The plant hormone auxin plays that role as a signalling molecule during embryo development of the plant thale cress (Arabidopsis thaliana).

Researchers from the Max Planck Institute for Developmental Biology and the University of Tübingen were already familiar with important intervals where auxin exerts its influence. They have now recreated a regulatory network where an increasing concentration of auxin can “switch on” genes for normal embryo development. After a certain point is reached, the genes do not halt their increased activity, even if the auxin concentration drops. Similar switching mechanisms are also known to exist in the animal kingdom.

In an early embryo, auxin increases in the cells located at the top of the embryo, leading to the formation of above-ground parts of the plant. After those cells are formed, auxin is transported into the lower cells. But a full explanation of the exact role of auxin was still needed.

Instead of carrying out experiments with thale cress (Arabidopsis thaliana) embryos, the scientists worked with thale cress protoplasts: living plant cells without a cell wall that offer a less complex environment for study. In test conditions using protoplasts, it is relatively easy to measure gene activity. Using the protoplasts, the scientists tested the effects of a gene-activating factor called MONOPTEROS and its inhibitor BODENLOS.

This and subsequent experiments showed that MONOPTEROS promotes its own production, as well as its inhibitor BODENLOS. A system of two linked feedback loops - and that linked system is controlled by auxin as seen through computer simulations.

“Everything points to the fact that auxin triggers a switch in the system,” says Steffen Lau.

It appears as the concentration of auxin increases, the breakdown of the inhibitor BODENLOS also increases. As a result, MONOPTEROS is weakly blocked. At a specific auxin concentration, the MONOPTEROS-BODENLOS system is boosted to a higher level of activity.

This regulatory mechanism in the embryonic development of plants was not previously understood, and displays similarities to a signalling pathway in embryonic stem cells in mammals.

The Max Planck Institute for Developmental Biology conducts basic research in the fields of biochemistry, genetics, and cell and evolutionary biology. It employs about 325 people and is located on the Max Planck Campus in Tübingen.

Steffen Lau, Ive De Smet, Martina Kolb and Gerd Jürgens from the Department of Cell Biology and Hans Meinhardt, are all from the Max Planck Institute for Developmental Biology in Tübingen. The Max Planck Institute for Developmental Biology is one of 80 research institutes of the Max Planck Society for the Advancement of Science in Germany.