ORNL Review Vol. 34, No. 1, 2001

Controlling Carbon in Hybrid Poplar Trees

Using carbon dioxide from the atmosphere, as well as sunlight and water, hybrid poplar trees grow fast and tall, up to 12 feet per year. They also harbor a considerable amount of carbon in their stems, branches, leaves, and roots. Plant geneticists would like to design hybrid poplar trees that maximize the amount of carbon they store in their cell walls. These trees could then be used to more effectively sequester carbon dioxide, a greenhouse gas, through increased carbon storage in their roots and, after the roots decay, in soil. Alternatively, when harvested and digested microbially, these "designer" trees could offer an increased yield of commodity chemicals (e.g., polylactic acid, furfural, and acetic acid) and ethanol fuel.

To produce a hybrid poplar tree, flowers from the female, Populus trichocharpa, are isolated and inoculated with pollen from the male, Populus deltoides. Hybrid progeny grow from the resulting seeds. The flowers and pollen from the hybrid trees can be crossed to produce descendants of the "grandparent" trees. The genomes of the grandparent trees and their progeny can be compared to a deck of cards that is shuffled and reshuffled for each tree. In the grandparents, the male has all black cards and the female has all red cards. In the next generation, the cards are half red and half black. In the third generation, the percentages of black and red cards vary greatly for each tree, producing genetic diversity that allows the linkage of genes to highly desirable traits.

In trees, carbon is "allocated" between aboveground stems, branches, and leaves and belowground roots. It is "partitioned," or divided, among three types of plant cell-wall components—cellulose, hemicellulose, and lignin. A plant could be designed to have an unusually high cellulose content above ground, if increased ethanol production is desired. In addition, if carbon sequestration is the goal, its roots could be designed to have unusually high lignin content, which is resistant to degradation by microbes, increasing the residence time of carbon in the soil.

"In five years, we hope to determine which genes control carbon allocation and partitioning in hybrid poplar trees," says Gerald Tuskan, a plant geneticist in ORNL's Environmental Sciences Division (ESD). "Our research indicates that carbon allocation is controlled by a small number of regulatory genes, that separate genes controlling cell-wall chemistry operate independently above ground and below ground, and that genes controlling carbon allocation affect carbon partitioning."

Tuskan is working on a three-year project to enhance bioenergy conversion and carbon sequestration in woody plants with his ESD colleagues Stan Wullschleger, Tim Tschaplinski, and Lee Gunter; Brian Davison of the Chemical Technology Division; and several researchers from DOE's National Renewable Energy Laboratory. The team is studying wood tissue samples from some 300 hybrid poplars grown in Washington that are the progeny of trees from Minnesota and Oregon parents.

Tuskan and his colleagues are mapping the hybrid poplar genome by finding genetic "markers" unique to trees that have a desirable trait, such as higher-than-normal cellulose content above ground. A marker is a known DNA sequence associated with a particular gene or trait; in this study, it consists of two unique, non-repeating DNA sequences flanking simple sequence repeats, such as GAGAGAGAGA. Some 150 markers have been found so far; the project's goal is 400 markers.

"Each hybrid poplar tree has a unique genetic fingerprint," Tuskan says. "We look for an association between markers unique to each tree and variations in the allocation and partitioning of carbon content. Once we find the marker that controls the trait we are interested in, such as high lignin content in the roots, then we will try to locate the genes responsible. Such genes could be used to design tree root systems that are high in lignin content."

Tuskan is also interested in finding the genes that control the size and thickness of a tree's cell walls, the substructure of wood that determines its usefulness and commercial value. "It's because of differences in cell sizes and wall thicknesses that oak floors are stronger than pine floors, maple furniture is more attractive than aspen furniture, and white oak rather than red oak is used to make barrels to store wine," he says. "Cell dimensions also determine whether a tree's wood is suitable for combustion or production of paper or ethanol."

Use of a light microscope or scanning electron microscope to determine wood cell dimensions in samples from various trees is expensive and time consuming. So, Tuskan sought help from Mike Paulus of ORNL's Instrumentation and Controls Division. Paulus is a co-developer of the high-resolution, X-ray-computed tomography system called a MicroCAT scanner. Although used mostly to image internal defects in small animals, the MicroCAT scanner also offers a faster, better, and cheaper way to measure the lengths and diameters of cell walls in wood. (See MicroCAT "Sees" Hidden Mouse Defects.)

"With the MicroCAT, we can get cell measurements from an intact block of wood, whereas for microscope studies, we have to slice wood into very small pieces," Tuskan says. "With the light microscope, we were getting 100-micron resolution, but with the modified MicroCAT, we get 10-micron resolution and may be able to get down to a resolution of one to two microns. The MicroCAT is a great tool for rapidly screening for wood-cell dimensions in the context of a large genetic mapping study."

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