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The dark side of plant growth: the role of plant hormones in apical hook development of Arabidopsis thaliana

Petra Zadnikova UGent (2012)
abstract
In the beginning, there was the Seed. The seed is a small embryo enclosed in a seed coat, from which an entire plant emerges. During seed germination, by which a new plant's life begins, the first organ that appears is the little embryonic root, called the radicle. Through the radicle, the emerging seedling becomes anchored in the ground and starts absorbing water and important nutrients from the soil. Subsequently, the embryonic shoot arises from the seed that consists of cotyledons (preliminary leaves), the hypocotyl (stem of germinating seedlings) and the shoot apical meristem (SAM), the founder cell population on which the entire future life of the plant depends. The just emerged young seedlings are very fragile. Because plants germinate in the soil in the dark, their tissues (especially the SAM) could be damaged when they penetrate the soil to reach the surface and to get into the light. For that reason, etiolated plants have evolved a strategy to protect their fragile tissues from damage by bending their hypocotyls, thus, hiding the important delicate tissues below an arch. This arch, so-called the apical hook, is formed by differential cell division and elongation on the opposite side of the hypocotyl. To guarantee a proper apical hook development, this process must be tightly regulated. Several plant hormones have been identified as the key regulators of cell division and elongation, resulting in differential growth. The individual hormonal action is governed principally interactions with other hormones and their signaling pathways. Although the principles of most plant hormonal interactions have been examined extensively, the molecular basis of the hormonal crosstalk remains largely unknown. The apical hook is a suitable model system to study the hormonal crosstalk that controls differential growth because it is a well described developmental process that can be followed easily in time and is regulated by several hormones, including auxin, cytokinin, and ethylene. Therefore, the aim of this thesis was to investigate the hormonal regulation of differential growth in the apical hook, to characterize the hormonal interaction during this process, and to gain insights into the molecular mechanisms of these interactions. At the start, experimental methods had to be established to study the apical hook development. First, real-time phenotypes were analyzed by means of the continuous monitoring of seedling growth in the dark. By using a camera Canon Rebel T2i-550DH, the whole process of the apical hook development could be recorded. Likewise, the determination of the parameters for the confocal microscopy and the quantitative analysis of green fluorescent protein (GFP) reporter signals for the apical hooks was also a prerequisite. Lastly, a method based on transversal sections of banded hooks with paraformaldehyde fixed GFP-expressing material was instituted. The introductory chapter 1 gives an overview of the current knowledge on the apical hook development, with focus on the role of individual hormones and their crosstalk. Chapter 2 assesses the role of auxin in apical hook development, specifically the contribution of auxin efflux carriers to regulate the asymmetric auxin distribution. Moreover, in chapter 2, the involvement of ethylene during apical hook development is presented. Chapter 3 combines computational and experimental approaches to determine (i) which are the requirements for spatial auxin redistribution during the formation of apical hook, (ii) how does ethylene impact on the auxin redistribution, and (iii) what is the role of cell division during the apical hook development. Subsequently, chapter 4 explores the involvement of cytokinins in the regulation of differential growth in apical hook development. In this chapter 4, the cytokinin and ethylene crosstalk is investigated and their impact on the asymmetric auxin distribution as well. Concluding remarks and future perspectives are presented in chapter 5, with a final summary highlighting the most relevant findings.
Please use this url to cite or link to this publication:
author
promoter
UGent and UGent
organization
year
type
dissertation (monograph)
subject
keyword
cytokinin, differential growth, auxin, hormonal crosstalk
pages
168 pages
publisher
Ghent University. Faculty of Sciences
place of publication
Ghent, Belgium
defense location
Zwijnaarde : Technologiepark (FSVM building, Jozef Schell seminar room)
defense date
2012-11-08 16:30
language
English
UGent publication?
