By Revving Up Key Gene, Researchers Discover They Can Manipulate The Size Of Cells In Plants

CHAPEL HILL – By revving up a key gene in plants –- making it produce more protein than it would naturally -- University of North Carolina at Chapel Hill scientists and colleagues have discovered that they can manipulate the size of plant cells without changing the overall size of the plant itself.

The discovery is something of a scientific breakthrough, the researchers say, because one day it could lead to food crops more resistant to overly dry or wet conditions and enable horticulturists to manipulate plants in other useful ways.

"Before long, we should be able to control the size of wood cells in trees," said Dr. Alan M. Jones, associate professor of biology at UNC-CH. "Since soft wood is better for making paper, for example, we could grow softer woods by increasing the size of cells in a tree trunk, or we could produce harder woods for furniture by making their cells smaller."

A report on the discovery appears in the Nov. 6 issue of the journal Science. Besides Jones, UNC-CH authors are Drs. Kyumg-Hoam Im and Ming-Jing Wu, postdoctoral fellows in biology, and graduate student Gregory DeWitt. Others are Drs. Andrew Binns and Michael Savka of the University of Pennsylvania, and Dr. Raymond Shillito, of AgrEvo, a Pikeville, N.C., agricultural development company.

In research completed in 1989, Jones identified a protein he believed to be the receptor to auxin, a major plant hormone that causes cells to grow, expand and change form. At the time, he could not link the protein to the hormone by genetic methods to prove that it was a genuine auxin receptor.

Now, using what is called a reverse genetic approach, he and his colleagues have been able to demonstrate the protein is exactly what they thought.

"We over-expressed -- or excessively turned on -- a special gene we created that makes this important receptor in tobacco," the scientist said. "Expression of this special gene, which we call a transgene, can be turned on or off by feeding plants the drug tetracycline. When we added tetracycline to the transgenic plants, what we found in both cultured cells and whole plants was remarkable."

The team discovered it could produce an over-abundance of the auxin receptor, but more interestingly, they found they had created normal-looking tobacco plants that were not normal at all. Individual plant cells were two to four the size of natural tobacco cells. As a result, plants contained fewer cells.

"We also found we could control the size of the cell by controlling expression of our transgene," Jones said. "We didn’t get any effect unless we also added the hormone auxin, and that proves the protein is definitely the auxin receptor. It is the hormone and the receptor that bind together that cause the cell expansion response."

The work is a breakthrough in the field of plant hormones that will allow the field to move forward, he said.

"Farmers have been doing genetic engineering since before the time of Jesus Christ by selecting seeds from larger fruits or particular fruits to plant," he said. "That is essentially all we are doing today in plant biotechnology, except that now we have become much more sophisticated. We are extremely excited about this work."

Although plants and animals produce some similar hormones, no hormone receptor comparable to the auxin receptor exists in human or other animals, Jones said. He and his colleagues work with tobacco cells because they are easy to transform genetically.