Totipotent stem cells allow plants to build new organs throughout their whole life. But it has been unclear how hormones and genetic factors work together to prevent plants from having growth that is either stunted, or uncontrolled and tumor-like. Scientists from the Max Planck Institute for Developmental Biology have now uncovered a feedback mechanism, involving a growth-enhancing hormone and a regulatory protein, which controls the number of stem cells the plant produces. (Nature, December 22, 2005). The results are of great importance for all of stem cell research.
All above ground parts of a plant - leaves, stem, flowers, and seeds - ultimately are derived from cells of a small tissue at the tip of the shoot. Biologists call this tissue the "apical meristem", and it contains totipotent stem cells that are active throughout the life of the plant. Unlike the stem cells of animals, which can only produce specific kinds of tissue after the animal is past its embryonic stage, plant stem cells remain their totipotency and, therefore plants can continue growing over many years, developing new organs.
But this ability comes at a price. If the number of meristematic stem cells increases too quickly, then there could be uncontrolled growth, similar to cancer. On the other hand, if the stem cell pool shrinks too quickly, the plant could have stunted growth. In order to stay alive and reproduce, the plant needs to find the right balance in the number of its stem cells. Two regulatory mechanisms were found to be important for this process. The first involves growth-promoting hormones like auxin and cytokinin, known already for more than half a century. The second involves genetic factors, which were discovered at the University of Tübingen, Germany about a decade ago. Here it was shown that a gene called WUSCHEL has a key influence on how many cells in the apical meristem actually stay stem cells. However, until now, it was unclear how hormones and regulatory genes, such as WUSCHEL work together to maintain this fine balance at the tip of the shoot.
The working group led by Dr. Jan Lohmann at the Max Planck Institute for Developmental Biology in Tübingen, Germany has now solved this problem. The object of investigation was Arabidopsis thaliana, the "favorite plant" for molecular and genetic research, whose genome was sequenced years ago. Lohmann’s team now carried out elaborate genetic and biochemical experimentation, and thereby identified four genes, which might serve as a mechanistic connection between plant hormones and the genetic regulatory elements in meristem.
The researchers in Tübingen used gene expression analysis to show that the genes ARR5, ARR6, ARR7 and ARR 15, "Arabidopsis Response Regulators", are subject to genetic regulation via the WUSCHEL gene. In particular, WUSCHEL restricts the activity of ARR7 in the apical meristem. The ARR genes in turn carry out a particularly important task in hormonal regulation: they are part of a negative feedback loop, by which the growth-inducing plant hormone cytokinin limits its own influence. The study shows that the ARR genes play a direct role in regulation of the stem cell pool.
The hormone itself instigates the meristematic stem cells to split; at the same time, it activates various ARR genes, which break the cytokinin signal chain. Jan Lohmann explains that "WUSCHEL supports the cytokinin effect by stopping its negative feedback." That is also the reason for earlier observations, that Arabidopsis samples with defective WUSCHEL genes only develop very small meristems, and have trouble growing. The researchers in Tübingen have now discovered the same effect in mutants whose ARR7 gene is constitutively active.
Cytokinin can only have its full growth-promoting effect in tissue in which the WUSCHEL regulatory gene is active. "Meristematic regulation is a fabulous example of how the effects of free circulating hormones can be limited to a particular tissue," Lohmann says. Only with this kind of mechanism, is it possible that the same hormone has different effects in different tissues, depending on which genetic conditions it encounters.
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