Study Offers Insight into Delicate Biochemical
Balance Required for Plant Growth
Implications for producing sustainable biomass,
biofuels, and food-processing agents
January 13, 2012
Compared with control plants (left) transgenic plants with
overexpression of a gene for pectin acetylesterase had altered leaf
shape as well as deformed anther sacs and pollen grains. These
findings imply that pectin acetyl esters are essential for normal
plant growth and reproduction.
UPTON, NY — In an ongoing effort to understand how
modifying plant cell walls might affect the production of biomass
and its breakdown for use in biofuels, scientists at the U.S.
Department of Energy’s (DOE) Brookhaven National Laboratory
have uncovered a delicate biochemical balance essential for
sustainable plant growth and reproduction. Their research on
pectin, a sugary component of plant cell walls commonly used as a
gelling and stabilizing agent in foods, might also suggest new ways
to improve its properties for industrial and food applications.
The research findings appear
online in the journal The Plant Cell.
“Pectin is the most structurally complex polysaccharide
(sugar) component of plant cell walls, and is mainly associated
with cell walls that form in fast-growing tissues that are
important for plant growth and development,” said Brookhaven
biologist Chang-Jun (C.J.) Liu, lead author of the paper.
“Our aim was to understand how small molecules, such as
acetyl esters, that commonly bind to the sugar backbone affect
pectin’s structure and its biological and biophysical
properties.” The team included postdoctoral research
associate Jin Ying Gou and former postdoctoral associate Xiao-Hong
Yu in C. J. Liu’s group, and collaborators Lisa M. Miller of
Brookhaven's National Synchrotron Light Source (NSLS), Guichuan Hou
of Appalachian State University, and Xiao-Ya Chen of the Institute
of Plant Physiology and Ecology, Shanghai.
By analyzing gene sequences available for poplar, a dedicated
bioenergy crop and common experimental plant species, the team
isolated and characterized a gene encoding what they thought might
be an enzyme able to split acetyl esters from the pectin in cell
walls. Biochemical experiments revealed that this enzyme, which
they named pectin acetylesterase, was indeed able to specifically
liberate the acetyl ester from cell wall pectins.
They then inserted the gene into tobacco, another experimental
plant, to see what effects “disturbing” the acetyl
esters would have on pectin in a growing plant, and examined the
consequences for plant growth and biomass digestibility.
They used a laser scanning confocal microscope at
Brookhaven’s Center for Functional Nanomaterials (CFN) to
identify where the enzyme, fused with a green fluorescent protein,
was being expressed within the plant cells. Studies using a form of
infrared microspectroscopy aided by Miller at the NSLS allowed them
to precisely monitor the changes in chemical composition of the
plant cell walls.
The findings were dramatic: Removing acetyl esters from pectin
drastically impaired the ability of cell walls to elongate with
dire consequences for plant growth.
Chang-Jun (C.J.) Liu
“During plant growth, cell-wall components are constantly
changed or remodeled, thus enabling the plant cells to continuously
expand, build their biomass, and become bigger and taller,”
Liu explained. In many fast-growing plant tissues, the major cell
wall component is pectin. So disrupting pectin by expressing the
pectin acetylesterase gene severely impeded cell growth.
“The most dramatic case that we observed was that removing
the acetyl esters retarded the germination of pollen grains and the
growth of pollen tubes. Eventually, the plants were completely
sterile, unable to produce seeds,” Liu said.
Equally dramatic — but unexpected — was the effect
on biomass digestibility.
“Previously, many in vitro studies had
demonstrated that acetylesters on the polysaccharide backbone of
cell walls act as a physical barrier, preventing the breakdown of
cell-wall polysaccharides,” Liu said. Consequently the
scientists thought that removing those acetyl esters might be
helpful for enzymatic digestion of cell-wall biomass, therefore
facilitating the production of biofuels.
“In contrast, we found that reducing acetyl moieties from
pectin actually impairs its digestibility, making it more
difficult to break down with digestive enzymes,” Liu said.
“This suggests that precise acetylation patterns in cell-wall
polysaccharides — at least for pectin — are required
for the action of the digestive enzymes in breaking down those
cell-wall polymers.”
Understanding the details of this delicate biochemical balance
will be essential as attempts are made to manipulate plants to
maintain the sustainability of plant biomass and improve cell wall
biomass digestibility for applications such as biofuel
production.
Though not the direct focus of Liu’s research, the current
findings might also offer insight into a more delectable aspect of
“digestion” — the application of pectin as a
food-processing agent. According to Liu, altering acetyl ester
content in pectin can dramatically affect its properties, such as
solubility and its ability to form gels (as in jellies and jams).
“Therefore, characterization of this pectin-specific
deacetylase provides a valuable molecular tool to manipulate pectin
properties for improving applications in industry and food
processing,” he said.
This research was funded by the DOE Office of Science and the
National Science Foundation.
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