Plants' management of nutrient suggests environmental remedies

“Our findings should help scientists understand how plant ecosystems respond to soil calcium depletion and design appropriate strategies to protect the environment,” said Zhen-Ming Pei, a Duke University assistant professor of biology who led the study, to be published in the Friday, March 9, issue of the journal Science.

The research was supported by the National Science Foundation, the U.S. Department of Agriculture and Xiamen University in China.

Calcium enters plants dissolved within the water that roots take in from surrounding soil. As the water circulates through a plant, its dissolved calcium gets shuttled where it is needed to give the plant's cells their structural rigidity. To grow, a plant needs a reliable supply of calcium. But calcium supplies coming into the plant cycle up and down over the course of the day, dropping to a minimum at night.

Plants use molecular sensors and flows of chemical messengers to detect and regulate the storage and distribution of vital nutrients such as water and calcium.

To track the calcium sensors in the model mustard plant Arabidopsis, the team used molecules originally found in jellyfish that emit light in response to calcium's presence. To deduce what the sensor does and does not do, the researchers also introduced an “antisense” version of the calcium-sensor protein that abolishes the sensor's effects.

The calcium-sensing molecule in plants, called CAS, was first identified by Pei's group and described in the Sept. 11, 2003, issue of the journal Nature. Arabidopsis is favored for such experiments because it has a relatively short life cycle of eight weeks and its genome has been completely sequenced.

By tracking the glow of the jellyfish molecules, the researchers learned that CAS plays a number of roles in plants. The scientists initially thought it simply monitored changes in levels of dissolved calcium that enters the plant from the outside. They discovered instead that CAS also triggers the release of internal calcium that is stored within the plant via a chemical signaling system.

This coupled system, the researchers deduce, ensures that constant levels of calcium remain available to a plant's cells despite widely varying amounts of the nutrient coming in during each day and night cycle.

“The sensors try to detect how much calcium is there, and they coordinate that level with growth and development,” Pei said. “If they detect there is not enough calcium, the plant may elect to hold off on growth and development until it has more calcium. The plant may thus appear not to be doing well.”

The findings have prompted Pei to begin a new research program aimed at altering this calcium balancing act to help plants adjust to the ravages of acid rain.

Produced by interactions between water vapor and human-created pollutants, acid rain can disrupt plants' calcium balance by leaching significant amounts of calcium from agricultural and forest soils as well as from plant leaves, according to Pei.

“It has been found that some soils have lost as much as 75 percent of their calcium during the past century,” he said. “One way to respond is to add new calcium to the soil. But we can't do that everywhere that it's needed and it is also expensive.”

Although acid rain robs soil of much of its calcium, enough is still left for plants to live on, Pei added. But he suspects that sensors like CAS may misinterpret “less” as “too little” in those plants and unnecessarily signal for growth shutdowns. Perhaps a plant's calcium sensors could instead be tricked into interpreting “less” as “still enough” and keep building new cell walls, he suggested.

As a preamble to such genetic engineering, Pei is now leading a study in his native China that will evaluate the physiology of various plants affected by acid rain. “It is in the south of China where acid rain is huge because of industry,” he said. “China is becoming the factory for the United States.

“We will monitor calcium changes in the soil there, and then clone calcium receptors from various plant species to see whether those receptors are responsible for growth and how they respond to acidity,” he said. “Some plants grow terribly under acid rain, but others grow very well.”