Not just another brick in the wall
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Plant cell walls are central to cellulosic ethanol because they are the raw material for next-generation biofuels. For this reason, the Plant Sciences Institute is investing in plant cell wall research, laying a foundation to advance feedstock development for cellulosic ethanol production in Iowa.
Plant cells make both primary and secondary cell walls. But it is the cellulose-rich secondary wall that interests biofuel innovators because they contain the most sugar-laden (polysaccharide) fibers.
Olga Zabotina, assistant professor in the Department of Biochemistry, Biophysics and Molecular Biology is helping to breach some of biofuel's brick walls.
Cellulose and hemicellulose are polysaccharides--sugars bonded together like beads on strings. They form complex webs in cell walls and it is this array of polysaccharide linkages that determine cell wall structure and the usefulness of a plant in bioprocesses.
The woody secondary walls contain cellulose and hemicellulose as do the primary cell walls that surround the delicate growing plant tissue. What makes the secondary walls tougher is the relative percentages of these components plus one more material--the nearly indigestible binding material called lignin--thought to also help protect cells from pests and pathogens as well as aid in water transport.
The cell wall can be described in civil engineering terms where secondary walls are much like reinforced concrete. Cellulose represents reinforcing steel rebar; hemicellulose and lignin, the cross-links and cement.
Understanding the intricate biochemical pathways that lead to the synthesis of these valuable and sometimes frustrating compounds lies at the heart of Zabotina's research.
To glimpse the complexity involved when plants make fibrous cell wall materials, Zabotina explains “it takes at least seven different enzymes to synthesize one unit of xyloglucan. So when it comes to all the fibrous compounds, given the spectrum of linkages in polysaccharides, it is estimated that more than 400 different enzymes (transferases) should be involved.”
Plants regulate--turn synthesis on and off--using complex synthetic machinery involving different types of enzymes, including the synthases and transferases, epimerases, hydrolases and so on, Zabotina explains. “All of them should be well coordinated and a lot is redundant,” doing the same job but under different conditions as when the plant responds to certain stresses.
Zabotina's studies of cell wall composition stem from her desire to blend two competing interests--chemistry and biology. Her work has taken her from her native Russia to Europe, California, and now Iowa—while with each stop, looking at cell walls from varying view points.
Zabotina began her research at Kazan State University, earning a degree in organic chemistry, then in1987, completing her Ph.D. in plant biochemistry and physiology.
She began looking at the effects of microgravity upon plant cell division and cell wall development, studying the resulting biochemical modifications. Her germinating seedlings orbited Earth “on satellites for two weeks,” says Zabotina until they were recollected and analyzed. Zabotina found they “grew faster and spread out--not up--without any rigid structure.”
Eventual upheaval in Russia's scientific programs, “canceled all experiments in space,” explains Zabotina who then shifted her focus to freeze tolerance in winter plants.
Studying a winter variety of wheat, Triticum aestivum L., Zabotina began trying to understand the biochemical changes in hemicellulose and other cell wall components as plant cells adapt to fall's frosts and winter's freeze.
She discovered as the plants harden to frost conditions, a certain oligosaccharide (smaller sugar polymer) accumulating in the plant can function as a signaling molecule increasing freeze tolerance—changing relative ratios of plant cell wall components like hemicellulose, will make the walls more rigid and resistant to damage if ice forms in the plant tissues.
Expanding her examination of cell wall biochemistry, Zabotina studied properties of pectin and the hemicellulose matrix biosynthesis while in Europe and at the University of California, Riverside.
This diverse perspective has now positioned Zabotina well as a cell wall expert able to collaborate with a variety of researchers tackling problems from plant protection to thermochemical conversion to nutraceutical construction.
