WEST LAFAYETTE, Ind. — Genetically modifying a key protein
complex in plants could lead to improved crops for the production of cellulosic
biofuels, a Purdue University study found.
Clint Chapple, distinguished professor of biochemistry, and
fellow researchers generated a mutant Arabidopsis plant whose cell walls can be converted
easily into fermentable sugars, but does not display the stunted growth patterns
of similar mutants.
The finding could maintain yield while reducing the need for
costly pretreatment processes that make cellulosic biofuels more inefficient to
produce than corn ethanol.
“This study opens the door to a whole new set of
technologies we never could have imagined,” Chapple said. “This finding is not
the silver bullet that will suddenly make the wide-scale production of
cellulosic biofuels possible, but it is a very important step forward.”
Cellulosic biofuels are made from the sugars in the cell
walls of wood, grasses and the inedible parts of plants. But production of
cost-efficient cellulosic biofuels is limited by lignin, the compound that gives
plants strength and structural integrity.
Lignin binds tightly to the main component of plant cell
walls, cellulose, which is made of simple sugars. Freeing cellulose from lignin
so that it can be broken down into sugars and fermented into fuel requires
expensive and complicated pre-treatment processes.
Scientists have probed ways of genetically modifying plants
to weaken lignin’s grip, but disrupting the lignin biosynthetic pathway in
plants often leaves them dwarfed and low-yielding.
“We’ve always known we couldn’t eliminate lignin entirely,”
Chapple said. “If there isn’t enough lignin in the cell walls, the plant’s water
conducting system will collapse. For the plants, it’s like trying to drink a
milkshake through a paper straw. They need the strength lignin provides to pull
water up from their roots.”
The challenge, he said, was to find a way of preserving
lignin’s key structural functions while preventing it from interfering with the
use of cellulosic materials.
Chapple and his fellow researchers took an
Arabidopsis mutant plant in
which the lignin biosynthetic pathway had been interrupted — and was, therefore,
weak and dwarfed — and made two additional mutations by knocking out two plant
genes known as MED5a and MED5b.
The triple mutation resulted in a healthy plant with normal
growth and wild-type levels of lignin.
“When I saw this plant, I thought it couldn’t possibly be
true,” Chapple said. “We thought plants like this would have too many
architectural problems to grow properly. But what this indicates is that plants
with a blocked lignin pathway can grow — they’re just ‘choosing’ not to.”
The study showed that suppressing the MED5 genes took away
the mutant plant’s ability to sense the disruption in its lignin biosynthetic
It responded by growing normally and producing a highly
unusual type of lignin that interfered far less with the breakdown of cell walls
into their sugar components than lignin in wild-type plants. When treated with
enzymes, the triple mutant yielded significantly more glucose than normal
“We’ve never worked with plant material like this before,”
Chapple said. “It gives up its sugars quite easily and eliminates the need for
pretreatment, which is a big component of the expense of making cellulosic
While lignin is considered a waste material in current
biofuels production, the simple composition of the novel lignin — which is made
up almost exclusively of a single type of alcohol — could make it a potential
fuel source in the future, the professor said.
The discovery also could lead to the development of more
digestible forage crops, which could improve weight gain in livestock.
Chapple cautioned that suppressing genes in MED5 is not
necessarily a quick fix to the complications of producing cellulosic biofuels.
Mutations in MED5 can negatively affect other important plant processes.
But the study gives important insights into plant metabolism
and how plant cell walls are made, Chapple said.
“Learning that this pathway exists gives us an opportunity
to find out how it works. These plants aren’t dwarfed for the reason we thought
they were,” he said. “Now we can re-examine other ways of modifying lignin that
we’d thought were useless.”
The study was published March 16 in Nature.
The Office of Basic Energy Sciences of the U.S. Department
of Energy and the Life Sciences Research Foundation provided funding for the