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 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.”
By analyzing gene sequences available for poplar, a dedicated
bioenergy crop and common experimental plant species, they 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
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 at the National Synchrotron
Light Source (NSLS), aided by collaborator Lisa A. Miller, 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.
“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
“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
“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
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. “Therefore, characterization of this
pectin-specific deacetylase provides a valuable molecular tool to manipulate
pectin properties for improving applications in industry and food processing,”