Plants know how to avoid shade

Researchers from the University of Freiburg demonstrate in the journal CELL how plants shift the action peak of a light receptor from red to far red, allowing them to grow in the shade of other plants.

Light affects many processes in the life of plants. They not only need to sense whether or not the sun is shining, but many different reactions are also triggered depending on the intensity and composition of light. Plants take advantage of the fact that in the shade of other plants the light composition is altered because neighbours filter the blue and red rays of sunlight, leaving a higher fraction of far-red light. They are able to sense the composition of light through their photosensitive proteins, so-called photoreceptors.

The red region of the light spectrum is absorbed by the phytochromes, which exhibit maximal absorption in red light. However, several decades ago, a biologist from the University of Freiburg, Prof. Dr. Karl-Max Hartmann, demonstrated that the maximal growth inhibition in plants is in the far-red region of the light spectrum. However, the reason for this shift and the fact that growth inhibition strongly depends on light intensity was not unravelled yet.

Using a joint experimental-theoretical approach, researchers at the Universities of Freiburg and Tübingen were able to unravel this phenomenon, called high intensity reaction (HIR). Dr. Julia Rausenberger of Dr. Christian Fleck’s research group at the Center for Biological Systems Analysis (ZBSA) and Dr. Andreas Hiltbrunner from the Center for Plant Molecular Biology (ZMBP) at the University of Tübingen presented their latest research results in the scientific journal CELL.

The results of their research reveal that higher plants have evolved the ability to sense far-red light although they have a photoreceptor that is tuned to respond to red light.

Whereas Prof. Dr. Eberhard Schäfer from the Institute of Biology II and BIOSS published the first theoretical interpretation of the HIR in 1975, the principle mechanism of HIR was still a mystery until now. In the article in CELL, the researchers present the joint experimental and theoretical approaches used to explain the HIR. Dr. Hiltbrunner demonstrated that two helper proteins are necessary for the nuclear trafficking of phytochrome A. In the nucleus, these proteins detach from the photoreceptor and migrate back to the cytoplasm, where they remain ready for further trafficking. Based on the findings of these experiments, the researchers developed a mathematical model for the action of phytochrome A. This model then had to be tested to determine whether it can account for the HIR and to unravel the core reactions required for the action of phytochrome A in far-red light.

Dr. Rausenberger used computer simulations to test the behaviour of the reaction model for a total of one million combinations of reaction parameters. She found out that the reaction model can be used to comprehensively describe the HIR. Further mathematical analysis of the reduced abstract reaction models revealed which key components enable the far-red action of phytochrome A. Finally, the newly identified key components, two antagonistic photoconversion cycles, were found in plants’ reaction network. Genetic and cellular biological methods were employed to show  that the helper proteins for nuclear trafficking connect the two antagonistic  photoconversion cycles.

Title of original publication: Julia Rausenberger, Anke Tscheuschler, Wiebke Nordmeier, Florian Wüst, Jens Timmer Eberhard Schäfer, Christian Fleck, and Andreas Hiltbrunner. (2011). Photoconversion and nuclear trafficking cycles determine phytochrome A’s response profile to far-red light. CELL 146, 5

Contact:
Dr. Christian Fleck
Zentrum für Biosystemanalyse
Universität Freiburg
Tel.: 0761/203-97198
E-Mail: christian.fleck@fdm.uni-freiburg.de