Getting Inside Plants
PET scans have been used for decades to help doctors diagnose disease in people - from cancers to heart problems. Now, the technology is returning to one of its early roots: the study of plants.
The work is supported by the Department of Energy's Office of Biological and Environmental Research. It is featured in BER's portfolio of basic ecological research to understand how rising amounts of carbon dioxide, or CO2, in the atmosphere will affect the environment.
Previous research has found that plants grow more vigorously if exposed to additional carbon dioxide, at least up to a point. Now scientists want to know what factors limit plants' ability to use additional CO2, how plants pass on additional CO2 to bacteria in the soil (carbon sequestration), and in what ways potential biofuel-producing plants can be optimized to take advantage of additional CO2 in the atmosphere.
"I think it's a good idea to take expertise that the labs have and to direct it toward research that could benefit from it, such as biofuels development, carbon sequestering, and agricultural crop improvements," said Drew Weisenberger, group leader of Jefferson Lab's Radiation Detector and Imaging group.
The group, in collaboration with Duke University, has just started work on developing specialized plant imaging systems, funded by a Scientific Focus Area grant from BER. The grant provides $600,000 each year for three years, beginning with the new fiscal year that began Oct. 1.
The group began preliminary work in this area back in April of this year. So far, group members have participated in two brief studies, along with collaborators, to begin testing the feasibility of its detectors for providing meaningful information for plant researchers.
Barley is not only used to make malt beer and chewy bread, it's also an ideal plant for photosynthesis and plant physiology research. The study was conducted at the Duke University Phytotron, which offers plant growth chambers and environment-controlled greenhouses.
"We decided to continue with an idea that Duke University had to find out what is the best detector arrangement that would be useful," Weisenberger said.
In the study, barley plants were exposed to controlled levels of CO2. The CO2 used in the study was a special type generated at the Triangle Universities Nuclear Laboratory. The carbon atoms in the CO2 emit positrons, which are the anti-particles of electrons. When a positron encounters an electron in one of the atoms of the plant, the two particles annihilate and produce gamma rays that are detected by PET detectors. This is a similar way in which medical PET scans are done on humans.
"It's easy to get CO2 into the plants - they just breath it in. The Phytotron group used a plastic cuvette, which seals over the leaf with a rubber seal. This is the only part of the leaf that's getting the CO2," Weisenberger explained. Much like a mask, the cuvette provides high levels of the special CO2 to the plant.
Green plants use photosynthesis to create their own food, in the form of sugars called carbohydrates (the same carbs we learn about from dietitians). If there is sunlight once the plant inhales the CO2, it is used immediately for photosynthesis, where the special carbon is incorporated into the carbs. The plant then stores the carbs in its roots.
The researchers wanted to see if they could image the special carbon as it was sent to the roots. To image the carbs' route, the team adapted components of a PET system that had previously been designed specifically for imaging the human breast.
"We literally just took it apart, and we arranged the detectors," Weisenberger said.
The detectors were placed on either side of the thin, green leaves of the barley plant. Another, smaller system built for the study was placed around the container holding the roots. The roots were suspended in hydroponic solution in a flat Plexiglass container.
The researchers found that they could clearly image the path of the carbs as the plant stored them in the roots.
In another study, conducted at West Virginia University, the researchers purchased a bouquet of roses from a local grocery store to test whether PET imaging systems could be used to image the plants.
In ordinary PET scans of the human body, a radiopharmaceutical is injected, such as fluorodeoxyglucose or FDG. The drug mimics sugar in the human body, so the body transports the drug to organs that require sugar for fuel. All the while, the drug is emitting positrons as the special carbon mentioned earlier.
To get the FDG into a plant, the researchers stuck a rose stem with two leaves on it in a water solution dosed with FDG. As a result, the FDG-toting water was sucked up by the plant and sent into the leaves for photosynthesis.
Initially, researchers attempted to image the plant with a PET detector system. They found that they weren't getting very good images of the leaves, because the leaves were so thin, that the positrons escaped the plant before ever encountering an electron.
To better detect where the FDG goes in the leaves, the team then tried to image the positrons directly. They taped the rose leaf directly to the top plate of a positron-imaging system. They found that they could image the leaf almost perfectly.
"What it's definitely showing is where water goes, where the FDG went," Weisenberger explained.
Growing Into Imaging Plants
Weisenberger said the next step is to apply the successes the team has had to develop new systems for imaging plants.
"What we'll do is probably make a detector that allows us to bring a direct positron detector to the leaf and a PET detector for the roots and larger structures. So, it won't be one-size-fits-all. You're going to have to use different technologies for the different aspects of the plant," he said.
In undertaking this new research direction, the Radiation Detector and Imaging group joins a very small circle of laboratories. Other facilities that conduct this type of research include Brookhaven National Laboratory, the Japan Atomic Energy Agency, Jülich Research Centre and the group's collaborators at the Duke University Phytotron.
This work is supported by BER, the Office of Nuclear Physics and the National Science Foundation.