The Fight Against Light

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UC Berkeley researchers have recently discovered that a key molecule called zeaxanthin, which is present in all plants, serves as a defense mechanism that carries away excess energy, preventing plant damage.

The researchers were able to identify specifics of the highly sophisticated system of regulating the amount of light absorbed at any time in the process of photosynthesis. Zeaxanthin prevents plants from oxidation damage, which occurs when a plant absorbs too much light. Oxidative damage can ultimately lead to plant death.

Researchers believe that zeaxanthin is a type of carotenoid. In addition to protecting chlorophyll molecules from damage, carotenoids also are necessary for the protein structure of photosynthetic complexes and are able to absorb solar energy in regions of the solar spectrum that chlorophyll cannot. Chlorophyll molecules are present in plants to control photosynthesis. While chlorophyll absorbs light from the sun and then convert this light to chemical energy, excess energy can lead to the formation of reactive oxygen species that decreases in photosynthetic capacity, researchers said.

"Carotenoids are there for protection to stop the chlorophyll molecules from getting damaged. It's made when the light gets too bright and is especially to protect against a situation in which there is too much light." said Graham Fleming, lead researcher of the study, UC Berkeley professor of chemistry, and director of Lawrence Berkeley Lab's Physical Bioscience Division.

"The fact that chlorophyll molecules absorb more energy than they can use at times has been known for a rather long time," said Nancy Holt, a recent doctoral graduate from the UC Berkeley physical chemistry department. "What was not known was the biophysical mechanism by which this occurs."

According to Holt, plants solve the problem of energy overflow by dissipating excess solar energy through an excitation interaction between a chlorophyll molecule and a zeaxanthin molecule.

The zeaxanthin molecule acts as a feedback de-excitation mechanism that stops excitation from reaching the reaction center where oxidizing compounds are formed. The spinach plant and the Arabidopsis thaliana plant were used in the study, since the genetics of these two plants have been worked out. This makes it possible to get mutants that lack the ability to make zeaxanthin or mutants that are stuck with the molecule all the time.

Plant biologists in Sweden compared the growth of mutant Arabidopsis thaliana plants that lack zeaxanthin to normal plants, and found that the mutants produced as little as 50 percent of the number of seeds as the normal plants.

"In ecological terms, this is a huge difference, and the mutants would quickly get ‘weeded out' by natural selection," said Kris Niyogi, associate professor in the UC Berkeley department of plant and microbial biology and faculty scientist in the Physical Biosciences Division at the Berkeley lab.

While it has been long known that plants receive more light energy than they can use, and scientists understand the role of chlorophyll molecules in the dissipation of energy, there had not been any definitive evidence concerning the biophysical mechanism by which the molecules functioned.

"The problem with previous measurements was that they did not utilize high time resolution experiments," Holt said.

Since the excitation interaction occurs so quickly, on the order a few hundred picoseconds, scientists could not detect its mechanism. They used femtosecond laser spectroscopy to measure the process, which allowed them to measure the high sensitivity of the defense mechanism.

"(The defense mechanism) is very sensitive. It even reacts when a cloud goes over them," Fleming said. "What is really remarkable is that we think plants just sit there and absorb sunlight, but they actually are constantly responding and reacting to changes."

While the identity and mechanism of zeaxanthin has been discovered, researchers hope that further studies will allow them to learn where the action of the molecule occurs in plants. They also hope to apply the results to artificial lighting systems.

"Our results have implications for efforts to mimic nature and produce artificial systems for photosynthesis. We need to think about how to engineer regulatory and protective mechanisms for new solar energy systems, and it's very informative to see how nature has solved similar problems," Niyogi said.

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