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The great 18th-century botanist Carl Linnaeus is said to have planted a 'timekeeping' garden in which he could tell the hours of the day by the opening and closing of various flowers. Like Linnaeus' flowers, most plants and animals have internal biological clocks called 'circadian rhythms,' which synchronize daily activities such as awakening or petal spreading.
Prof. Meir Edelman of the Plant Sciences Department, working with Dr. Autar Mattoo of the USDA Agricultural Research Service in Maryland, has recently found that a 24-hour circadian clock regulates the most basic function in plants - photosynthesis (the process in which plants produce sugar and oxygen - see box).
Edelman and Mattoo have been collaborating on studies of photosynthesis for over 20 years, ever since Mattoo arrived at Edelman's lab from India for a two- year stint as a guest researcher. Since that time, their research has focused on an unusual protein central to photosynthesis.
The protein, called D1, sits right at the heart of the plant's energy centers, and is the dynamo of the photosynthetic process. Edelman and Mattoo were intrigued by their finding that a phosphate molecule regularly binds to, and is later released from, D1. They demonstrated that this process (called phosphorylation) does not occur at a steady rate; rather, its magnitude rises and falls in a 24-hour cycle. This swing between peak and ebb met all of the criteria for circadian rhythms: It continued for several days even when night and day cycles were artificially interrupted in the lab, and the cycle could be reset - just as the body of a person traveling across different time zones resets its sleep cycle after a few days. (The cycle did break down, however, when plants were kept in total darkness, as photosynthesis cannot take place without light.)
Interestingly, they saw that the high point of this cycle did not coincide with the time of peak sunlight. Instead, phosphorylation climaxed at about 10 a.m., several hours before high noon, and afterward began to drop off sharply. This meshed in with the scientists' belief that D1 acts as a 'light meter' for the plant's energy centers. The scientists theorize that the phosphorylation cycle may be timed to help plants protect themselves against a sunlight 'overdose': Although the plant depends on sunlight for nourishment, too much sunlight can damage plant cells. D1 is tuned to work in a wide range of light conditions, including the weak light of early morning and cloudy days. But when the intensity of the light passes a certain level, the system has more incoming energy than it can handle. To prevent overload, the plants must suppress the reaction.
Continuing their research into the mystery of circadian rhythmic control of the D1 protein, Edelman and Mattoo have isolated an enzyme they suspect may be the main agent of daily D1 phosphorylation. Now they are performing further experiments using this enzyme to see if it is, indeed, the mainspring of the photosynthesis clock.
From sunlight to sugar
Photosynthesis is the process in which green plants and certain other organisms produce carbohydrates using light, water and carbon dioxide. The green pigment in plants (chlorophyll) collects energy from sunlight.
The plant then uses the sunlight to split water into hydrogen and oxygen. The oxygen is released to the atmosphere as a byproduct, while the hydrogen adheres to molecules of carbon dioxide that the plant has soaked up from the air, producing sugar. D1, the protein studied by Edelman, plays a central role in this process, using the collected energy to split water.
Prof. Edelman's research was supported by the Avron-Wilstaetter Minerva Center for Research in Photosynthesis. He is the incumbent of the Sir Siegmund Warburg Chair of Agricultural Molecular Biology.
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