWeb. By the integration of complex data in a framework of existing knowledge, GabiPD provides new insights and allows for new interpretations of the data.
About 2,000 of the more than 27,000 genes of the genetic model plant Arabidopsis thaliana encode for transcription factors (TFs), proteins that bind DNA in the promoter region of their target genes and thus act as transcriptional activators and repressors. Since TFs play essential roles in nearly all biological processes, they are of great scientific and biotechnological interest. This thesis concentrated on the functional characterisation of four selected members of the Arabidopsis DOF-family, namely DOF1.2, DOF3.1, DOF3.5 and DOF5.2, which were selected because of their specific expression pattern in the root tip, a region that comprises the stem cell niche and cells for the perception of environmental stimuli. DOF1.2, DOF3.1 and DOF3.5 are previously uncharacterized members of the Arabidopsis DOF-family, while DOF5.2 has been shown to be involved in the phototrophic flowering response. However, its role in root development has not been described so far. To identify biological processes regulated by the four DOF proteins in detail, molecular and physiological characterization of transgenic plants with modified levels of DOF1.2, DOF3.1, DOF3.5 and DOF5.2 expression (constitutive and inducible over-expression, artificial microRNA) was performed. Additionally expression patterns of the TFs and their target genes were analyzed using promoter-GUS lines and publicly available microarray data. Finally putative protein-protein interaction partners and upstream regulating TFs were identified using the yeast two-hybrid and one-hybrid system. This combinatorial approach revealed distinct biological functions of DOF1.2, DOF3.1, DOF3.5 and DOF5.2 in the context of root development. DOF1.2 and DOF3.5 are specifically and exclusively expressed in the root cap, including the central root cap (columella) and the lateral root cap, organs which are essential to direct oriented root growth. It could be demonstrated that both genes work in the plant hormone auxin signaling pathway and have an impact on distal cell differentiation. Altered levels of gene expression lead to changes in auxin distribution, abnormal cell division patterns and altered root growth orientation. DOF3.1 and DOF5.2 share a specific expression pattern in the organizing centre of the root stem cell niche, called the quiescent centre. Both genes redundantly control cell differentiation in the root´s proximal meristem and unravel a novel transcriptional regulation pathway for genes enriched in the QC cells. Furthermore this work revealed a novel bipartite nuclear localisation signal being present in the protein sequence of the DOF TF family from all sequenced plant species. Summing up, this work provides an important input into our knowledge about the role of DOF TFs during root development. Future work will concentrate on revealing the exact regulatory networks of DOF1.2, DOF3.1, DOF3.5 and DOF5.2 and their possible biotechnological applications.
Pectic polysaccharides, a class of plant cell wall polymers, form one of the most complex networks known in nature. Despite their complex structure and their importance in plant biology, little is known about the molecular mechanism of their biosynthesis, modification, and turnover, particularly their structure-function relationship. One way to gain insight into pectin metabolism is the identification of mutants with an altered pectin structure. Those were obtained by a recently developed pectinase-based genetic screen. Arabidopsis thaliana seedlings grown in liquid medium containing pectinase solutions exhibited particular phenotypes: they were dwarfed and slightly chlorotic. However, when genetically different A. thaliana seed populations (random T-DNA insertional populations as well as EMS-mutagenized populations and natural variations) were subjected to this treatment, individuals were identified that exhibit a different visible phenotype compared to wild type or other ecotypes and may thus contain a different pectin structure (pec-mutants). After confirming that the altered phenotype occurs only when the pectinase is present, the EMS mutants were subjected to a detailed cell wall analysis with particular emphasis on pectins. This suite of mutants identified in this study is a valuable resource for further analysis on how the pectin network is regulated, synthesized and modified. Flanking sequences of some of the T-DNA lines have pointed toward several interesting genes, one of which is PEC100. This gene encodes a putative sugar transporter gene, which, based on our data, is implicated in rhamnogalacturonan-I synthesis. The subcellular localization of PEC100 was studied by GFP fusion and this protein was found to be localized to the Golgi apparatus, the organelle where pectin biosynthesis occurs. Arabidopsis ecotype C24 was identified as a susceptible one when grown with pectinases in liquid culture and had a different oligogalacturonide mass profile when compared to ecotype Col-0. Pectic oligosaccharides have been postulated to be signal molecules involved in plant pathogen defense mechanisms. Indeed, C24 showed elevated accumulation of reactive oxygen species upon pectinase elicitation and had altered response to the pathogen Alternaria brassicicola in comparison to Col-0. Using a recombinant inbred line population three major QTLs were identified to be responsible for the susceptibility of C24 to pectinases. In a reverse genetic approach members of the qua2 (putative pectin methyltransferase) family were tested for potential target genes that affect pectin methyl-esterification. The list of these genes was determined by in silico study of the pattern of expression and co-expression of all 34 members of this family resulting in 6 candidate genes. For only for one of the 6 analyzed genes a difference in the oligogalacturonide mass profile was observed in the corresponding knock-out lines, confirming the hypothesis that the methyl-esterification pattern of pectin is fine tuned by members of this gene family. This study of pectic polysaccharides through forward and reverse genetic screens gave new insight into how pectin structure is regulated and modified, and how these modifications could influence pectin mediated signalling and pathogenicity.
