A Key Bridge Needed for Complete Modeling of Photosynthesis Is Established

The Science

Photosynthesis consists of three stages of reactions—photophysical, photochemical, and biochemical. Photophysical reactions harvest photons in light to generate excitation energy in chlorophyll molecules. Photochemical reactions trap excitation energy via electron transport, and biochemical reactions use products from electron transport to assimilate carbon dioxide. The photophysical, photochemical, and biochemical reactions must work collaboratively to convert photon energy in light to chemical bond energy in sugars. Previously, these different stages of reactions could not be modeled together because a model for the middle stage—photochemical reactions—was lacking. In this study, a team of researchers developed a photochemical model of electron transport to improve understanding of light capture to carbon assimilation.

The Impact

With the development of the photochemical model of electron transport, it is now possible to couple previously developed photophysical and biochemical models to model the complete system of photosynthesis. A complete photosynthesis model will enable many advances that have not been possible previously. For example, carbon cycle modelers can now use a broad scope of measurements including fluorometry and gas exchange to improve carbon cycle predictions. Bioengineers can quantitatively determine how components of photophysical, photochemical, and biochemical reactions can be modified to improve the overall efficiency of the photosynthetic machinery.

Summary

A photochemical model of photosynthetic electron transport (PET) is needed to integrate photophysics, photochemistry, and biochemistry to determine redox conditions of electron carriers and enzymes for plant stress assessment and mechanistically link sun-induced chlorophyll fluorescence to carbon assimilation for remotely sensing photosynthesis. Toward this goal, a team of researchers derived photochemical equations governing the states and redox reactions of complexes and electron carriers along the PET chain. These equations allow the redox conditions of the mobile plastoquinone pool and the cytochrome b₆f complex (Cyt) to be inferred with typical fluorometry. The equations agreed well with fluorometry measurements from diverse C₃/C₄ species across environments in the relationship between the PET rate and fraction of open photosystem II reaction centers. The team found the oxidation of plastoquinol by Cyt is the bottleneck of PET, and genetically improving the oxidation of plastoquinol by Cyt may enhance the efficiency of PET and photosynthesis across species. Redox reactions and photochemical and biochemical interactions are highly redundant in their complex controls of PET. Although individual reaction rate constants cannot be resolved, they appear in parameter groups which can be collectively inferred with fluorometry measurements for broad applications. The new photochemical model developed enables advances in different fronts of photosynthesis research.

Principal Investigator

Lianhong Gu
Oak Ridge National Laboratory
[email protected]

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
[email protected]

Funding

This research is supported by the Biological and Environmental Research Program within the U.S. Department of Energy’s (DOE) Office of Science. Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for DOE under contract DE-AC05-00OR22725. Additional support was received from the National Science Foundation Macrosystem Biology (Award 1926488), U.S. Department of Agriculture–National Institute of Food and Agriculture Hatch Fund (1014740), Cornell Initiative for Digital Agriculture Research Innovation Fund, and Ontario Ministry of Agriculture, Food and Rural Affairs for two Alliance Tier 1 Awards (UofG2016-2732 and UG-T1-2021-100932).

References