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West Lafayette, Indiana
April 11, 2003
Improved digestibility of
livestock feed, hardier crops and higher yield of biofuels may
result from information that Purdue University researchers are
learning about the sorghum gene that controls plant cell wall
hardness.
The scientists have cloned the gene and also developed markers
that allow molecular identification of three mutations of the
gene, which is involved in forming lignin, a plant cell wall
hardening substance. When the gene is not functioning, or
mutated, the cell walls are softer.
The study detailing the gene cloning and marker development is
published in this month's issue of Molecular Genetics and
Genomics and is currently available on the journal's Web site at
http://link.springer.de/link/service/journals/00438/contents/tfirst.htm.
"Our research focuses on finding
solutions to increase the productivity of plants," said
senior study author Wilfred Vermerris, assistant professor
of agronomy and agricultural and biological engineering.
"The value of cloning this gene is to help us better
understand what has changed in these mutants so that we can
introduce similar changes in other crops, such as rye grass
and corn."
The gene, Brown midrib (Bmr), encodes caffeic acid
O-methyltransferase (COMT), a lignin-producing enzyme.
Sorghum mutants in which the gene is defective had reduced
amounts of COMT, Vermerris said. This results in the mutants
containing significantly lower lignin in their leaves and
stems
compared with their wild-type or normal counterparts. Plants
with these genetic changes have brown vascular tissue,
rather than the normal green, and they are softer.
Vermerris and his co-author,
research technician Siobhán Bout, cloned Bmr after comparing
the chemical composition of the mutants' cell walls. They
also identified the part of the sorghum Brown midrib gene
that is different in the three mutants, bmr12, bmr18 and
bmr26. The small changes in DNA that make this gene inactive
can be identified with tools called molecular markers. |
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Purdue University assistant
professor Wilfred Vermerris and a research team have cloned
a sorghum gene that controls plant cell wall hardness. The
sorghum research could contribute to the plant's hardiness
and digestibility. Vermerris is an assistant professor of
agronomy and agricultural and biological engineering.
(Purdue Agricultural Communication photo/Tom Campbell) |
The plant-softening mutations
improve the digestibility of the food, and livestock also seem
to like the taste better.
"I have seen some sheep trials conducted by Purdue agronomy
professor Keith Johnson where they let the sheep select their
forage," Vermerris said. "They first eat all the mutants. When
they are done, they go for the normal plants."
Since similar mutants exist in corn, researchers can use what
they learn about lignin development in sorghum to compare the
two species, better understand plant cell wall formation and
develop new varieties of these and other species.
For instance, sorghum is better able to cope with environmental
stresses, such as heat or drought, than is corn, Vermerris said.
"We're interested in discovering how corn and sorghum differ in
their cell wall biosynthesis," he said. "If we can figure it
out, then we may be able to make corn more stress tolerant."
Molecular markers that make it possible to distinguish between
the wild-type and mutant plants are valuable for future genetic
studies of the mutants and also to help breeders select plants,
Vermerris said.
Breeding more stress-resistant plants will increase crop yield,
he said. Development of more productive plants and ones that can
grow under difficult environmental conditions is imperative
because the world population is rapidly increasing while
available agricultural land is decreasing. The population is
expected to jump from the current 6.3 billion to about 8.9
billion by 2050, according to the Population Reference Bureau.
At the same time, some evidence exists that the average
temperatures are rising and rainfall patterns are changing,
Vermerris said. This contributes to plants being challenged by
new diseases and insects, while water and land for agricultural
uses is becoming scarcer, he said.
"I envision exploiting plants' natural mechanisms in order to
develop a new generation of crop plants that hopefully will meet
the demands of the future," he said.
These plants will be better able to handle harsher conditions
such as drought and heat, he said. They also will be more
digestible, so more meat and milk can be produced per pound of
livestock feed.
"Currently for every pound of meat you produce, you have to feed
the animal five pounds of plant products," Vermerris said.
"Essentially you're spending four pounds of plant products in
the conversion process."
One disadvantage to reducing lignin in plant cell walls so they
are tastier and more digestible for the livestock is that they
also may be tastier to insects, resulting in more crop damage,
Vermerris said.
He noticed that one of the mutant lines is a magnet for Japanese
beetles.
"I wasn't very surprised by that because the pheromone that
attracts Japanese beetles to the commercial traps is very
closely related to some of the chemicals in lignin," Vermerris
said. "We would have to test that but it's possibly an
explanation of why insects might like these plants better - they
can smell them. On top of that, they may be easier for the
insects to chew."
Vermerris and Bout also are investigating how changes in the
amount of lignin in the cell wall may improve production of
biofuels. A decreased amount of lignin, resulting in softer cell
walls, should make it easier to break down the plants to form
fuel, Vermerris said. This could mean producing more fuel from
each acre of crop.
The Showalter Foundation provided funding for this study
Writer: Susan A. Steeves, (765) 496-7481,
ssteeves@purdue.edu
Source: Wilfred Vermerris, (765) 496-2645,
Vermerris@purdue.edu
Related Web sites:
Molecular Genetics and Genomics online:
http://link.springer.de/link/service/journals/00438/contents/tfirst.htm
Vermerris & Bout paper:
http://link.springer.de/link/service/journals/00438/contents/03/00824/paper/s00438-003-0824-4ch000.html
Wilfred Vermerris:
http://www.agry.purdue.edu/staffbio/vermerris.htm
Purdue Agronomy:
http://www.agry.purdue.edu
Population Reference Bureau:
http://www.prb.org
ABSTRACT:
A candidate-gene approach to clone the sorghum Brown midrib gene
encoding caffeic acid O-methyltransferase
S. Bout 1 and W. Vermerris 1,2 - (1) Department of Agronomy,
Purdue University, 915 W. State Street, West Lafayette, IN
47907-2054, USA (2) Department of Agricultural and Biological
Engineering, Purdue University, West Lafayette, IN 47907, USA
The brown midrib (Bmr) mutants of sorghum have brown vascular
tissue in the leaves and stem as a result of changes in lignin
composition. The Bmr mutants were generated via chemical
mutagenesis with diethyl sulfate (DES) and resemble the brown
midrib (bm) mutants of maize. The maize and sorghum brown midrib
mutants are of particular value for the comparison of lignin
biosynthesis across different, yet evolutionarily related,
species. Although the sorghum brown midrib mutants were first
described in 1978, none of the Brown midrib genes have been
cloned. We have used a candidate-gene approach to clone the
first Brown midrib gene from sorghum. Based on chemical analyses
of the allelic mutants bmr12, bmr18 and bmr26, we hypothesized
that these mutants had reduced activity of the lignin
biosynthetic enzyme caffeic acid O-methyltransferase (COMT).
After a northern analysis revealed strongly reduced expression
of the COMT gene, the gene was cloned from the mutants and the
corresponding wild types using PCR. In all three mutants, point
mutations resulting in premature stop codons were identified:
bmr12, bmr18 and bmr26 are therefore mutant alleles of the gene
encoding COMT. RT-PCR indicated that all three mutants express
the mutant allele, but at much lower levels relative to the
wild-type controls. Molecular markers were developed for each of
the three mutant alleles to facilitate the use of these mutant
alleles in genetic studies and breeding programs.
The study detailing the gene
cloning and marker development is published in this month's
issue of Molecular Genetics and Genomics and is currently
available on the journal's Web site at
http://link.springer.de/link/service/journals/00438/contents/tfirst.htm.
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