with photosynthesis, the plant cell wall is one of the features that
most set plants apart from animals. A structural molecule called
cellulose is necessary for the manufacture of these walls. Cellulose is
synthesized in a semi-crystalline state that is essential for its
function in the cell wall function, but the mechanisms controlling its
crystallinity are poorly understood.
research from a team including current and former Carnegie scientists
David Ehrhardt (Carnegie), Ryan Gutierrez (Carnegie), Chris Somerville
(U.C. Berkeley), Seth Debolt (U. Kentucky), Dario Bonetta (U. Ontario)
and Jose Estevez (U. de Buenos Aires) reveals key information about this
process, as well as a means to reduce cellulose crystallinity, which is
a key stumbling block in biofuels development. Their work is published
online by Proceedings of the National Academy of Sciences for the week of February 20-24.
plant’s cell wall serves several essential functions including
mechanical support: Allowing the plant to withstand the onslaughts of
wind and weather, and permitting it to grow to great heights—hundreds of
feet for trees like the giant Redwood—and providing an essential
barrier against invading pathogens. The cell wall is also the source of
materials that have long been utilized by humans, including wood and
cotton, in addition to serving as a potential source of biofuel energy.
is the primary constituent of the cell wall and as such is the most
abundant biopolymer on the planet. It is also the key molecule providing
the cell wall its essential mechanical properties.
address the question of its manufacture in plant cells, the research
team, led by Seth DeBolt of the University of Kentucky, focused on
different aspects of cellulose-synthesizing complexes.
in conjunction with Chris Somerville, Ehrhardt developed a method for
observing this complex by tagging it with a fluorescent marker derived
from jellyfish and imaging the tagged protein using a technique called
spinning disk confocal microscopy. This technique allows individual
biosynthetic complexes to be seen and studied in living cells, producing
an unusually high level of resolution.
Bonetta of the University of Ontario Institute of Technology, Debolt,
Somerville and Ehrhardt all participated in screening a large number of
small molecules to determine which ones interfere with cell wall
building. Those that interfered were then examined at the cellular
level—using the fluorescent marker—in order to see how they affect the
interesting candidates were identified, a search was undertaken to look
for mutant plants that showed reduced responses to these molecules. It
was assumed that, because these plants were either unaffected or
differently affected by these molecules, then they would have plant cell
walls that are compromised or in some way unusual.
this process of elimination, two mutations, called CESA1 and CESA3,
were found in the genes that encode certain cellulose synthase proteins
and these mutated genes were further studied. Both of these mutations
are predicted to be found in the part of these proteins that cross the
plant cell’s membrane, which forms just inside the cell wall.
members of the team analyzed the cellulose manufactured by plant cells
that had these mutations and found defects in the structure of cellulose
that these altered proteins produced.
the individual sugar chains that make up cellulose bond to each other
to make a semi-crystalline fiber. This crystalline structure gives
cellulose its essential mechanical properties, such as rigidity and
tensile strength. This structure is also is responsible for cellulose’s
resistance to digestion, which provides a key barrier to utilizing
cellulose as a source to produce liquid fuel.
mutant CESAs, 1 and 3, produced cellulose with lower crystallinity.
This cellulose was also more easily digested, a process that is needed
to liberate sugars from cellulose so they can be converted to useful
team made a connection between the structure of the proteins that
produce cellulose, and the structure of their product,” Ehrhardt said.
“This is a first step in understanding how this important property of
cellulose may be regulated, opening possibilities for development of
useful biomaterials and for cellulosic biofuel crops.”