According to Tokyo Institute of Technology, it was discovered that the “light reaction” that occurs in plant photosynthesis is controlled by changes in the calcium ion concentration in chloroplasts due to the intensity of light energy as well as the “dark reaction”. A joint research group at Setsunan University announced. It was newly found that when there is not enough light, the calcium ion concentration rises and the signal transmitters increase, suppressing the light reaction. Plant cells have changed the mechanism of metabolic control brought in by bacteria in symbiosis in ancient times.

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    In the process of evolution, plants coexisted with cyanobacteria in cells (provided by Tokyo Institute of Technology).

Plants use the energy of light to break down water in a light reaction, producing oxygen and two other substances. In the subsequent dark reaction, these two substances and carbon dioxide taken in from the atmosphere are used to make carbohydrates and continue life activities. It is said that the roots of chloroplasts, which are the work sites of photosynthesis, were about 1.5 billion years ago when a type of bacterium “cyanobacterium” entered into primitive plant cells and coexisted.

Photosynthesis is controlled according to the intensity of light. In cyanobacteria, genes for light and dark reactions are written in a single genome. When there is not enough light, the substance “guanosine pentaphosphate (ppGpp)” that conveys information inside the cell is actively produced, and the expression of genes related to the light reaction and the dark reaction is suppressed.

On the other hand, in plant cells, the genes for the light reaction and the dark reaction are written across the genomes of the nucleus and chloroplast, and it was a mystery how the light reaction and the dark reaction are linked. It was known that when there was not enough light, the calcium ion concentration increased and the dark reaction was suppressed. On the other hand, the mechanism of controlling the light reaction was unknown.

Therefore, the research group investigated the photosynthesis of the model plant “Arabidopsis thaliana” used in the experiment in detail. As a result, it was found that ppGpp was actively produced when the calcium ion concentration increased and suppressed the light reaction, and the enzyme that synthesizes ppGpp was also identified. When I made Arabidopsis thaliana lacking this enzyme by genome editing technology, it did not grow well in the situation where the brightness changed.

From this, it was found that, unlike cyanobacteria, plant cells control the light reaction and the dark reaction to be linked by changes in calcium ion concentration.

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    Comparison of light and dark reactions between cyanobacteria and plant cells (provided by Tokyo Institute of Technology)

The results will deepen the understanding of endosymbiotic theory that occurred in the process of evolution, and will also lead to the development of plants that efficiently perform photosynthesis. Associate Professor Shinji Masuda (Plant Physiology) of the Research Group, Tokyo Institute of Technology, said, “It is really interesting how plant cells acquired such a mechanism in the process of evolution, and it will be a subject for future research.” Says.

The research group consists of Associate Professor Masuda and Professor Takashi Shiina of the Faculty of Agriculture, Setsunan University. The results were published in the international journal “Plant and Cell Physiology” of the Japanese Society of Plant Physiology on October 22, and both universities announced on the same day.

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