Institut für Pflanzenernährung

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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.
Physiological, molecular, and epigenetic aspects of early transient nitrogen deprivation recovery in maize
(2024) Hernandez Pridybailo, Andres; Ludewig, Uwe
Maize is a widely cultivated crop and a primary food source for humans and livestock. Along with its substantial contribution to biomass for fuel production, maize is the most produced cereal globally. However, despite its prevalence, improving nitrogen (N) use efficiency in maize presents ongoing challenges. Understanding how maize plants utilize nitrogen is crucial for identifying traits that could aid breeders in addressing this efficiency gap. When plants experience transient stress and return to previous conditions, a recovery phase is initiated, acclimating the plants and potentially enhancing their responses to subsequent stress events. This mild or transient stress is termed "eustress," and its intentional manipulation is referred to as "crop priming." Crop priming, extensively studied in the context of drought, was explored in our previous research, which revealed that nitrogen supplementation during recovery from early transient water deficits enhances priming effects. This highlights the pivotal role of N in priming and that impaired N supply might negatively affect any acclimation to future stresses. Notably, there is limited literature on transient N deprivation, with most studies focusing on plant responses under conditions of maintained N deficiency. Furthermore, epigenetic regulation of plant mineral nutritional responses via DNA methylation has been reported, like many aspects of plant development that are regulated by such mechanism. Epigenetics is the study of phenotypic changes that can be inherited through mitosis or meiosis, which cannot be explained by changes in the DNA sequence. DNA methylation is the covalent modification of cytosines by the addition of a methyl group, which, depending on its location in the genome, can affect genomic stability and gene expression. Previous results have revealed that N deficiency modifies the methylome of maize roots, hypomethylating transposable elements in a nutrient-specific way. Weak correlations between DNA methylation and gene expression were observed, but a deeper insight into how this covalent modification of cytosines is related to plant mineral nutrition is lacking. DNA methylation patterns are heritable in a sort of “epigenetic memory” and may arise after an environmental stimulus. Here, the role of DNA methylation and its relationship with physiological responses during transient N deprivation were studied. The Fast-Flowering Mini-Maize inbred line A (FFMM-A) was chosen for this study because it can be easily grown hydroponically under controlled conditions. Maize ear leaves recovering from initial nitrogen deprivation (without water limitation) exhibited tissue-specific differences in rapidly dividing meristematic and mature photosynthetically active tissues. Through a series of experiments, a 60 h N deprivation period at the V1 stage was established as a nutritional stress that allowed plants to show mild N deprivation symptoms from which the plants fully recovered. Kinematic analysis of the recovering ear leaf revealed a slight decrease in leaf blade length due to reduced leaf meristem size and leaf number, resulting in a reduced cell production rate. Interestingly, the leaf elongation rate (LER) was maintained, causing a higher specific leaf area, which affected the leaf blade structure. Transcriptomic analysis of the cell division and maturation zones of the recovering ear leaf showed altered gene expression related to cell cycle, lipid metabolism, and secondary metabolism, including phytohormone regulation. Notably, cell cycle-related proteins are also involved in DNA repair and may be involved in somatic memory effects. If plants that had been exposed to early transient N-deficiency stress were later subjected to a second N-deprivation stress at the V5 stage, physiological measurements showed that early N deprivation affected upper leaf chlorophyll accumulation during late N-deprivation recovery. These findings suggest that early N deprivation has long-term effects, especially in dividing tissues, from which young and mature tissues are produced by replication. Furthermore, I identified that the loss of DNA methylation makes maize plants more susceptible to early N deprivation. Loss-of-function zmet2 allele was used in this study. Examination of the methylome of the zmet2 and zmet5 mutants provided insights into the relationship between DNA methylation patterns, nitrogen responses, and the expression of developmentally regulated genes in the leaf. These alleles correspond to CHROMOMETHYLASE3-like maintenance methyltransferase genes, and their mutation causes generalized genome hypomethylation, mostly in the CHG and CHH contexts. After a 96 h N deprivation period at the V1 stage, the zmet2 mutants failed to modify their root-to-shoot ratio and to maintain the LER, with respect to the isogenic B73 line. Moreover, machine learning identified interesting clusters according to their expression patterns from the leaf transcriptome of the dividing and maturation zones of FFMM-A plants. Reanalysis of publicly available datasets from B73 seedling leaf methylation patterns and those of the zmet2 and zmet5 mutants was used to compare the expression patterns of the gene clusters, their methylation patterns, and selected genome features. Interestingly, the methylation pattern of the gene body was highly correlated with the level of expression in the dividing zone of the developing ear leaf. Collectively, mutants in the methylation maintenance pathway, rather than being susceptible to N deprivation, failed to develop symptoms of recovery from such stress, which might be related to the regulation of N metabolism-related genes by DNA methylation. Additionally, the closely related gene expression pattern in the dividing zone and intron non-CpG methylation suggests an intimate role of maintenance of DNA methylation in N nutrition.
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.

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