Hormonal regulation of primary and secondary growth in the root of Arabidopsis thaliana

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http://urn.fi/URN:ISBN:978-951-51-2196-7
Title: Hormonal regulation of primary and secondary growth in the root of Arabidopsis thaliana
Author: Siligato, Riccardo
Contributor: University of Helsinki, Faculty of Biological and Environmental Sciences, Department of Biosciences
Institute of Biotechnology
Date: 2016-06-17
Language: en
Belongs to series: URN:ISSN:2342-3161
URI: http://urn.fi/URN:ISBN:978-951-51-2196-7
http://hdl.handle.net/10138/163086
Thesis level: Doctoral dissertation (article-based)
Abstract: Plants possess the rare capability to shape the own architecture according to biotic and abiotic stimuli received from the environment. Spatially defined groups of cells, called meristems, contribute to the division and differentiation processes continuously occurring inside the organism. Meristems can be classified as primary meristems, if they are specified during embryogenesis, or secondary meristems, if they form from undifferentiated, quiescent cells outside the primary meristems. Primary meristems, like the Root Apical Meristem (RAM) and the Shoot Apical Meristem (SAM), coordinate the apical growth of the plant in opposite directions, while secondary meristems shape the radial architecture, regulating the thickness and branching of the primary root and shoot. Cambium is a secondary meristem which produces the vascular tissues xylem and phloem. Xylem transports water and minerals from the root to the photosynthetic tissues; it comprises lignified dead conducting cells called tracheary elements, living parenchyma cells, and lignified dead cells, called fibres, which confer mechanical support and strength. Phloem distributes glucose, RNA, viruses, and proteins from the photosynthetic sources to the sink cells; it consists of empty living sieve elements, supporting companion cells, and parenchyma cells. In order to investigate the regulation of primary and secondary growth, we developed a new chemically inducible system to control the timing and location of the induction of an effector or gene of interest. This enables us to avoid deleterious effects such as seed lethality or sterility when studying the role of a gene in a particular cell type. For example, the meristem cambium is difficult to access through normal techniques, since mutations affecting cambial cell divisions often inhibit the primary growth, too. We developed the inducible system by combining the Multi-Site Gateway cloning technology with the already extant XVE inducible system. This system was used to perform part of the research presented in the thesis. Phytohormones are involved in virtually every aspect of plant life, from development to stress response. They are small molecules which act cellautonomously or non-cell-autonomously to mediate the majority of developmental and environmental responses and, consequently, the activity of the meristems throughout the plant life cycle. Auxin and cytokinins, which were among the first phytohormones discovered, regulate almost every aspect of plant life, such as the division and differentiation processes occurring continuously in the RAM and SAM. The two phytohormones have long been known to interact, and recent studies have uncovered significant crosstalk on the level of biosynthesis, transport, signalling and degradation. We investigated the dynamic role of auxin in maintaining the balance between division, elongation, differentiation in the RAM of the model organism Arabidopsis thaliana. Our results confirm that an optimal level of auxin response is required for division and elongation, while differentiation mechanisms require just a minimal concentration of auxin to proceed normally. We discovered that auxin and cytokinin responses interact synergistically to specify the stem cells and to regulate the timing of divisions in the cambium of Arabidopsis thaliana. The auxin and cytokinin signalling pathways both have a positive role in triggering secondary growth, but the hierarchy of the crosstalk between them is still unclear. Finally, auxin transported via the AUX1/LAX auxin influx carriers regulates the differentiation of vessel elements in the later stages of root cambium development. In summary, we confirm that auxin and cytokinins behave as master regulators of meristematic activities throughout the root, as the signalling pathways associated with both phytohormones heavily influence primary and secondary growth.
Subject: plant Biology
Rights: This publication is copyrighted. You may download, display and print it for Your own personal use. Commercial use is prohibited.


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