Institut für Pflanzenernährung

Permanent URI for this collectionhttps://hohpublica.uni-hohenheim.de/handle/123456789/41

Browse

Now showing 1 - 5 of 5

Identification of regulatory factors determining nutrient acquisition in Arabidopsis
(2011) Giehl, Ricardo Fabiano Hettwer; von Wirén, Nicolaus
The acquisition and translocation of mineral nutrients involves the orchestrated action of a series of physiological and biochemical mechanisms, which are, in turn, regulated by nutrient availability and demand. Furthermore, root morphological changes play an outstanding role for nutrient acquisition, especially when the availability of a certain nutrient is low. Although for most nutrients the molecular mechanisms involved in their acquisition from soils have been described, much less is known about the regulatory pathways underlying the uptake and translocation of nutrients in plants. Thus, the main aim of the present study was to characterize root morphological responses to nutrient supply and to identify novel regulatory components. The first part of the present thesis describes the morphological response of Arabidopsis roots to the essential element iron (Fe), which has a particularly low solubility in soils. Relative to a homogenous supply of Fe, localized Fe supply to horizontally-separated agar plates doubled lateral root length without a particular effect on lateral root number. The internal tissue Fe rather than external Fe triggered the local elongation of lateral roots. In addition, the Fe-stimulated emergence of lateral root primordia and root cell elongation was accompanied by a higher activity of the auxin reporter DR5:GUS in lateral root apices. A crucial role of the auxin transporter AUX1 in Fe-triggered lateral root elongation was indicated by Fe-regulated AUX1 promoter activities in lateral root apices and by the failure of aux-1 mutants to elongate lateral roots into Fe-enriched agar patches. Furthermore, a screening was designed to identify novel regulatory components involved in the Fe-dependent stimulation of lateral roots. One member of the GATA family of transcription factors was found to play a role in the local, root-endogenous regulation of lateral root development in response to local supplies of Fe. It was concluded that a Fe sensing mechanism in roots regulates lateral root development by modulating auxin transport. The second part of the thesis describes the use of multi-elemental analyses to identify novel regulators of nutrient accumulation in Arabidopsis. Firstly, it is shown that the disruption of transcription factors expression can lead to significant alterations in the accumulation of one or more nutrients in shoots. In addition, this approach allowed the identification of a so-far uncharacterized transcription factor ? NGAL1 ? that regulates primary root elongation in response to phosphorus (P) supply. The loss of NGAL1 resulted in hypersensitive inhibition of primary root growth under low P and a P-independent increase in lateral root elongation. The results presented here indicate that NGAL1 participates in a signaling pathway that modulates meristematic activity by controlling the expression of important root patterning regulators according to the local availability of P.
Interactions between non-symbiotic N2-fixing bacteria and plant roots in plant-microbial associations
(2009) Calvo Alegre, Olga-Cristina; von Wirén, Nicolaus
The development of biofertilizers on the basis of plant growth promoting rhizobacteria (PGPR) may be a promising approach to partially substitute costly and energy-consuming mineral fertilizers in agricultural plant production and to support agriculture in developing countries. A successful and competitive rhizosphere colonization of PGPR strains has been identified as a prerequisite for the expression of plant growth promoting effects. Apart from a wide range of external factors with an impact on the colonization process, such as soil properties, temperature, soil moisture and fertilization levels, in particular plant-microbial interactions may play an important role for the successful establishment of compatible associations. In this context, certain plant root exudates may act as signals to mediate bacterial responses with importance for root colonisation (e.g. motility and chemotaxis, production of extracellular polysaccharides). On the other hand, the induction of bacterial plant growth promotion may also depend on ability of the host plant to respond to the presence and the activity of the associated bacteria. It was therefore the aim of this thesis to investigate the contribution of putative PGPR to growth and N uptake in wheat plants and characterize the underlying mechanisms in root-bacterial associations. In the first part of this thesis, the contribution of various non-symbiotic diazotrophic rhizobacteria to plant growth promotion and N nutrition has been studied in a series of greenhouse pot inoculation experiments with wheat (Triticum aestivum L.). Different bacterial inoculants, plant genotypes, soil properties, water regimes and N fertilization levels have been varied as factors with potential impact on plant growth promotion by diazotrophs. The contribution of biological nitrogen fixation was assessed by the 15N dilution method. Plant growth and grain yield were influenced by the different N fertilization levels but no stimulation of growth or N uptake was note upon bacterial inoculation. These observations suggested a high degree of specificity or limiting factors, determining a successful plant-microbial