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Too Much of a Good Thing

21 January 2005 (All day)
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Sun screen. The carotenoid zeaxanthin (as seen under a microscope) is key to protecting plants from excess sunlight.

Plants are supposed to crave sunlight, but too much sunlight can create potentially deadly free radicals. Now researchers have identified a mechanism plants use to protect themselves. The discovery could aid the development of genetically modified crops that adapt to different light conditions.

Plants harvest energy from the sun with the help of chlorophyll and carotenoid, two photon-capturing molecules. But if the plants are exposed to too much sun, these molecules absorb more energy than they can handle and generate reactive species of oxygen that can destroy the plant. Researchers have known for more than 20 years that plants have a way to protect themselves against this damage, but the actual mechanism has remained a mystery.

It turns out plants use a unique safety valve, according to research published this week in Science. Utilizing instruments that were able to record chemical reactions lasting one millionth of a billionth of a second, a team of researchers from the University of California at Berkeley and the Lawrence Berkeley National Laboratory in Berkeley, California, followed the path of absorbed solar energy in spinach leaves and Arabadopsis. The researchers found that excessive sunlight triggers the formation of zeaxanthin, a member of the carotenoid family. The molecule transfers an electron to chlorophyll, rendering it incapable of generating reactive oxygen. The positively charged zeaxanthin then combines with the negatively charged chlorophyll in a reaction that disperses away excess energy as heat.

Team member Graham Fleming, a biochemist affiliated with both Berkeley and the Lawrence Berkeley National Laboratory, says that the zeaxanthin defense mechanism is so sensitive, it even turns itself off when clouds pass overhead.

The new report is an important study that offers the first clear-cut evidence of the mechanism plants use to protect themselves, says Yale University biophysical chemist Gary Brudvig. Robert Blankenship, a biochemist at Arizona State University, adds that the findings could aid the development of artificial photosynthesis systems for efficiently harnessing solar energy, or high yielding genetically modified crops that can be fine-tuned to different light environments.

Related Links
The Fleming Lab homepage
More information on photosynthesis

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