User research at BESSY II: Insights into the visual perception of plants

Inside the 3D-structure of a phytochrome a bilin pigment absorbs the photon and rotates, which triggers a signal.

Inside the 3D-structure of a phytochrome a bilin pigment absorbs the photon and rotates, which triggers a signal. © Jon Hughes

Plants use light not only for photosynthesis. Although the plant cell does not have eyes, it can still perceive light and thus its environment. Phytochromes, certain turquoise proteins, play the central role in this process. How exactly they function is still unclear. Now a team led by plant physiologist Jon Hughes (Justus Liebig University Gießen) has been able to decipher the three-dimensional architecture of various plant phytochrome molecules at BESSY II. Their results demonstrate how light alters the structure of the phytochrome so that the cell transmits a signal to control the development of the plant accordingly.

Plants use light to live, via a process called photosynthesis. Yet, they do use light also by so called phytochromes - special molecules that give plants a kind of sight and can thus control the biochemistry of the cell and the development of the plant. It is now known that phytochromes regulate almost a quarter of the plant genome.

However, it was unclear how phytochromes function exactly: How is the light absorbed? What happens in the molecule afterwards, how is the light signal transmitted?

Prof. Jon Hughes' research group at the Institute of Plant Physiology at the Justus Liebig University Gießen (JLU) has now taken a big step towards understanding this, together with scientists at HZB in Berlin. Their results have been published in the scientific journal "Nature Plants".

Phytochromes: the "eyes" of plants

Phytochromes are turquoise coloured proteins that are able to absorb red and infrared light. Although plants cannot create images of their environment, their phytochromes enable them to perceive extremely weak light and even distinguish colours. They can therefore recognise leaves in their neighbourhood and can react to threats from competitors.

3D-architecture of phytochromes deciphered

The teams from Gießen and Berlin have now succeeded in deciphering the three-dimensional structures of various plant phytochrome molecules. They can see the bilin pigment with which the photon - i.e. light - is absorbed. The chemical bonds between the bilin and the protein can also be identified. Part of the bilin pigment rotates when excited by light energy. This changes the interaction with the protein, so that part of its structure is torn apart and re-formed. These changes, in turn, switch on the signal transmission.

MX-Beamlines at BESSY II
 
The phytochrome structures were created using X-ray crystallographic measurements at the BESSY II synchrotron in Berlin. The researchers from Gießen were able to cause various phytochrome molecules to form microscopic, sapphire-like crystals in small droplets. If these crystals are irradiated with high-intensity X-ray light, as produced at BESSY II, so-called diffraction patterns are obtained from which the 3D structures can be calculated and, with the help of further information, details of the molecular function can be elucidated.

Prof. Hughes thanks the participating scientists in Gießen and Berlin. "With our basic research we want to find out how phytochromes function. We have now taken a big step forward, but there is still a lot to do," said Hughes. "However, we are already able to use genetic engineering methods to modify the phytochrome system of crops in such a way that the plants grow better and better harvests can be achieved.

The work was funded by the German Research Foundation (DFG) through the DFG Collaborative Research Centre SFB 1078 "Protonation Dynamics in Protein Function", which is coordinated by the FU Berlin and in which Hughes' research group is involved.

arö/Uni Gießen