Photosystem I -- An Intermediate Step -- May Not Be Necessary For Plants

July 19, 1996

ITHACA, N.Y. -- Plant biologists have long held the view that photosynthesis -- the process by which cells in green plants convert the energy of sunlight into chemical energy and use carbon dioxide to produce sugars -- needs two intermediate light-dependent reactions for successful energy conversion: Photosystem II and Photosystem I.

But according to a new study reported today (July 19), that dogma may have gone for a walk, and scientists may have to rethink some aspects of how photosynthesis works. "Scientists have thought they understood photosynthesis for a long time, and this revelation flies in the face of what we know," said Thomas G. Owens, Cornell University associate professor of plant biology. "What does this all mean? No one knows yet."

The Z scheme is the classic example of how scientists describe the light-dependent reactions of photosynthesis. When photons -- particles of light -- enter a plant cell, their energy is used to power two sequential light-dependent reactions. In Photosystem II (PSII), the light energy is used to split water, producing oxygen and electrons. In Photosystem I (PSI), these electrons are used to produce compounds that ultimately react with carbon dioxide to produce carbohydrates, lipids, proteins and nucleic acids -- the fundamental components of life.

But Owens and researchers from the Oak Ridge National Laboratory in Tennessee found that in some cases photosynthesis can occur in the absence of PSI, they report in the journal Science (July 19, 1996). The report, "Oxygenic Photoautotrophic Growth Without Photosystem I," was authored by James W. Lee, C.V. Tevault and Elias Greenbaum, all of the Oak Ridge National Laboratory, and Cornell's Owens.

The scientists from Oak Ridge used artificially developed mutants of a green alga to find this new life process.

"It is a very important discovery; I didn't believe any of this stuff when it was first reported. I thought it was impossible, but cooperation between the two labs showed it was possible," Owens said.

Owens does not know how this discovery will fit into the larger puzzle of plant biology. As an analogy, he describes that when the laser was first discovered, no one had a clue that it could be used for reading CD-ROM disks or making the grocery store checkout faster. "The framework that we have for evaluating this discovery is incomplete," he said. "No one knows where it will lead."

The Oak Ridge researchers explained they found that mutant algal cells were able to grow and survive in near-normal conditions. "No pun intended, but this sheds new light on photosynthesis," Greenbaum said. "This is speculation, but this could be a crude prototype of primordial photosynthesis."

Wanting to know whether these organisms really lacked the PSI complex, or whether it was working undetected, the researchers investigated a key protein in the PSI complex before and after the experiments. Owens was not able to detect the key PSI protein.

Does this mean that PSI is not necessary for normal photosynthesis? Owens gives an emphatic "no." This is just a biological alternative that had not been found until the Oak Ridge experiments. "It doesn't completely replace our thinking about photosynthesis; it simply demonstrates that our present knowledge is inadequate to explain this surprising observation," he said.

Cornell University

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