Freiburg plant biologist Prof. Dr. Thomas Laux and his research group have published an article in the journal Developmental Cell presenting initial findings on how shoot stem cells in plants form during embryogenesis, the process of embryonic development. Pluripotent stem cells can develop into any type of cell in an organism. In contrast to animals, plants can form completely new organs from their stem cells throughout their lives, which in the case of several species of trees may span a thousand years or more. Stem cell research promises to solve key problems in medicine and plant breeding. However, whereas the function of stem cells for the regeneration of tissue or for growth has been studied in detail, much less is known so far about how stem cells form in the course of embryonic development.

Several years ago, Laux and his team already discovered the transcription factor responsible for the continuous replacement of shoot stem cells in the model plant Arabidopsis, commonly known as thale cress. Called WUSCHEL, this protein is already present in the embryo when the stem cells form. "Much to our surprise, our PhD student Zhongjuan Zhang made the key observation that it is not WUSCHEL but the related transcription factor WOX2 that is responsible for the development of the stem cells," says Laux. All that was previously known about the protein WOX2 was that it controls earlier steps in the pattern formation of the embryo, the phase in which the embryo's cells arrange themselves in a particular structure. Zhang discovered that WOX2 prevents the cells in the region of the embryo in which the stem cells form from differentiating into specialized cell types and thus from losing their unlimited potential for development.

This means that plants follow similar strategies in the process of stem cell development as found in animals. In the case of Arabidopsis, WOX2 regulates the balance between the two plant hormones cytokinin and auxin by allowing relatively large amounts of the former and relatively small amounts of the latter to collect in the progenitor cells of the stem cells. Plant researchers have already been using this mechanism for several decades to regenerate a shoot from a root or a leaf. This method, originally found in tissue cultures, therefore ultimately reflects the same mechanism that evolution already found much earlier to develop stem cells during embryogenesis.

Thomas Laux is head of a laboratory at the Institute of Biology III and a member of the University of Freiburg's Cluster of Excellence BIOSS Centre for Biological Signalling Studies.

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The roots of a plant are constantly growing, so that they can provide the plant with water and minerals while also giving it a firm anchor in the ground. Responsible for these functions are pluripotent stem cells. In order to avoid differentiation and to remain pluripotent, these stem cells are dependent on signals from their neighbouring cells. These signals are generated by only a small group of slowly dividing cells in the so-called quiescent centre inside the root. An international consortium under the leadership of Prof. Dr. Thomas Laux, a biologist from the University of Freiburg, has identified the transcription factor WUSCHEL HOMEOBOX (WOX) 5 as the signal molecule, showing that it moves through pores from the cells inside the quiescent centre into the stem cells. The team of researchers has published their findings in the professional journal Developmental Cell. 'Solving the mechanism by which signals within the root control stem cell activity has implications for the general workings of the stem cell regulation in plants and humans,' Laux said. He also explained that this will allow scientists to study how plant growth adjusts to different environmental conditions, adding that, 'this is a fascinating field of research in the era of climate change.'

Of all the cells in plants and animals, pluripotent stem cells are the most multi-functional. When they divide, they produce two types of daughter cells: some become new stem cells, while others differentiate to replace tissue or form new organs. To maintain its stem cells, the organism generates the signals that block differentiation inside special stem cell niches. These niches are the only place where stem cells can exist. For blood stem cells, for example, the stem cells reside in the bone marrow.

Laux's group of researchers had previously discovered the transcription factor WOX5, which is necessary for generating signals, in the cells of the root's quiescent centre. However, what its precise role is has remained unclear until now. Laux's team studied the stem cells in the model organism of the Arabidopsis plant, or rock cress, which is part of the Brassicaceae family of plants, including mustard and cabbage. Studies have already shown, however, that many of these findings also apply to crops such as rice. When the signal WOX5 enters the stem cells through pores, it binds at specific DNA sequences, the promoters, of target genes and recruits an enzyme via a so-called adaptor protein. This enzyme changes the DNA's protein shell, the chromatin, causing the respective gene to be no longer effectively readable.

But why does WOX5 switch off its target gene CDF4 in stem cells? Laux's team of researchers has shown that the CDF4's function is to initiate the differentiation of the stem cell's daughter cells. If the concentration of the CDF4 protein would be too high in the stem cells, then the stem cells would also be forced to differentiate and the plant would have to stop root growth. Where the concentration of WOX5 is high enough, the stem cell niche is able to maintain the pluripotent stem cells. Where the concentration of WOX5 is low, the concentration of CDF4 rises and the cells differentiate into root tissue. This balance is the secret to the life-long activity of a stem cell niche.

Laux is the head of a laboratory at the Institute of Biology III and a member of the cluster of excellence BIOSS Centre for Biological Signalling Studies at the University of Freiburg.

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