yes
classification
D1
copyright statement
I have transferred the copyright for this publication to the publisher
id
3051602
handle
http://hdl.handle.net/1854/LU-3051602
date created
2012-11-12 18:29:46
date last changed
2013-12-20 08:51:38
@phdthesis{3051602,
  abstract     = {In the beginning, there was the Seed. The seed is a small embryo enclosed in a seed coat, from which an entire plant emerges. During seed germination, by which a new plant's life begins, the first organ that appears is the little embryonic root, called the radicle. Through the radicle, the emerging seedling becomes anchored in the ground and starts absorbing water and important nutrients from the soil. Subsequently, the embryonic shoot arises from the seed that consists of cotyledons (preliminary leaves), the hypocotyl (stem of germinating seedlings) and the shoot apical meristem (SAM), the founder cell population on which the entire future life of the plant depends. The just emerged young seedlings are very fragile. Because plants germinate in the soil in the dark, their tissues (especially the SAM) could be damaged when they penetrate the soil to reach the surface and to get into the light. For that reason, etiolated plants have evolved a strategy to protect their fragile tissues from damage by bending their hypocotyls, thus, hiding the important delicate tissues below an arch. This arch, so-called the apical hook, is formed by differential cell division and elongation on the opposite side of the hypocotyl. To guarantee a proper apical hook development, this process must be tightly regulated. Several plant hormones have been identified as the key regulators of cell division and elongation, resulting in differential growth. The individual hormonal action is governed principally interactions with other hormones and their signaling pathways. Although the principles of most plant hormonal interactions have been examined extensively, the molecular basis of the hormonal crosstalk remains largely unknown.
The apical hook is a suitable model system to study the hormonal crosstalk that controls differential growth because it is a well described developmental process that can be followed easily in time and is regulated by several hormones, including auxin, cytokinin, and ethylene. Therefore, the aim of this thesis was to investigate the hormonal regulation of differential growth in the apical hook, to characterize the hormonal interaction during this process, and to gain insights into the molecular mechanisms of these interactions.
At the start, experimental methods had to be established to study the apical hook development. First, real-time phenotypes were analyzed by means of the continuous monitoring of seedling growth in the dark. By using a camera Canon Rebel T2i-550DH, the whole process of the apical hook development could be recorded. Likewise, the determination of the parameters for the confocal microscopy and the quantitative analysis of green fluorescent protein (GFP) reporter signals for the apical hooks was also a prerequisite. Lastly, a method based on transversal sections of banded hooks with paraformaldehyde fixed GFP-expressing material was instituted.
The introductory chapter 1 gives an overview of the current knowledge on the apical hook development, with focus on the role of individual hormones and their crosstalk. Chapter 2 assesses the role of auxin in apical hook development, specifically the contribution of auxin efflux carriers to regulate the asymmetric auxin distribution. Moreover, in chapter 2, the involvement of ethylene during apical hook development is presented. Chapter 3 combines computational and experimental approaches to determine (i) which are the requirements for spatial auxin redistribution during the formation of apical hook, (ii) how does ethylene impact on the auxin redistribution, and (iii) what is the role of cell division during the apical hook development. Subsequently, chapter 4 explores the involvement of cytokinins in the regulation of differential growth in apical hook development. In this chapter 4, the cytokinin and ethylene crosstalk is investigated and their impact on the asymmetric auxin distribution as well. Concluding remarks and future perspectives are presented in chapter 5, with a final summary highlighting the most relevant findings.},
  author       = {Zadnikova, Petra},
  keyword      = {cytokinin,differential growth,auxin,hormonal crosstalk},
  language     = {eng},
  pages        = {168},
  publisher    = {Ghent University. Faculty of Sciences},
  school       = {Ghent University},
  title        = {The dark side of plant growth: the role of plant hormones in apical hook development of Arabidopsis thaliana},
  year         = {2012},
}

Chicago
Zadnikova, Petra. 2012. “The Dark Side of Plant Growth: The Role of Plant Hormones in Apical Hook Development of Arabidopsis Thaliana”. Ghent, Belgium: Ghent University. Faculty of Sciences.
APA
Zadnikova, P. (2012). The dark side of plant growth: the role of plant hormones in apical hook development of Arabidopsis thaliana. Ghent University. Faculty of Sciences, Ghent, Belgium.
Vancouver
1.
Zadnikova P. The dark side of plant growth: the role of plant hormones in apical hook development of Arabidopsis thaliana. [Ghent, Belgium]: Ghent University. Faculty of Sciences; 2012.
MLA
Zadnikova, Petra. “The Dark Side of Plant Growth: The Role of Plant Hormones in Apical Hook Development of Arabidopsis Thaliana.” 2012 : n. pag. Print.