Elusive intermediary: Newly discovered protein may help improve crop yields, solar cells

Photosynthesis is the process used by plants to convert atmospheric carbon dioxide into the energy-rich chemicals upon which all life-forms depend. The energy trapped in these compounds comes from sunlight, and photosynthetic organisms -- plants, algae and certain types of bacteria -- capture this energy in a usable form with the help of protein complexes called photosystems. Photosystems include antenna proteins that collect incident light, and green plants have two sorts of photosystems, which respond best to light of different wavelengths.

A team of researchers at LMU, led by Professor Dario Leister, has now identified a protein named PAM68 that is essential for the assembly of Photosystem II in green plants. The protein is also found in photosynthetic cyanobacteria, but there it serves a different function. "It turns out that PAM68 itself does not form part of the functional photosystem II at all," says Leister. In the longer term, the new finding may make it possible to improve the yields of important crops and might even form the basis for new types of solar cells.

The research is published online in the journal Plant Cell.

Photosynthesis can be thought of as the central pillar of the biosphere, because this set of biochemical reactions provides the oxygen and energy-rich foodstuffs upon which other organisms, including humans, subsist. The energy for the process comes from sunlight, and is captured by molecules that act as solar collectors in photosynthetic organisms, such as plants, algae and cyanobacteria. "All of these organisms possess two different photosystems, each of which responds most efficiently to light of a particular wavelength," says Professor Dario Leister of the Department of Biology I at LMU Munich.

The photosystems consist of light-absorbing chlorophyll pigments and a variety of proteins. "Assembly of these multiprotein complexes takes place in several steps and requires the participation of specific accessory proteins," explains Leister. In their latest study, he and his team set out to identify assembly factors necessary for correct formation of photosystem II in the model plant thale cress (Arabidopsis thaliana) and in the cyanobacterial species Synechocystis. They showed that a previously unknown protein, which they called PAM68, interacts with several of the components of photosystem II and is required to put the functional complex together.

"PAM68 is found in both the plant and the cyanobacterium," Leister points out, "but it has quite different functions in the two organisms." In both cases, the newly discovered assembly factor is essential for the first steps in the construction of Photosystem II. In thale cress mutants that lack PAM68, however, these early intermediates accumulate. Inactivation of the cyanobacterial protein, on the other hand, actually facilitates the assembly of larger complexes. Strikingly, although it is required in the building of Photosystem II, PAM68 is not a member of the fully assembled, functional complex. "This is one case where the whole is less than the sum of the parts," says Leister.

The new work has uncovered common features of plant and bacterial photosynthesis, but also points to distinct differences between the two. "In the long term, a comprehensive understanding of the function of Photosystems I and II should enable us to utilize solar energy more efficiently," says Leister. It could, for instance, contribute to the development of artificial systems that mimic photosynthesis, perhaps leading to new types of solar cell. The new results will also be of interest to agronomists, as they suggest that it should be possible to produce more robust strains of crop plants that can cope with higher levels of light stress and produce better yields. At all events, Leister and his team will continue their quest for the new factors involved in photosystem assembly. (CA/suwe)