Breaking Down Walls
Basic research on plant cell walls promises to boost not only dairy efficiency, but biofuels production as well. USDA Agricultural Research Service scientists may be on the edge of a breakthrough.
Featuring one of the most sophisticated digestive systems in nature, cows and other ruminants can convert rough, fibrous plant material into critical, life-sustaining energy and milk.
Yet, while herds of these natural plant processors are scattered across the country's vast bucolic landscape, there's not a single commercial facility in the United States capable of a similar feat: converting the Earth's most abundant renewable resource-plant cellulose-into fuel.
Lignin Locks Up Energy
Even though dairy cows are impressive plant-to-energy converters, they can't digest especially fibrous feed portions toughened up by lignin, the cementing agent that holds plant cell walls together.
For bioenergy researchers, lignin and other cell wall components are significant stumbling blocks to unlocking the enormous energy that's tied up in plants. "It's all about the sugars," says Michael Casler, a geneticist based at DFRC. "To draw energy from a crop, you've got to get to the sugars so that they can be fermented into fuel."
However, in cows and biofuels research, lignin almost always gets in the way. Plants use three main materials to build their cell walls: the polysaccharides cellulose, hemicellulose and the phenolic polymer lignin. Cellulose is a chain of glucose (sugar) molecules strung together. As these molecules multiply, they organize themselves in linear bundles that crisscross through the cell wall, giving the plant strength and structure.
The cellulose bundles are weakly bound to an encircling matrix of hemicellulose, which is strongly linked to lignin. The gluey lignin polymer further strengthens plants and gives them flexibility. Lignin is the reason plants can pop back up after heavy rains and winds, and it's how they made the leap from a life in the ocean to one on land eons ago.
Plants have invested great energy in crafting exquisite cell wall structures that resist degradation and loss of their precious sugars. Over the course of millions of years, they've had to fend off an insatiable crowd of energy-hungry fungi, bacteria, herbivores-and now, people.
A Sticky Plasticity
John Ralph, a DFRC chemist, is one of a handful of scientists in the world who are probing lignin's structural details. With the help of nuclear magnetic resonance (NMR), a technology that takes advantage of the magnetic fields surrounding atoms, Ralph and colleagues have been able to chip away at lignin's mysteries, including how plants make it through a process known as "lignification."
Many of Ralph's insights have come from years of scrutinizing the lignin structures in transgenic plants. He says there's much to be learned about a gene by watching what happens when it's altered.
For example, almost 10 years ago, Ralph and colleagues published a paper describing what happens to loblolly pine trees when they're deprived of the gene that codes for cinnamyl alcohol dehydrogenase-an enzyme that helps make vital lignin building blocks. Ralph says that even with extremely low levels of the important lignin-building enzyme, the trees compensated by incorporating novel monomers-small molecules that can bind with others to form polymers-to ensure that they had the necessary lignin-like glue to perform basic functions.
After using NMR and other methods to analyze many other genetically transformed plants-including tobacco, aspen, alfalfa, corn and the model plant Arabidopsis-Ralph and his colleagues and collaborators have laid a foundation of basic knowledge about how lignin production is orchestrated in plants.
Ralph belongs to a major camp of scientists who maintain that the formation of the lignin polymer is pretty much a random affair and isn't strictly controlled by proteins and enzymes like many other plant polymers. Another group argues that lignification is just like protein building, a process that's predictable and leaves few surprises.
But Ralph contends that there is a wider number of building blocks the plant has at its disposal for assembling lignified cell walls. He says the plant can put these components together in a virtually infinite number of ways, as did the pine trees and many other transgenic plants. Ralph calls it "metabolic plasticity." Lignification is "a remarkably evolved solution that allows plants considerable flexibility in dealing with various environmental stresses," he says.
Even if some don't appreciate lignin's evolutionary role in helping plants adapt, that's OK, Ralph says. "A greater awareness of these plant processes will increase our opportunities to modify lignin composition and content," he says.
Zooming In on Lignin
Another of DFRC's many lignin-related discoveries has been especially well received in scientific circles. Fachuang Lu, a research associate in Ralph's group, was the first to find a way to study the highly detailed chemical structure of the entire plant cell wall.
