Elucidation of electron flow pathways in chloroplasts

In plants, electrons are transferred in chloroplasts using light energy, as a result, they fix carbon dioxide into sugars. A group led by Associate Prof. Jun Minagawa of the Institute of Low Temperature Science successfully elucidated the cyclic electron flow (CEF) - a hitherto unknown pathway of the two modes of electron flow that exist in chloroplasts.

Linear electron flow (LEF) and cyclic electron flow (CEF) are known as modes of electron flow in photosynthesis. In the linear pathway, electrons are transferred from photosystem II (PSII) to NADP+ by way of the cytochrome b6f complex (Cyt b6f) and photosystem I (PSI). In the cyclic pathway, however, the exact pathway of electrons that originate in PSI and then return to PSI is not clear (Fig. 1).

The research group isolated PSI from phytoplankton cells in a controlled study in which energy was supplied only to PSII. The result was that they found a supercomplex with a molecular weight of approximately 1.5 million composed not only of PSI but also of other protein complexes that are encountered in linear electron flow, such as Cyt b6f. They confirmed that when this protein complex, named the Cyclic Electron Flow (CEF) supercomplex, is illuminated, electrons flow between PSI and Cyt b6f next to PSI. This finding reveals that cyclic electron flow takes place in protein complexes where steps in the linear pathway are rearranged to undergo an alternative pathway for the flow of electrons (Fig. 2).

This finding is expected to lead to a breakthrough in the understanding of how plants regulate photosynthesis in order to survive in natural environments as well as be useful in predicting the impact of climate change on the ecology of phytoplankton and in developing farm crops that can grow in harsh environments.

(Fig. 1) The two modes of electron flow

(Fig. 2) Electron flow by way of the CEF supercomplex (cyclic electron flow)

These results were first reported in Nature Vol. 464, p. 1210-1213, 2010 (published online on 4 April 2010). http://www.nature.com/nature/journal/v464/n7292/abs/nature08885.html

Contact information:
Associate Prof. Jun Minagawa
Institute of Low Temperature Science, Hokkaido University
E-mail: minagawa@lowtem.hokudai.ac.jp