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By Sean Henahan, Access Excellence

NASHVILLE, Tenn- An unusual bit of genetic engineering involving luminescent jellyfish and tobacco plants has revealed a key role for calcium in biological rhythms, report researchers at Vanderbilt University.

Calcium is known to regulate numerous intercellular events such as glucose metabolism, muscle contraction and ion movements in the brain. In an attempt to discover the role of calcium in circadian plant rhythms the researchers created a transgenic plant in which a gene from glow-in-the-dark jellyfish was transported into tobacco cells. The enzyme made from this gene emits light only when calcium is present, allowing the investigators to monitor calcium levels in the tobacco cells by tracking the amount of light emitted.

Lead investigator Carl Johnson, an associate professor of biology at Vanderbilt, had been conducting experiments with calcium for more than ten years. However, his previous experiments were unsuccessful because the techniques available to measure calcium at the time weren't conducive to long-term recordings. To adequately study biological rhythms, experiments must last at least several days at a time. The genetic engineering experiments allowed him to at last conduct longer term experiments, in this case averaging eight days.

"For the first time, we were able to examine the role the circadian clock plays in regulating calcium," Johnson said. "This, in turn, might explain how the clock controls all different kinds of observable rhythms that impact our lives."

Evaluated calcium transport in plant cells, tissues and seedlings. Luminescence testing showed a correlation between cytosolic calcium concentrations and circadian rhythms, with peak levels seen immediately after dawn. The researchers were able to shift the luminescence rhythm by extending the darkness interval in the light-dark cycle.

Plants have very complex circadian rhythms which involve, among other things, the movements of stems, petioles and stomatal apertures. All of these process are known to respond to turgor pressure which is itself regulated by potassium and calcium ions. Calcium also regulates such key process as growth and mitosis, protein kinases and gene expression of chlorophyll binding proteins, he said.

The current research suggests a mechanism linking calcium and plant biorhythms in processes including the timing of growth and flowering, plant movement and gene expression it also suggests that calcium is regulated differently in distinct compartments of the plant, he noted.

In nature, the circadian system stimulates processes such as flowering, animal reproduction and migration, Johnson said. For example plants annually flower in the spring and make seeds in the winter. The circadian clock is the timer they use to measure the day and night length that triggers this rhythm.

"I'm hoping that ultimately these studies on plant genes and on calcium's link to the circadian clock will reveal exactly how the clock works," said Johnson, adding it's likely that daily clocks drive seasonal clocks. Chronobiologist can then search for molecular mechanisms that answer questions such as what makes deer breed in the spring and human immune systems beef up their defenses in winter.

"Biological rhythms govern everything from alertness and sleep to body temperature and hormone production," Johnson said. "As chronobiologists we try and help people better understand jet lag, winter depression, adjusting to shift work, disrupted sleep patterns, accidents due to drowsiness and the optimal time of day to administer drugs."

Physicians have found that the course of diseases can change in daily cycles. Heart attacks, joint pain and migraine headaches are more likely to occur in the morning, for reasons that are not entirely clear. Johnson is one of few chronobiologists to teach medical students about circadian rhythms.

The Vanderbilt study appeared in Science, Vol. 269., Sept. 29, 1995.

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