Take Two!

Max Planck researchers in Cologne have discovered how protein kinases in plants regulate adaptation to changing light conditions

October 20, 2005

All life on earth depends on photosynthesis, a process in which light energy is used to build organic substances. When the amount and proportion of light changes, a plant has to adapt; we distinguish between three different kinds of adaptation. The plant makes changes to its photosynthetic machinery and important "protein gears". In the journal Nature (October 20, 2005), Max Planck researchers have explained how two protein kinases - that is, enzymes which transfer phosphate groups to other proteins - regulate how different kinds of photosynthetic machinery do this adapting and make it possible for the plant to adjust itself better under various lighting conditions.

The protein kinase STN8 is located in the chloroplasts of the leaf. Confocal laser microscopy makes the red marker protein visible (left). In green, the autofluorescence of the chloroplasts (middle). The pictures are superimposed (right).

Photosynthesis is a very complex process, and without it life on earth would be extremely difficult, or possible only for exotic micro-organisms. Life requires two molecules: carbon dioxide and water. The environment contains large amounts of both. A plant sends both of these components through its photosynthesis machinery, where they are combined, and sugar molecules are synthesised. This sweet supplier of energy provides nourishment for the plant and thus, indirectly, all forms of life.

There are three methods by which photosynthesis adapts when light conditions change. The first is short-term adaptation, in which antennas, collecting light, are altered within minutes. The second is long-term adaptation, in which within days the composition and the relationships of the photosystems to each other change. And the third is the phosphorylation of certain proteins of photosystem II, which until now scientists assumed was necessary for the replacement of defective photosynthetic proteins.

A small molecular helper, the protein kinase STN7, is responsible for the first and second form of adaptation. A related kinase, STN8, is responsible for the third. Although the function of STN7 in the first type of adaptation has already been known, the research team from Cologne and Munich, with support from Jena and Dusseldorf, were able to show that STN7 is also necessary for the second form of adaptation. They were also able to clarify the role of the enzyme STN8 for the third kind of adaptation. The team, led by Dario Leister of the Max Planck Institute for Plant Breeding Research, has thus reached a milestone in the research of the adaptation of the photosynthesis mechanism in altered light conditions.

STN8 fundamentally modifies photosystem II, in which it phosphorylates proteins. For a long time, this phosphorylation was considered important for the replacement of defective proteins in the photosystem II. The researchers were able to show, however, that the phosphorylation of proteins of photosystem II is not crucial to their replacement. This has led to a new question: why, then, is this phosphorylation necessary? The researchers hope, in the future, to pursue this line of enquiry, and how the STN7 kinase coordinates the short and long-term adaptation of photosynthesis. In their report, the researchers were able to present the first indications of an answer: phosphorylation of certain photosynthetic proteins seems to be important for the regulation of special genes in chloroplasts and cell nuclei.

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