Gene quartet controls stem cell number in plants

Newly discovered mechanism prevents Verk mmerung or cancerous proliferation

Arabidopsis seedlings © MPG
Read out

Thanks to their stem cells, plants have the ability to constantly form new organs throughout their lives. However, how hormones and genetic factors interact to prevent plants from becoming stunted or cancerous was previously unknown. Max Planck scientists have now deciphered a mechanism by which a growth-promoting hormone and a regulatory protein in plants are linked to control the number of stem cells. As the researchers report in the current issue of the scientific journal Nature, these findings are of fundamental importance for the entire stem cell research.

All aboveground parts of a plant - leaves, flowers, stems, seeds - ultimately arise from a tiny area of ​​tissue at the top of the shoot. This region, referred to as a branch meristem by biologists, contains totipotent stem cells that remain active throughout the lifetime of a plant. In contrast to animals that only have tissue-specific stem cells after the completion of embryonic development, plants can therefore continue to grow over many years and form new organs.

Messengers and genes work together

At the same time, however, this ability also poses dangers: If the number of meristematic stem cells increases too quickly, cancerous growths threaten. On the other hand, when the stem cell pool shrinks sharply, the plant atrophies. In order to remain viable and to ensure their own reproduction, the plant must therefore exactly balance the number of stem cells. As you know today, this is done through two sets of rules: on the one hand via growth-promoting plant hormones such as auxin and cytokinin, which have been known for more than fifty years.

On the other hand, genetic factors also contribute to stem cell regulation. About ten years ago, also in Tübingen, a central control gene called "Wuschel" was discovered, which has a decisive influence on how many cells remain as stem cells in the branch meristem. However, it has been puzzling how hormones and genes work together to maintain the fine balance in the shoot tip.

This puzzle has now been revealed by the research team led by Jan Lohmann at the Tübingen Max Planck Institute for Developmental Biology. The object of study was the "house plant" of botanical research, the thale cress Arabidopsis thaliana, whose genetic material was completely deciphered several years ago. Using elaborate genetic and biochemical experiments, Lohmann and his team have now identified four genes that can serve as a mechanistic link between the plant hormones and the genetic control elements in the meristem. display

Wuschel gene controls activity

As the gene expression analyzes of the T binger researchers show, the Arabidopsis Response Regulators (ARR) genes ARR5, ARR6, ARR7 and ARR15 are genetically engineered by the Wuschel gene. Under his influence, especially the activity of ARR7 in the branch meristem is significantly reduced. The current study proves that the ARR genes are directly involved in the genetic regulation of the stem cell pool. At the same time, however, they also fulfill an important task in the hormonal regulation: they are part of a negative feedback loop, with which the growth-promoting plant hormone cytokinin limits its own effect.

The hormone itself stimulates the division of the meristematic stem cells; however, it simultaneously activates several ARR genes, which in turn disrupt the cytokinin signaling pathway. "Wuschel supports the cytokinin effect by preventing its negative feedback, " explains Lohmann. This also explains the earlier observation that Arabidopsis specimens with a defective Wuschel gene form only very small meristems and are disturbed in their growth. The T binger researchers now also found the same effect with mutants whose ARR7 gene was overactive.

Accordingly, cytokinin can only fully develop its growth promoting activity in tissues in which the Wuschel control gene is active. "Meristematic regulation is an excellent example of how the effect of free-circulating hormones on certain tissues can be limited, " says Lohmann. It is only through such mechanisms that it becomes possible for one and the same hormone to produce different effects in different tissues - depending on which genetic conditions it meets there.

(idw - MPG, 22.12.2005 - DLO)