uni'wissen 02-2012 ENG

nowned journal Nature, his team demonstrated that genes similar to WUSCHEL and CLAVATA3 are active in the root, although the structure of the meristems located there differ greatly from those in the shoot. Other research groups dis- covered that this is also the case in the third meristem of plants, which is responsible for causing the stem to grow in thickness. Similar regulatory mechanisms are active in animal stem cells; the difference lies solely in the participat- ing molecules. “That is astounding when one considers that the evolutionary course of ani- mals diverged from that of plants at the unicel- lular stage, where there were not yet any stem cell niches.” The Freiburg scientist and his research team also discovered that genes related to WUSCHEL lead the way in the creation of the embryo as well. When the fertilized egg cell divides, the em- As Laux found out, the neighboring cells of the stem cells use the WUSCHEL gene to pro- duce a transcription factor that influences the transcription rate of other genes. It prevents stem cells from specializing and continuing to di- vide. Even the daughter cells that stay in the niche remain stem cells. “The remaining daugh- ter cells that leave the stem cell niche no longer receive the WUSCHEL signals,” says Laux. “In- stead, other messengers cause them to differen- tiate into specialized cell types.” These daughter cells divide several more times before finally finding their place in a leaf, a root, or the stem, where they fulfill their specific function. Thus, much as in animals their fate is determined by signal substances from the surrounding area of stem cells. However, whereas animal stem cells can only form cells for a specific type of tissue – stem cells in the brain of an adult animal, for in- stance, can only produce specialized brain cells – plant stem cells can still reconstruct any organ even after many years. In humans, this ability is limited to the embryonic stage. Laux and his team devoted their first experi- ments to the shoot, in which the gene CLAVATA3 acts the role of WUSCHEL’s antagonist. “The messengers produced by WUSCHEL and CLA- VATA3 are in balance in healthy plants,” says Laux. “In this way they keep the amount of stem cells constant.” In a study published in the re- “If all we see is that the plant is not doing well after we have manipulated a genetic switch, we do not yet comprehend the underlying relations” In plants stem cell niches are located in the shoot and in the root, in trees also in the trunk. In the stem cell niches, neighboring niche cells form WUSCHEL signals (WUS) that prevent the stem cells from specializing. At the same time, stem cells form the factor CLAVATA3 (CLV3), which prevents too many new stem cells from forming. Cells at the tip of the root protect the stem cells from damage. The stem cell niche of the root under a microscope: The cells colored in turquoise maintain the stem cells located below them by means of WUSCHEL signals. The specialized daughter cells of the stem cells in the root tip are visible in the violet-colored starch granules. Stem cells Stem cells Stem cells Root tip Signal-producing niche cells Signal-producing niche cells 34

uni'wissen 02-2012 ENG

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