Plants are the primary producers of biomass and thereby the basis of all life. Many varieties are cultivated, mainly to produce food, but to an increasing amount as a source of renewable energy. Because of the limited acreage available, further improvements of cultivated species both with respect to yield and composition are inevitable. One approach to further progress in developing improved plant cultivars is a systems biology oriented approach. This work aimed to investigate the primary metabolism of the model plant A.thaliana and its relation to plant growth using quantitative genetics methods. A special focus was set on the characterization of heterosis, the deviation of hybrids from their parental means for certain traits, on a metabolic level. More than 2000 samples of recombinant inbred lines (RILs) and introgression lines (ILs) developed from the two accessions Col-0 and C24 were analyzed for 181 metabolic traces using gas-chromatography/ mass-spectrometry (GC-MS). The observed variance allowed the detection of 157 metabolic quantitative trait loci (mQTL), genetic regions carrying genes, which are relevant for metabolite abundance. By analyzing several hundred test crosses of RILs and ILs it was further possible to identify 385 heterotic metabolic QTL (hmQTL). Within the scope of this work a robust method for large scale GC-MS analyses was developed. A highly significant canonical correlation between biomass and metabolic profiles (r = 0.73) was found. A comparable analysis of the results of the two independent experiments using RILs and ILs showed a large agreement. The confirmation rate for RIL QTL in ILs was 56 % and 23 % for mQTL and hmQTL respectively. Candidate genes from available databases could be identified for 67 % of the mQTL. To validate some of these candidates, eight genes were re-sequenced and in total 23 polymorphisms could be found. In the hybrids, heterosis is small for most metabolites (< 20%). Heterotic QTL gave rise to less candidate genes and a lower overlap between both populations than was determined for mQTL. This hints that regulatory loci and epistatic effects contribute to metabolite heterosis. The data described in this thesis present a rich source for further investigation and annotation of relevant genes and may pave the way towards a better understanding of plant biology on a system level.
The major aim of this thesis was to study the effect of nitrate on primary metabolism and in development of the model plant Arabidopsis thaliana. The present work has two separate topics. First, to investigate the GDH family, a small gene family at the interface between nitrogen and carbon metabolisms. Second, to investigate the mechanisms whereby nitrogen is regulating the transition to flowering time in Arabidopsis thaliana. To gain more insights into the regulation of primary metabolism by the functional characterization of the glutamate dehydrogenase (GDH) family, an enzyme putatively involved in the metabolism of amino acids and thus suggested to play different and essential roles in carbon and nitrogen metabolism in plants, knock out mutants and transgenic plants carrying RNA interference construct were generated and characterized. The effect of silencing GDH on carbon and nitrogen metabolisms was investigated, especially the level of carbohydrates and the amino acid pool were further analysed. It has been shown that GDH expression is regulated by light and/or sugar status therefore, phenotypic and metabolic analysis were developed in plants grown at different points of the diurnal rhythm and in response to an extended night period. In addition, we are interested in the effect of nutrient availability in the transition from vegetative growth to flowering and especially in nitrate as a metabolite that triggers widespread and coordinated changes in metabolism and development. Nutrient availability has a dramatic effect on flowering time, with a marked delay of flowering when nitrate is supplied (Stitt, 1999). The use of different mutants and transgenic plants impaired in flowering signalling pathways was crucial to evaluate the impact of different nitrate concentrations on flowering time and to better understand the interaction of nitrate-dependent signals with other main flowering signalling pathways. Plants were grown on glutamine as a constitutive source of nitrogen, and the nitrate supply varied. Low nitrate led to earlier flowering. The response to nitrate is accentuated in short days and in the CONSTANS deficient co2 mutant, whereas long days or overexpression of CONSTANS overrides the nitrate response. These results indicate that nitrates acts downstream of the known flowering signalling pathways for photoperiod, autonomy, vernalization and gibberellic acid. Global analyses of gene expression of two independent flowering systems, a light impaired mutant (co2tt4) and a constitutive over-expresser of the potent repressor of flowering (35S::FLC), were to be investigated under two different concentrations of nitrate in order to identify candidate genes that may be involved in the regulation of flowering time by nitrate.
Each organ of a multicellular organism is unique at the level of its tissues and cells. Furthermore, responses to environmental stimuli or developmental signals occur differentially at the single cell or tissue level. This underlines the necessity of precise investigation of the “building block of life” -the individual cell. Although recently large amount of data concerning different aspects of single cell performance was accumulated, our knowledge about development and differentiation of individual cell within specialized tissue are still far from being complete. To get more insight into processes that occur in certain individual cell during its development and differentiation changes in gene expression during life cycle of A. thaliana leaf hair cell (trichome) were explored in this work. After onset of trichome development this cell changes its cell cycle: it starts endoreduplication (a modified cell cycle in which DNA replication continues in the absence of mitosis and cytokinesis). This makes trichomes a suitable model for studying cell cycle regulation, regulation of cell development and differentiation. Cells of interest were sampled by puncturing them with glass microcapillaries. Each sample contained as few as ten single cells. At first time trichomes in initial stage of trichome development were investigated. To allow their sampling they were specifically labelled by green fluorescent protein (GFP). In total three cell types were explored: pavement cells, trichome initials and mature trichomes. Comparison of gene expression profiles of these cells allowed identification of the genes differentially expressed in subsequent stages of trichome development. Bioinformatic analysis of genes preferentially expressed in trichome initials showed their involvement in hormonal, metal, sulphur response and cell-cycle regulation. Expression pattern of three selected candidate genes, involved in hormonal response and early developmental processes was confirmed by independent method. Effects of mutations in these genes on both trichome and plant development as well as on plant metabolism were analysed. As an outcome of this work novel components in the sophisticated machinery of trichome development and cell cycle progression were identified. These factors could integrate hormone stimuli and network interactions between characterized and as yet unknown members of this machinery. I expect findings presented in this work to enhance and complement our current knowledge about cell cycle progression and trichome development, as well as about performance of the individual cell in general.