Researchers at Michigan Technological University have discovered a gene that may have played a key role in the evolution of hardwood trees such as oaks and maples. Their work is featured on the cover of the July 11 issue of The Plant Cell.
Millions of years ago, gymnosperms--including conifers such as pines and redwoods--were the only type of plants on earth. Then angiosperms--the flowering plants--appeared, among them hardwood trees.
While angiosperms are considered more advanced than gymnosperms, their origins largely remain a mystery. At least a part of that mystery may now be solved, thanks to the work of the lead author, Dr. Laigen Li, and researchers at the Plant Biotechnology Research Center in Michigan Tech's School of Forestry and Wood Products.
The Michigan Tech researchers, including Dr. Vincent Chiang, the center's director, have described the genetic pathway used to create syringyl lignin, a type of lignin that is unique to angiosperms. Lignin is found in all trees and is the substance that makes them stiff. But in gymnosperms, also known as softwoods, only guaiacyl lignin is present. In hardwoods, both guaiacyl and syringyl lignin are found.
The researchers identified and, for the first time, cloned a gene from aspen, an angiosperm, which they suspected was responsible for producing syringyl lignin. They introduced the gene into E. coli bacteria, and found that it produced a protein with a very specific purpose: It assembled hardwoods' syringyl lignin.
For years, most scientists have believed that another gene controlled the production of both lignin types. "But we thought it didn't make sense for plants to evolve new proteins and still use the old gene," Chiang said. "Our discovery of a syringyl-specific gene overturns that traditional model; it's been very exciting."
The researchers also have identified two characteristics of syringyl lignin that could give hardwoods an evolutionary advantage. Before angiosperms appeared, the function of lignin in plants was primarily to conduct water and other nutrients. In angiosperms, however, syringyl lignin took on an important mechanical role, serving as the "skeleton" for angiosperm trees. In addition, Chiang notes, the syringyl lignin molecule includes additional methoxyl (H3CO) groups that the researchers suspect increase its toxicity. A tree with this type of lignin could be less vulnerable to disease, Chiang said.
The article, "The Last Step of Syringyl Monolignol Biosynthese in Angiosperms Is Regulated by a Novel Gene Encoding Sinapyl Alcohol Dehydrogenase," is the first on the subject of a tree species to appear in The Plant Cell since the journal was first published twelve years ago. In addition to Li and Chiang, the coauthors are Dr. Xio Fei Cheng and Dr. Scott Harding of Michigan Tech, Dr. Jacqueline Leshkevich, formerly of Michigan Tech, and Toshiaki Umezawa of Kyoto University.
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