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Control centre for light and cold

Plants grow particularly well under optimal light conditions – but only if they are not under stress. For example, if the soil contains too much salt or if the temperatures are too low, then plant tissue will grow less. Cell biologists have long assumed that information is exchanged between the molecular signalling pathways that mediate light perception and stress tolerance. A group of researchers under Prof. Gunther Neuhaus at the University of Freiburg has now substantiated this assumption and even discovered a potential molecular interface.

Plant cells are not blind. They possess specialised receptor molecules with which they perceive light. They then transfer the information on light intensity and duration to the cell nucleus by way of complicated cascades. Specific nuclear genes react to the information and either induce or suppress plant growth. The plants have also developed similar signalling pathways for the perception of abiotic stress. High salt concentrations, cold or excess dryness lead to a broad range of reactions in the nucleus and protect the cells from damaging effects. The individual networks of signalling molecules have to constantly exchange information for the plant to be able to reduce growth even under optimal light conditions, for example when the soil suddenly gets more salty or when the ambient temperature decreases. A discovery by a team of researchers under Prof. Gunther Neuhaus at the Institute of Biology II at the University of Freiburg has now confirmed the cell biologists’ long-standing assumption with its investigations on the plant Arabidopsis thaliana (thale gress).

Circadian control of a protein

Yeast colonies achieve minimal growth at high salt concentrations (left row); recombinant yeast cells with the two salt tolerance genes, STH and STO (central and right row), have fewer problems with elevated salt concentrations. (Figure: Work group Prof. Gunther Neuhaus)
Neuhaus and his former work group in Zurich were looking at how calcium and calcium-binding proteins regulate abiotic stress in yeast, when they accidentally came across salt tolerance in plants. They switched off the calcium channel gene in the yeast membrane and thus undermined the yeast’s capacity to tolerate high salt concentrations. As a result, the yeast cells grew less under salt stress. They subsequently transferred different Arabidopsis genes into the yeast cells and some of these genes “saved” the life of the salt-sensitive mutants. The cells were able to grow normally despite high salt concentrations. The researchers then concentrated on one of these genes – the SALT TOLERANCE (STO) gene. “It was pure chance that we picked STO,” said Neuhaus. “However, since then we have found out that this gene can do a lot more.”

After moving to the Department of Cell Biology at the University of Freiburg, Neuhaus and his group of researchers led by Dr. Marta Rodríguez Franco wanted to investigate the role of STO in Arabidopsis, from where it originated.
Arabidopsis seedlings grow badly if the STO gene is switched off (the four seedlings in the middle) (Figure: Work group Professor Gunther Neuhaus) © Work group Professor Gunther Neuhaus
Although a Japanese team had already shown that the gene also conferred salt tolerance in Arabidopsis, the researchers nevertheless faced an unexpected problem. They were unable to show that elevated salt concentrations led to more protein products of the STO gene in the cell. Instead, they found that light affected the quantity of STO protein, which accumulated during the day and then vanished at night. Thus, Neuhaus and his group of researchers discovered the circadian rhythm of the protein and assumed that the protein was also involved in the light-signalling pathway. It soon became possible to prove this assumption.

Integration of cold and light

“In the meantime, we have found out that STO is a negative regulator of light-induced plant growth,” said Neuhaus. “During the day, it inhibits the signalling pathway that is normally induced by light and which stimulates plant growth. In other words, it blinds the plant.” At night, specialised enzymes degrade the STO gene product. Detailed analyses of the STO protein in the test tube also showed that the protein bound and most likely regulated genes that controlled the plants’ reaction to cold. The Freiburg researchers were fascinated because this finding placed STO at the interface of two signalling pathways. It seemed that they had discovered a molecular control centre that sets information from the light and cold signalling chains off against each other.
“We do not yet know how the protein integrates information about cold stress into the light-controlled signalling pathway,” said Neuhaus. “But we have started working in cooperation with a number of specialist research groups to find out more about this.” One of these groups is in Finland and is investigating the cellular signalling mechanisms initiated upon cold shock. The “light specialist group” is in closer proximity to Neuhaus’ group who only have to go down one floor to talk to the plant light perception experts led by Prof. Eberhard Schäfer. The cell biologists under Neuhaus are working with Schäfer’s group in the “Centre for Applied Biosciences” (ZAB) in Freiburg that is currently managed by Neuhaus.
The goal of the research groups is to clarify the molecular processes occurring during the interaction of the two signalling pathways. “However, success is still a long way off,” said Neuhaus, in the belief the investigations they are carrying out will slowly but surely bring them closer to their goal.

mn – 17th June 2008
© BIOPRO Baden-Württemberg GmbH
Further information:
Prof. Dr. Gunther Neuhaus
Cell Biology 
Institute of Biology II
University of Freiburg
Tel.: +49 (0)761/2032673
Fax: +49 (0)761/2032675
E-mail: gunther.neuhaus@biologie.uni-freiburg.de
Website address: https://www.gesundheitsindustrie-bw.de/en/article/news/control-centre-for-light-and-cold