Schlicht, Markus: Polar Auxin Transport And Auxin Induced Development : Root System And Signaling Molecules Give The Clue. - Bonn, 2008. - Dissertation, Rheinische Friedrich-Wilhelms-Universität Bonn.
Online-Ausgabe in bonndoc: https://nbn-resolving.org/urn:nbn:de:hbz:5N-16357
@phdthesis{handle:20.500.11811/3724,
urn: https://nbn-resolving.org/urn:nbn:de:hbz:5N-16357,
author = {{Markus Schlicht}},
title = {Polar Auxin Transport And Auxin Induced Development : Root System And Signaling Molecules Give The Clue},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2008,
note = {

Nearly every developmental phase and every growth process of a plant is affected by auxin. Auxin is transported over long distance via the phloem but also polarly from cell-to-cell, along the entire plant body; from the shoot tip downwards and from the root tip upwards. This rather unique feature is not shared by any other known signaling molecule in plants. Auxin transport is always directional, energy dependent, and substrate specific. The mechanism of polar auxin transport (PAT) is described by the chemiosmotic model, proposed some 33years ago by Raven and independently by Rubery and Sheldrake. Since then it has obtained paradigm status receiving only marginal modifications by the discovery of some molecular components such as auxin influx and efflux transporters. However, the current study is describing and discussing observations, which contradict several predictions of the classical chemiosmotic theory of PAT, which makes it desirable to update our view of cellular auxin efflux.
For example, the chemiosmotic theory cannot explain the rapid inhibition of PAT by Brefeldin A, an inhibitor of secretion. Under these conditions, i.e., while PAT is inhibited, PIN auxin transporters are still present at the plasma membrane. This observation is the motivation and starting point in the current study to investigate the relationship between PAT and endosomal membrane recycling with some consideration of auxin action on root hair formation, and upstream effectors of auxin such as D´orenone and indole butyric acid (IBA).
A new IAA specific antibody has been used here to re-investigate several key aspects of polar auxin transport. The conclusions from this work are: (1) Endosomes and vesicle recycling are essential parts of the auxin transport machinery. Auxin is enriched at cross wall domains (end poles) of IAA transporting cells, but not of cells impaired in PAT either due to inhibitors or to genetic lesions. (2) The mere presence of PIN proteins at the plasma membrane is not sufficient to sustain auxin transport. (3) Continuous F-actin-dependent vesicle recycling between the plasma membrane and endosomes is necessary for polar positioning of PIN.
It is further shown, that auxin transport provides vectorial information for the localization of root hair initiation sites close to apical ends of hair-forming cells (ends that are oriented towards the root tip). Once initiated, root hair tip growth is based on a threshold level of auxin in the trichoblasts. Auxin export out of trichoblasts is driven by the PIN2 auxin efflux transporter to the next basal epidermis cell. This study provides evidence, that one cleavage product of ß-carotene, called D’orenone, inhibits root hair formation and modifies the root architecture. Analysis of cellular changes after D´orenone treatment reveal an effect of this substance on PAT. In the D´orenone treated roots more PIN2 protein is found in root apices and the PIN2 expression zone is increased in size and shifted basally (away from the root apex). Root hair growth inhibition by D´orenone is rescued by externally applied IAA. Moreover, PIN2 knock-out mutants are insensitive towards D´orenone. Taken together, D´orenone targets root hair growth by upregulating PIN2 and thereby increasing the rate of PAT.
It is important to note, that auxin based modulation of root architecture involves second messengers such as reactive oxygen species and reactive nitrogen species (ROS/RNS). It is shown here, that mutants which are insensitive to the naturally occuring auxin, indole butyric acid (IBA), proove to be useful experimental tools to examine the relevance of the ROS/RNS dependent activity mechanism. Apparently there are two pathways involved in the action of IBA. One is that IBA is converted to IAA, which then feeds IAA into intracellular signalling pathways, and the other is that the IBA-conversion induces ROS/RNS production, though in a similar pattern as IAA within root tissues. Both generated IAA and alongside the IBA-to-IAA conversion produced nitric oxide are necessary for any IBA related root architecture altering effects. The comparison of IBA and IAA activity in this study implicates that all mentioned auxin-dependent changes of root growth and development are formed by complex feedback interactions between auxin and stress-related signalling molecules, which together underly environment-dependent phenotypic plasticity of the plant root system architecture.

},

url = {https://hdl.handle.net/20.500.11811/3724}
}

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