Browsing by Subject "Root morphology"

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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.
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    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.