Photosynthesis generates adenosine triphosphate (ATP), which is the universal molecular fuel in living organisms. An international research team led by Dr Boon Leong Lim from the School of Biological Sciences of The University of Hong Kong could visualize ATP concentrations in chloroplasts and cytosol of living plants. The team included research groups from Sweden, USA and Germany. The Swedish participant was professor Per Gardeström from UPSC, Department of Plant Physiology at Umeå University. The results highlight how different parts of the photosynthetic cell are interconnected in order to optimize the efficiency of photosynthesis and is of interest for future crop breeding. The study is now presented by the journal PNAS.
All life on earth ultimately relies on energy from the sun and photosynthesis in plants is the vital link. The researchers around Boon Leong Lim showed that in mature plants the chloroplastic ATP pool is separated from the rest of the cell. A surplus of reducing equivalents can be exported from the chloroplast and used by mitochondria to supply ATP to the cytosol but the rate of ATP import into mature chloroplasts to support CO2 fixation was negligible. Only chloroplasts of very young developing leaves of Arabidopsis thaliana could import ATP from the cytosol to support their development. This developmental transition could be important in order to restrict futile ATP consumption at night when photosynthesis is not operating.
“We saw a significantly lower concentration of ATP in the chloroplast than in the cytosol of mature photosynthetic cells,” said study lead author Dr Boon Leong Lim. “Although the chloroplast is the key energy harvester and producer in a plant cell, its demand for ATP is also extremely high. Illumination increases chloroplast ATP concentration instantly, but it drops to a basal level very quickly after illumination stops. Our results suggest that there was a need to restrict ATP consumption in mature chloroplasts in the dark. A primary job of mature mesophyll chloroplasts is to harvest energy and export sugar to support plant growth in the light. Nevertheless, wasteful energy consumption must be avoided in the dark.”
Co-authors Dr Wayne K. Versaw and Abira Sahu of Texas A&M University stated: “Live imaging of intact plants provided the spatial and temporal resolution to reveal important changes in how different cell compartments collaborate to manage photosynthesis and overall cellular energy.”
The results also have important implications for the understanding of energy flow in plant cells. Using energy harvested from sunlight, water molecules are split into protons, oxygen and electrons. The electrons pass through photosystems to reduce NADP+ to NADPH that acts as a carrier for the electrons. Together with water splitting, this so called linear electron flow (LEF) also creates a pH gradient across the thylakoid membrane, the inner membrane of the chloroplast. This pH gradient is the driving force for ATP synthesis. To fix one CO2 molecule in a chloroplast, 3 ATP and 2 NADPH molecules are consumed. However, only 2.57 ATP molecules per 2 NADPH are generated by LEF. The shortfall of ATP must be met for photosynthesis to operate efficiently.
A paper published in Nature in 2015 (524:366–369) showed that chloroplasts in unicellular diatoms can import cytosolic ATP to support carbon fixation. Chiapao Voon, who joined the lab as a PhD student, explained: “Unlike unicellular diatoms, mature plant chloroplasts are unable to import ATP from the cytosol to supplement the demand for CO2 fixation. Rather, the export of reducing equivalents is the key to maintaining the optimal ATP/NADPH ratio required for photosynthesis. Otherwise, the build-up of NADPH in chloroplasts will impede photosynthesis”.
“The ability to study metabolism in the living cell with a spatial resolution between the different cellular compartments is a big step forward and will significantly increase our understanding on how the cell is operating. I have in particular been interested in the implications for mitochondrial contributions to photosynthetic metabolism” complements co-author Prof. Per Gardeström from Umeå University.
Co-author Prof. Markus Schwarzländer of Münster University added: “The study brings us a step closer to understanding how carefully cells optimize the operating conditions in their different organelles. I find it particularly intriguing how efficiency of plant energy metabolism can be maintained, and how this appears to be dynamically adjusted.”
ATP compartmentation in plastids and cytosol of Arabidopsis thaliana revealed by fluorescent protein sensing
Chia Pao Voon, Xiaoqian Guan, Yuzhe Sun, Abira Sahu, May Ngor Chan, Per Gardeström, Stephan Wagner, Philippe Fuchs, Thomas Nietzel, Wayne K. Versaw, Markus Schwarzländer, Boon Leong Lim
Proceedings of the National Academy of Sciences (PNAS) Oct 2018, 201711497; DOI: 10.1073/pnas.1711497115
Link to the publication
Live images of a plastid-localized ATP sensor in an Arabidopsis seedling. Red and green panels show the emission of the ATP sensor at 470 nm – 507 nm, and 526 nm – 545 nm, in a 3-day-old seedling. The ratio between both emission channels is represented in a rainbow color scale in the lower left panel, which corresponds to ATP concentration (higher levels in red and lower levels in green). The lower right panel shows a brightfield image of the same seedling.
For questions please contact:
Professor Per Gardeström
Department of Plant Physiology
Photo: Chia Pao Voon
Text: Boon Leong Lim, Chia Pao Voon, Wayne K. Versaw, Per Gardeström, Markus Schwarzländer