In the past, the job of extracting the various polymers from cell walls for detailed analysis required the deftness of a brain surgeon. There was always a tradeoff between the integrity of the material extracted and the speed with which it could be done.
Now, entire cell walls can be dissolved in a special solution in which all their contents-cellulose, hemicellulose and lignin-are dissolved in a matter of hours instead of weeks, as with traditional methods. Once all the polymers are in the solution, NMR can provide a structural picture of them. "Traditionally, we could only get a portion of the cell wall into solution," Ralph says. "By using this new solution and NMR method, we can get a chemical fingerprint of the major and minor structures of the entire cell wall. The amount of detail is striking."
Researchers interested in running cell wall samples from either conventionally bred or genetically modified energy crops can use the tool to get a zoomed-in view of what their plants' modified cell walls look like. With such powerful capabilities, the method can serve as an important gauge of progress.
Low-Input Plants for Energy
In addition to probing minute cell-wall structures, DFRC scientists are also breeding plants that possess energy-friendly qualities. Casler is hanging his hopes on grasses-the perennials that cover an estimated one-third of the nation's acreage.
Aside from switchgrass, on which he's built an entire breeding program, Casler is also eyeing the promise of other low-input grasses, such as smooth bromegrass, orchardgrass and reed canarygrass. He thinks they have the potential to feed both cows and the country's enormous energy appetite.
Casler and colleague Hans Jung, a DFRC dairy scientist based in St. Paul, Minn., have been selecting grasses that possess either less lignin or fewer ferulates, which are chemicals that help bind lignin to hemicellulose in the cell wall, impeding access to the sugars. "When we started these studies, we wondered ‘Is it lignin that's most responsible for binding up the carbohydrates, or is it the way ferulates link the lignin to hemicellulose?'" Casler says.
After running studies in several grass species, Casler, Jung and collaborators have proved that either approach works when it comes to breaking down tough cell walls. Hoping to breed plants whose cell walls are more easily degraded, Casler and Jung will soon begin crossing promising grass lines.
Focusing on Alfalfa
Other DFRC researchers are focused on alfalfa-a crop that, unlike corn and other grasses, fixes its own nitrogen and therefore requires less fertilizer. Plant physiologist Ronald Hatfield and molecular geneticist Michael Sullivan are working to boost alfalfa's biomass by altering genes that affect its development. "We're looking at alfalfa's developmental structure, how it branches," Hatfield says. "We're also trying to reduce leaf abscission, or leaf drop."
Because alfalfa plants are grown close together, many of their understory leaves fall off from lack of sunlight. Hatfield and Sullivan would like to minimize loss of this valuable plant material.
Hatfield, Sullivan and Ralph are collaborating with the Noble Foundation in Ardmore, Okla., to build the ideal alfalfa plant. "The Noble Foundation usually engineers the plants with reduced lignin," Hatfield says. "Then we use NMR and other analytical techniques to see what the modified cell walls look like and how easily they can be processed either by the cow or for biomass conversion to energy."
The alfalfa research team has already discovered that when they transform plants by down-regulating enzymes called "methyl transferases," they can reduce lignin content, boost cellulose content and enhance cell wall digestibility.
Part of the Big Picture
In the end, DFRC researchers believe that agriculture's role in supplying renewable energy to the country is crucial. However, Hatfield cautions that the bioenergy movement mustn't miss the forest for the trees. "We need to consider the whole agricultural picture," he says. "You can't convert everything into bioenergy."
There are other biobased products and niche industries to consider. Take alfalfa, for instance. DFRC researchers have found that, in addition to providing great grist for the ethanol mill, alfalfa is a source of quality protein and health-promoting nutraceuticals. Plus, its fiber fractions have value as a water-filtering agent, and it's an ideal substrate for making an all-natural glue.
"We also have to think in terms of sustainability for the sake of local agricultural economies and our natural resources," Hatfield says.
Erin K. Peabody is a member of the USDA-ARS' information staff. Reach her at firstname.lastname@example.org or (301) 504-1624. This article was published in the April 2007 issue of Agricultural Research.
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