April 11, 2003
Cloned gene may help crops and livestock meet future needs
WEST LAFAYETTE, Ind. 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.
"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.
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, firstname.lastname@example.org
Source: Wilfred Vermerris, (765) 496-2645, Vermerris@purdue.edu
Ag Communications: (765) 494-2722; Beth Forbes, email@example.com; http://www.agriculture.purdue.edu/AgComm/public/agnews/
A publication-quality photograph is available at ftp://ftp.purdue.edu/pub/uns/vermerris.sorghum.jpeg.
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.
Keywords: Bmr (brown midrib), Caffeic acid, O-methyltransferase, Cell wall, Lignin, Sorghum bicolor