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Research team rewriting book on photosynthesis

PULLMAN — Toss out that old biology book you were hanging onto for reference. The chapter on photosynthesis is about to be rewritten, thanks to work by David Kramer and his research team at Washington State University’s Institute of Biological Chemistry.

Using instruments they designed and built themselves, Kramer and his group have discovered that plants adjust their light intake to match their metabolic needs by regulating the level of protons within sealed chambers in the chloroplasts.

Traditionally, plant physiologists have focused on the role of electrons in photosynthesis. Kramer’s lab has changed the field by looking at what’s happening with the protons inside chloroplasts in intact living leaves.

“That’s a whole half of the way that the plant works that had never really been observed in a living plant,” he said.

Their work also provides the first clear demonstration of a new link between what have usually been called the light reactions and dark reactions. Kramer’s latest work, co-authored by Thomas Avenson, Jeffrey Cruz and Atsuko Kanazawa, was reported in the June 20 edition of the Proceedings of the National Academy of Sciences.

Green plants use energy from light to split water molecules into protons, electrons and oxygen. Textbook diagrams of the process show the electrons skimming through a series of chlorophylls and other photosynthetic pigments, eventually producing an energy-storing compound called NADPH. At the same time, the protons move into sealed compartments within the chloroplasts. The resulting energy gradient powers an enzyme called ATP synthase, which makes the energy-storing compound ATP. These are the “light reactions,” so called because they can only occur in the presence of light.

The “dark reactions,” which may occur either in light or dark, use the energy stored in NADPH and ATP to drive the assembly of carbon dioxide into sugars. Together, the light and dark reactions create nourishment for the plant and, through the food chain, human beings and all other animals as well.

Despite their key role in supporting most forms of life on the planet, the link between light reactions and dark reactions has eluded scientists. A plant somehow balances them so it incorporates energy at a rate suited to its needs. If a plant takes in too much light energy, it could die; if it takes in too little, it will not thrive as well as it could.

Kramer’s team showed that a plant achieves that balance by changing the level of protons within the sealed compartments. When the compartments fill to a certain level, the photosynthetic machinery becomes less responsive to light.

Kramer likens the process to filling a bathtub. You can vary the water level by turning the faucet or by opening or closing the drain. His research shows that plants do it at the drain; and the drain, in this case, is the enzyme ATP synthase.

“It is as if the hole just gets smaller,” said Kramer. “The system is still working fine, but it’s like plugging up a little bit of your bathtub drain.”

Their hand-made instruments allowed Kramer and his colleagues to deliver a pulse of light to a leaf and then sense slight changes in the color of light emitted by the leaf in response. That let them track protons moving in and out of the compartments.

They found that when a plant needs to slow its rate of photosynthesis, its ATP synthase lets fewer protons leave the compartments. The protons pile up, and photosynthesis slows or stops. When they looked at a mutant strain of the plant that didn’t down-regulate well, they found that protons didn’t accumulate to the level needed to trigger a shut-down.

“That’s a big story, because we think we now understand what the connection is between what we call the light reactions and the dark reactions,” Kramer said. “Nobody knew. We’ve found maybe the major regulatory connection between the light and dark reactions.”

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