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uni'wissen 02-2012 ENG

The fertilization of an egg cell triggers a ge- netic program that causes the organism to grow and prepares it for the adverse conditions of the environment. Stem cells play an integral role in this process, not only during the embry- onic stage but throughout the entire life of the organism. They are the generalists in the world of cells: They can transform themselves into oth- er types of cells to form organs or take the place of damaged cells. Doctors and patients are thus pinning their hopes on stem cell therapies for the treatment of diseases like Parkinson’s or impair- ments like paraplegia. The Freiburg biologist Prof. Dr. Thomas Laux has shown that stem cells are not unique to humans and animals, but that they are also present in special niches in plants, where they are regulated by the neighboring cells. In the course of a plant’s life, which in the case of certain trees may last longer than a thou- sand years, the stem cells enable the plant to grow roots under the earth and develop branch- es, leaves, and blossoms. Only 20 years ago, the mechanisms plants use to keep growing and form new organs over long stretches of time were entirely unknown. Laux wanted to use the experience he had gained writing his dissertation on the biochemis- try of animals to solve this mystery. “The preva- lent belief at that time was that plants don’t have any special stem cells – which would be aston- The Developmental Biologist Thomas Laux Is Decoding Genetic Mechanisms That Explain How Plants Grow and Mature Stem Cells Help Plants Sprout by Hinnerk Feldwisch-Drentrup ishing when one considers their capacity for growth,” says Laux. He chose the inconspicuous plant Arabidopsis thaliana, also known as thale cress. It is regarded as a weed but has enjoyed great success in science as a model organism: Thale cress reproduces rapidly, is easy to grow, and has very few genes. A Niche for Stem Cells If plants actually do possess stem cells, they must be located in the meristems. These tissues give rise to new organs, such as blossoms, leaves, stems, and roots. They are thus located at the tip of the roots and shoots as well as in the stem. In order to determine which genes regulate the stem cells, Laux asked himself how plants would develop if they could not form or maintain any stem cells due to a genetic defect. By study- ing classical experiments described in the litera- ture, Laux discovered that the first two leaves generated by a meristem grow without the help of the stem cells, which aren’t activated until the plant has produced additional leaf primordia. Laux thus reasoned that the plant could not con- trol this second step without stem cells and would instead revert repeatedly to the original program and grow two leaves. He found that there actually are mutated versions of thale cress that behave in this manner. These plants are missing a gene that protects a part of their stem cells from differentiating into special cell types. Laux’ team named the gene WUSCHEL after the disheveled outward appearance of the mutated plant – in German, “wuschelig” means shaggy or fuzzy. “Somatic embryogenesis enables breeders to preserve particular varieties in a precise state” 33