Biologists Discover New Pathway Into Plant Cells

The scientists have identified for the first time a protein that can cross plant cell membranes, where it functions as a toxin to kill the cell. It had been known that viruses and bacteria can penetrate cell wall barriers and disrupt plant cells, but never before has a protein been found that could do this by itself.

When more research is done, this may provide a new tool to penetrate plant cells and possibly manipulate their behavior in some beneficial way - to grow faster, resist disease or increase yields.

The findings were published today in two articles in The Plant Cell, a professional journal.

Also of considerable interest is that the biological mechanism discovered here bears striking similarity to the way proteins can function in mammalian cells - scientists say they may have found a characteristic that has been preserved for more than 600 million years, when plants and animals diverged from a common ancestor on their separate evolutionary paths.

"This is a doorway into plant cells that we never knew existed," said Lynda Ciuffetti, an OSU professor of botany and plant pathology. "Viruses and bacteria have been known to bring proteins into cells, but this is just a protein by itself crossing the cell wall barrier without disrupting its integrity. This is a significant fundamental advance in our understanding of plant biology."

The research was done with a pathogenic fungus that causes tan spot of wheat, a costly plant disease that is found around the world, and in some places can cause crop losses ranging up to 50 percent. These fungi produce multiple toxins that attack wheat plants, reducing yields and ruining wheat used as seed. In the United States, it's a particular problem in the Great Plains and Midwest. Ciuffetti has spent much of her career studying these "host-selective" toxins.

"Until now, we didn't really know exactly how the protein produced by this fungus was causing disease, whether it was from inside or outside of the plant cells," said Viola Manning, an OSU faculty research assistant and co-author of both publications. "No one had ever shown before that a protein could move, without a pathogen's assistance, from outside a plant cell to the inside. But in this case, the protein does penetrate the cell membrane and interacts with chloroplasts, ultimately leading to cell death."

The scientists said this mechanism probably will be found in other cells besides wheat, and with other proteins. And while it may lead ultimately to some way to help address this plant disease problem in wheat, the more important discovery is the new pathway into plant cells.

"We still don't know exactly how the protein penetrates the cell, but it's clear that it does," said Andrew Karplus, an OSU professor of biochemistry and biophysics. "And with work done by a graduate student, Ganapathy Sarma, we also now have a clear understanding of what the molecular structure of the toxin looks like. With continued research, we should not only be able to determine how the protein is getting into the cell, but also remove the toxic effect associated with it.

"What that would leave us with is a type of delivery vehicle, a completely new way to deliver compounds inside of a plant cell and target specific genes. This is a new and unprecedented insight into how plants can work."

The process of proteins getting inside of cells and affecting their behavior is common in animal cells, the scientists said. For instance, that's how the AIDS virus causes its damage. But the same process had never before been shown to exist in plant cells, which have been evolving separately from animals for hundreds of millions of years.

With a new delivery mechanism such as this, applied research could be done either to help or harm plant cells - by increasing or controlling their growth, or introducing new characteristics.

The research was supported by the National Science Foundation and the National Research Initiative of the Cooperative State Research, Education and Extension Service, an agency of the U.S. Department of Agriculture.