association. The second part describes possible mechanisms that may be involved in the establishment of diazotrophs in the rhizosphere of suitable host plants. As an initial step of the colonization process, a targeted movement of the bacteria to the root surface is required and root exudates may act as attractants. Since dicarboxylic acids are known to exert chemotactic activity on diazotrophic bacteria, seed and root exudates of two graminaceous crops (Triticum aestivum L. and Zea mays L.) and for comparison also of a non-graminaceous plant species (Phaseolus vulgaris L.) were collected in hydroponic culture with and without N supply, and organic acid profiles in these root exudates were analysed. Bacterial motility assays were conducted with the major carboxylates detected in the root exudates of the selected plant species and compared to glucose and water, using Brevibacillus reuszeri as a model bacterium. Pure malate, which was found at high levels in root exudates of bean and wheat, and particularly malonate (bean) and t-aconitate (maize) stimulated the motility of Brevibacillus reuszeri as compared with glucose or water. A particularly intense promotion of bacterial motility was recorded in the presence of crude root exudates of wheat and maize plants grown under N limitation, which was not observed for root exudates of bean. However, this was not related with comparable changes of malate or t-aconitate concentrations in the root exudates. In wheat exudates, malate concentrations even decreased in response to N limitation. These findings suggest the presence of specific factors released in root exudates of N-deficient cereals, promoting the rhizosphere colonisation with B. reuszeri. For an identification of the respective factors, a more comprehensive profiling of the root exudates is necessary. In associations with diazotrophic bacteria, host plants are supplied with ammonium by the bacterial partner. This raised the question whether plant ammonium uptake systems have an impact on the efficiency of the association. To address this problem, an antisense approach was conducted with tomato, with characterised ammonium transporters (LeAMT1;1 and LeAMT1;2). The final goal was the inhibition of the ammonium transporters by production of LeAMT antisense lines to study their putative role in plant associations with diazotrophic bacteria. Northern blot analysis revealed a strong repression of LeAMT1;2 expression in three independent antisense lines associated with a lower ammonium uptake capacity under N-sufficient and N-deficient growth conditions. In contrast, LeAMT1,1 expression was only weakly repressed in antisense lines and there was no impact on N uptake. A faster decline of chlorophyll in older leaves indicates a physiological function of LeAMT1;1 and LeAMT1;2 in ammonium uptake and retrieval in shoot and root cells. The absence of consistent effects on N acquisition of the investigated antisense lines limited the suitability of this approach for studies on associations with diazotrophic bacteria.
Iron and ammonium sensing differentially modulate root plasticity in Arabidopsis thaliana
(2010) Lima, Joni Esrom; von Wirén, Nicolaus
Modulation of root system architecture by plants has an impact on water, nutrient acquisition and anchorage during plant development. In a given environment, root plasticity is a favorable feature to react according to abiotic and biotic factors. Under nutrient limited conditions, the root plasticity is essential for a better soil volume exploitation. This response can vary according to the plant species and the given environment in which they evolved. Moreover, nutrient mobility in the soil plays an important role for the response of plants to nutrient limitation. Thus, root plasticity is a nutrient-specific response during plant development. In fact research on the effect of nutrient availability on root system architecture is scarce. Furthermore, the mechanism how plants sense nutrients and the signaling upon nutrient availability remains a challenge. Therefore, identification of which nutrient can affect the root system architecture and investigating the molecular components involved in the signaling pathway is certainly relevant for agronomical practices. The first part of the present work aimed to identify how the root architecture is affected by iron (Fe) supply. Due to the low mobility of Fe in soils, the morphological response of lateral roots from Arabidopsis plants to localized Fe supply and its regulation were investigated. Increasing Fe concentrations in a homogenous or localized supply on separated agar plates enhanced lateral root number in a similar manner. Lateral root length, however, was twofold higher under localized relative to homogenous Fe supply. With further increasing Fe concentrations lateral root length was repressed even though shoot growth was unaffected. In the Fe uptake-defective mutant irt1, the formation of lateral roots required higher local Fe supplies, which restored wild type levels only with respect to the number but not to the length of lateral roots. Moreover, IRT1 transcript levels were strongly enhanced under localized Fe supply. In the frd3-1 mutant, which is defective in root-to-shoot translocation of Fe, lateral root development was similar to wild type plants although frd3-1 shoots were Fe deficient. These results show a differential regulation of lateral root initiation and elongation in response to localized Fe supply and that lateral root elongation is under control of a local rather than a