Professor probing the evolution of tropical orchids

7/12/2007 7:00:00 AM

John Cushman, Department of Biochemistry and Molecular Biology professor, is making great strides in the ongoing research of plants with the metabolic ability to use less water than other plants.

Cushman, along with researchers from the University of Florida and the Smithsonian Tropical Research Institute in Panama, was awarded a three-year grant of $750,000 from the National Science Foundation to study the evolution of plants with a form of photosynthesis called crassulacean acid metabolism or CAM.

CAM plants take up carbon dioxide, a major part of their metabolism, through small pores called stomata during the night instead of during the day, as plants that have C3 photosynthesis do. CAM plants than store the carbon dioxide as organic acids and introduce them into the rest of the photosynthesis process during the day.

Plants with C3 photosynthesis, which occurs in 90 percent of all plants, lose a great deal of water by opening their stomata during the day while CAM plants lose much less water by storing carbon dioxide at night.

"CAM plants lose 10 times less water C3 plants do," Cushman said. "It is a very 'water-efficient' way of taking up carbon dioxide in a metabolic sequence."

This type of photosynthesis occurs in about 7 percent of vascular plant species, a very minimal amount of plants compared to the large number of species that conduct C3 plants. To study this metabolic phenomenon, Cushman and his colleagues are studying the orchid family, 50 percent of which have CAM.

Orchids are important in many other ways, according to Cushman. Orchids produce vanilla, a widely used flavoring, and many different species are sold as potted plants or as cut flowers.

"Orchids are horticulturally significant," Cushman said. "There are many different types and the orchid family is one of the largest families of higher plant species."

The group is interested in tracking the changes that led C3 plants to evolve into CAM plants by using molecular phylogeny, a sort of family tree that traces the evolutionary history of a group of organisms. By seeing where CAM plants occur in the tree, the researchers can identify the circumstances under which the switch from C3 to CAM took place.

The group of researchers is also studying a set of targeted genes they believe are critical to the process of CAM. The genes are duplicated in three different steps along the evolutionary pathway: orchids with C3, orchids with CAM and orchids that are in between, called "weak CAM".

"We'll look for changes in these plants in order to understand the changes in gene structure and expression that occurred during the progression from C3 plants to CAM plants," Cushman said.

Despite the progress that is being made with this project, Cushman says the ultimate goal of being able to replicate this metabolic process in crops is too far off in the future.

"CAM is regulated on many different levels," Cushman said. "We won't be able to create a CAM plant any time soon. There are too many other complexities that remain poorly understood."


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