Antennas on the move
Plants can thrive under widely differing light regimes. For instance, at sunny locations in Central Europe, plants are exposed to 180 times more light than they would receive if they were growing on the floor of a tropical rainforest. But the quantity and quality of light available at a given site can also vary considerably during the course of the day. Plants and green algae are known to possess two mechanisms that enable them to adapt to changes in light. In the chloroplasts, the compartments of the cell where photosynthesis takes place, there are two so-called photosystems. Each consists of a core complex and a set of molecular antennas, the light-harvesting complexes. The molecular antennas associated with the two photosystems differ in the efficiency with which they “collect” light of different wavelengths.
If red light with wavelengths around 680 nm predominates, photosystem II (PSII) is preferentially stimulated, because its antennas absorb light energy most efficiently at that wavelength. Within minutes, in what is called a state transition, a fraction of the antenna molecules detaches from PSII and moves to photosystem I (PSI). In this way, the available light energy is evenly distributed between the two photosystems and can be utilized optimally by the plant, says Professor Leister, who leads the research group on “Photosynthesis, Intracellular Signalling and Genome Evolution” at the LMU Munich’s Department of Biology I. Long-term adaptation, in contrast, takes days: Further copies of the core complexes of photosystems I or II, are produced as required.
In cooperation with researchers in Jena and Milan, Leister’s team has now succeeded in dissecting, for the first time, the mechanisms that regulate long- and short-term adaptation. Mutants of thale cress (Arabidopsis thaliana) provided the crucial clues. The antennas in these mutants were either incapable of detaching from PSII or unable to dock onto PS, and consequently could not undergo state transitions. “Nevertheless, we were able to detect long-term adaptation”, says Leister. Clearly, long-term adaptation proceeds independently of the short-term response. The only feature that is common to both processes is their dependence on the protein STN7, an enzyme discovered in Leister’s laboratory a few years ago.
In green algae some 80 % of light-harvesting complexes can migrate between photosystems; in green plants only 20 % possess this capability. “This is why it had long been thought that state transitions are of only minor significance in flowering plants, only coming into play when lighting conditions are poor”, says Leister, who has now refuted this hypothesis. “We looked at plants that were defective either in short- or long-term adaptation, and were also impaired in overall photosynthetic capacity. We found, to our surprise, that plants in which short-term adaptation was inhibited grew much more slowly. “They were less capable of adapting to changes in light, and correspondingly exploited the solar energy much less efficiently.” Thus, flowering plants also depend on short-term state transitions for optimal growth.
The researchers were also able to show that the long-term adaptation response has an effect on gene activity, ensuring that rates of production of the photosystems are modulated as appropriate for the prevailing light regime. This process acts directly upon the transcription of chloroplast genes, but takes longer to influence nuclear gene expression: Here long-term adaptation sets in only after the first step on the way to protein synthesis. All these findings are important pieces of the puzzle of how plants adapt to lighting conditions, says Leister. “This knowledge could help us to develop plants that can grow optimally under demanding light conditions.” (CA/suwe)
“Arabidopsis STN7 Kinase Provides a Link between Short- and Long-Term Photosynthetic Acclimation”;
Paolo Pesaresi, Alexander Hertle, Mathias Pribil, Tatjana Kleine, Raik Wagner, Henning Strissel, Anna Ihnatowicz, Vera Bonardi, Michael Scharfenberg, Anja Schneider, Thomas Pfannschmidt, Dario Leister;
Plant Cell, 25 August 2009;
Prof. Dr. Dario Leister
Department of Biology I
Phone: +49 (0) 89 / 2180 - 74550
Fax: +49 (0) 89 / 2180 - 74599