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How plants develop their axis

When does a plant embryo decide which cells will later become the shoot and which the roots? Professor Dr. Thomas Laux from the University of Freiburg and his team of researchers have just shown that this already happens at the two-cell stage of the plant embryo. The researchers have discovered genes that play a key role in the development of the polar axis of Arabidopsis plants.

The shoot of plants grows upwards and the root grows deep into the soil. But how does a zygote determine what becomes shoot and what becomes root? The fate of the plant cells is determined right after the first cell division. The zygote divides asymmetrically. The apical cell remains small and divides into different directions, giving rise to a three-dimensional embryo. The large basal cell expands and divides horizontally, thereby forming a thread-like structure (suspensor) which connects the embryo with the maternal tissue. The top cell of this cell thread develops part of the pre-root tissue. The development of a root on one end and a shoot on the other end of the embryo is a complex process that needs to be controlled. But how? The researchers led by Prof. Dr. Thomas Laux from the Department of Developmental Biology and Biotechnology of Plants at the University of Freiburg has now found the answer to this question. “Three members of a gene family discovered at our institute play a key role in this tightly regulated process,” said Laux. “These genes are active very early and affect the subsequent processes.”
Professor doctor Thomas Laux with an Arabidopsis thaliana plant, a very inconspicuous, but popular laboratory plant.
Prof. Dr. Thomas Laux with an Arabidopsis thaliana plant, a very inconspicuous, but popular laboratory plant. (Source: Matthias Nawrat)

Elementary developmental steps

A few years ago, Laux and his former research team discovered the gene Wuschel, which became the eponym for the family of WOX genes (Wuschel-related homeobox genes). A homeobox is a DNA sequence found within genes that are involved in the regulation of patterns of development in plants and animals. All proteins derived from WOX genes are most likely transcription factors and can thus regulate the transcription rate of other genes. The lack of Wuschel genes in Arabidopsis thaliana results in shoot deformations. The plant lacks stem cells at the end of the shoot where a pool of stem cells is normally stored. “Wuschel is a factor that turns some cells into shoot stem cells,” said Laux. “At that time we did not know that the other members of this gene family had a similarly important role.”
Early Arabidopsis thaliana embryo; meristem tissue is dyed yellow; the stem cells of a stem cell niche are dyed blue. (Photo: Work group Prof. Dr. Thomas Laux)
A recent paper, published by Laux and his former doctoral students Holger Breuninger and Enno Rikirsch, shows that WOX genes are also active in the early embryo. After the first division of the zygote, the WOX2 gene is expressed in the apical cell; the WOX8 and WOX9 genes are active in the basal cells. WOX2 starts the genetic programmes that lead to the formation of the shoot and WOX8 and WOX9 those for the formation of the root. “The WOX genes are required for the normal development of basal and apical cell lineages and have parallels with the key regulators in anterior-posterior axis formation in animal embryos,” said Laux.

Trees for the industry

Now, Laux and his researchers are working to identify the molecular effectors of the WOX genes in the early embryo. Which molecules control the complex development? Further experiments already suggest that these molecules might affect the distribution of the phytohormone auxin in the early embryo. This hormone is unevenly distributed in the cells of an intact embryo; this is achieved by auxin transport proteins in the cell membranes which “pump” the molecule in one direction. In WOX8/WOX9 double mutants, the production of these transporters is greatly reduced. Auxin is distributed evenly and no polarity is achieved. This might be one of the reasons why a correct body axis does not develop.

“We will carry out further experiments to investigate further molecules that might be involved in this process,” said Laux. The research of the Freiburg researchers already has practical implications: in a project carried out in cooperation with researchers from the University of Nanjing in China, in which the Chinese partners hope to produce economically important trees from somatic embryos, i.e. without fertilisation. This works in Arabidopsis although the plant does not commonly produce somatic embryos. “Understanding how stem cell niches and early embryonic development are regulated enables such practical applications,” said Laux, highlighting the fact that the project seeks to allow the rapid and extensive production of rapidly growing trees. This might be able to compensate for the demand for wood by the Chinese industry - without having to destroy vast areas of forest.

mn – 18 August 2008
© BIOPRO Baden-Württemberg GmbH
Further information:
Prof. Dr. Thomas Laux
Institute of Biology III
University of Freiburg
Schänzlestr. 1
79104 Freiburg
Tel: ++49-(0)761/203-2943
E-mail: laux@biologie.uni-freiburg.de
Website address: https://www.biooekonomie-bw.de/en/articles/news/how-plants-develop-their-axis