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This issue...

   Brieflies

  Plants with Backbone: A Promising Mystery

  Laboratory Plasmas Shed Light on the Sun

  Eureka! California Discovers Plutonium...Again!

  Working Science

  People

  Subscribe Free

Plants with Backbone: A Promising Mystery

by Michaela Mann

Fierce competition drove prehistoric plants upward in search of sunlight. This skyward impulse was facilitated by lignins, compounds that reinforce cell walls, serve as the "backbone" for all upright plants, and provide structural support for rigid components such as branches and roots. The biochemical pathway to lignins evolved hundreds of millions of years ago; mapping of that pathway has been a very recent phenomenon.

In spite of lignin's evolutionary popularity—approximately 30% of all biomass is in the form of lignins—their formation and structures have long been poorly understood. Breakthrough research at Washington State University now points to mechanisms that control lignin formation. The research is particularly important to two Office of Science missions: producing biomass energy crops and developing biomass-based fuels. It also has implications for a variety of modern dilemmas, from protecting plants from disease to the manufacture of plastics, and from improving paper products to breaking down and recycling plant-based waste.

Lignins are highly polymerized, complex carbon-based compounds. In 1874 coniferyl alcohol was identified as a component of lignin, beginning the long process of unraveling its chemical structure. By 1933 it was known that lignin originated from a coupling process that involved alcohol-derived free-radicals (reactive molecules or compounds that have a free electron).

Free radicals are often associated with cancer cell formation and aging. Yet free radical reactions are critical to many animal and plant biological processes, including formation of pigments in insects, cell walls in algae and fungi, as well as several biosynthetic pathways in vascular plants. The implications of understanding free-radical coupling, then, extend far beyond lignin formation.

Until recently, it was assumed that the free-radical coupling process was uncontrolled; that is, that it could occur at random at different sites within the molecules during lignin assembly. This assumption bothered Norman Lewis and other members of a research team at Washington State University. It failed to explain the formation of lignans, which are closely related to lignins and help protect plants from disease.

Perhaps more importantly, the contemporary presumptions about lignin formation contradicted what is known about the specific, precise manner in which all other biochemical systems work. Thus, the team began researching lignin and lignan formation using the considerable arsenal of molecular biology analysis such as comparative genetic analysis, electrophoresis and electron microscopy.

In 1997, Lewis and the team definitively identified the existence of a previously unknown form of proteins in plants that control free-radical coupling. These dirigent proteins (from the Latin "dirigere," to guide) are not, in themselves, catalysts. In the plants studied by the Lewis team, they appear to selectively capture free radicals derived from coniferyl alcohol and guide the outcome of their coupling.



Horsetail (Equisetum telmateia) was one of the earliest plants to use lignins for structural support. Photo courtesy of UC Berkeley Digital Library

Using the analytical molecular biology approach that was so effective in uncovering the role of dirigent proteins, Lewis and the research team have now identified a link between these proteins and lignin formation and have isolated the genes that encode dirigent proteins.

The research team first cloned the portion of the gene they suspected of encoding the lignan encoding dirigent protein, then analyzed it for homologues (similar gene structures) to see if any other gene produced similar proteins of known function. None did, suggesting that the gene sequence was, indeed, producing a unique protein for a specific function.

The research team then tested the function of the protein. They predicted that, if the free-radical coupling were not random, the protein would moderate the outcome and produce a specific compound--in the case studied by the team, a substance called Pinoresinol. This prediction proved accurate, and the team had strong evidence that the dirigent protein controlled free-radical coupling very precisely.

The next piece of this particular puzzle was to link the action of the dirigent sites with lignin production. It was already known that the process of lignification, in which lignin builds up in certain plant cells, occurs very precisely and moves from the outer wall of the cell inward towards the plasma membrane. Lewis and the team assumed that this would occur only if lignification were initiated at specific sites in the outer cell wall. If dirigent proteins or a close equivalent controlled lignin formation, they reasoned, these lignification initiation sites would be home to the dirigent sites.

The team treated lignifying plant cells with polyclonal antibodies that cross-reacted with dirigent proteins. When they examined the treated cells under the microscope (light and electron), they could see that dirigent sites resided in the same area in which lignification was occurring.

Lewis is confident that the new model of lignin production and the paradigm of free-radical coupling guided by dirigent sites will open a new area of scientific investigation and knowledge. "The field," he says, "is at a turning point."

This research is funded by SC's Division of Energy Biosciences, Washington State University, and the University of Minnesota.

Contact: Norman Lewis, WSU, 509-335-3412, e-mail lewisn@wsu.edu


Related Information:

"A 20th Century Roller Coaster Ride: A Short Account of Lignification." Current Opinion in Plant Biology 1999 2:153-162

"Regiochemical Control of Monolignol Radical Coupling: A New Paradigm for Lignin and Lignan Biosynthesis." Chemistry and Biology 16 February 1999

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