systemic regulatory loop involving the high-affinity Fe transporter IRT1. In the second part of the thesis, a remarkable and an unknown feature of root morphology dependent on localized ammonium supply is described. Arabidopsis plants were able to increase lateral root initiation and higher-order lateral root branching. Since ammonium-stimulated lateral root number or density decreased after ammonium or glutamine supply to a separate root fraction and did not correlate with cumulative uptake of 15N-labeled ammonium, lateral root branching was not purely due to a nutritional effect but most likely a sensing event. Moreover, a detailed investigation has shown that ammonium and nitrate co-ordinate root morphology in an additive and complementary way. By a genetic approach, the ammonium-induced lateral root branching was demonstrated to be dependent on AMT1;3 activity in the root.
The AtIREGs - characterization of a new family of metal transporters in Arabidopsis thaliana
(2009) Kirchner, Silvia; von Wirén, Nicolaus
Essential transition metals are required in all plant cells for the activities of numerous metal-dependent enzymes and proteins, but can become toxic when present in excess. For the detoxification of heavy metals and to adjust to changes in micronutrient concentrations in the environment, plants possess a tightly controlled metal homeostasis network. In this regard, transition metal transporters are of central importance. Many metal transporters have already been identified, but a large number of candidates for heavy metal transport proteins still have to be analyzed at the biochemical level and within the plant metal homeostasis network. Based on the description of the animal IREG1 metal transporter as an iron exporter in vertebrates, a phylogenetic analysis of eukaryote and prokaryote sequences with similarity to IREG1 showed three homologous genes in Arabidopsis, which were named AtIREG1, AtIREG2 and AtIREG3. As these AtIREG family members were candidates for yet uncharacterized metal transporters, the main objective of this thesis was to investigate the physiological function of this newly identified transporter family in plants.
Transcriptional profiling of Bacillus amyloliquefaciens FZB42 in response to seed and root exudates collected under different nutrient regimes
(2010) Carvalhais Costa, Lilia; von Wirén, Nicolaus
Plant growth-promoting rhizobacteria (PGPR) live in close association with plants and improve their growth. Bacillus amyloliquefaciens strain FZB42 is a prominent plant root-colonizing bacterium that is able to stimulate the growth of maize. To decipher the molecular cross-talk between B. amyloliquefaciens and crop plants, an exploratory analysis of the effect of seed and root exudates on the transcriptome of Bacillus amyloliquefaciens FZB42 was performed. Root exudates were collected from maize plants grown in an axenic hydroponic system under nutrient sufficiency or under deficient supply of nitrogen (N), phosphorus (P), iron (Fe) or potassium (K). An analysis of primary metabolites in the exudates was carried out, compared between treatments, and correlated with the transcriptional profiles of Bacillus amyloliquefaciens FZB42 that were gained after incubation of the bacterial culture with the root exudates. Higher exudation rates of citrate were found under Fe deficiency and greater release of ã-amino butyric acid under P deficiency. Based on a negative correlation observed between the average diffusion coefficient of N, P, K, and Fe in soils and the exudation rates of primary metabolites under conditions of N, P, K, or Fe deficiency, it was hypothesized that the exudation of sugars, amino acids and organic acids may reflect the availability and mobility of plant nutrients in soils. In the presence of seed and root exudates collected from nutrient-sufficient plants, genes involved in spore germination, transport and utilization of nutrients, biosynthesis pathways, multidrug transporters, motility and competence development were differentially expressed. In comparison to P, Fe and K, N-deficient maize root exudates caused a more distinguished change in the transcriptome of bacteria when they were in the logarithmic growth phase. During this growth phase, a number of genes coding for ribosomal proteins were down-regulated by N-deficient maize root exudates, indicating that bacterial activity was repressed. Exclusively in the presence of P-deficient maize root exudates, several genes associated to bacterial motility were induced. Moreover, a gene involved in the biosynthesis of the auxin precursor tryptophan was up-regulated by all deficiency treatments. In the transitional growth phase of Bacillus amyloliquefaciens FZB42, several genes were commonly down-regulated in different deficiency treatments. This finding is in agreement with previous studies showing that quorum-sensing and starvation-sensing are integrated to regulate cell entry into the transient phase. Taken together, this is the first study comparing the effect of different nutrient deficiencies on the composition of primary metabolites in root exudates of one plant species and evaluating systematically the transcriptional response of a Gram-positive PGPR to seed and root exudates collected from plants grown under different nutrient regimes. This analysis provides new information about the early communication between plant roots and PGPR and points to involved genes and processes that merit further investigation.

Browse

Browsing Institut für Pflanzenernährung by Person "von Wirén, Nicolaus"