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.
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 componentsócellulose, 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
"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
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
"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."