Hunt for a botanical gene for all diseases.

In the last 5 years, biologists have struck a genetic bonanza in the armamentarium that plants use to avoid getting sick. They've found genes for resistance to bacterial infections in tomatoes, to viruses in cauliflower, to fungi that attack barley, and, just this year, to nematodes that devour sugar beets.

Each of these specific plant genes defends against a single disease-causing organism or, more often, a specific strain or variant of the organism.

Researchers have been looking for a more general defense mechanism that might allow important crops to be bred or genetically engineered to withstand a wide range of assaults. They've found the gene for what seems to be part of such a defense in the simple, widely used experimental plant Arabidopsis.

Plant biologists Karen S. Century of San Francisco State University, Brian J. Staskawicz of the University of California, Berkeley, and their colleagues screened plants grown from mutated seeds to find the first evidence of the gene about 2 years ago. They have now come up with its DNA sequence, as well as more evidence that the gene is involved in warding off very different kinds of pathogens.

The gene, labeled NDR1, "plays a really central role in plant disease resistance," says Century.

"If you knock it out, you knock out resistance to several strains" of a bacterial pathogen, Pseudomonas syringae, that attacks tomatoes. "Even more important," she adds, "it knocks out resistance to fungal pathogens," namely, several strains of the mildew-causing organism Peronospora parasitica.

This more general plant response offers some clues about the molecular bucket brigade that swings into action when a microbe or nematode attempts to infect a leaf or shoot.

A strain-specific plant gene seems to maintain a constant supply of a protein that recognizes some marker on the invader. The recognition proteins pass the signal down the line, where it eventually reaches NDR1, which is structurally unlike the strain-specific genes. The NDR1 gene encodes a protein whose amino acid sequence suggests that it's associated with a membrane somewhere in the cell.

Another difference is that production turns on only when a pathogen is present, the researchers report in the Dec. 12 Science. "It's novel," says Century, "kind of a pioneer protein."

She and her colleagues are now working to determine exactly what the protein does and how it manages to thwart such different invaders. They are also testing it for activity triggered by still more pathogens, including viruses.

Undoubtedly there are other important genes to be found in the general biochemical pathway by which plants resist disease (SN: 8/14/93, p. 103), says molecular biologist Xinnian Dong of Duke University in Durham, N.C., but NDR1 "is significant."

The gene "may be tying together many different receptors," says Christopher R. Somerville of the Carnegie Institution of Washington in Stanford, Calif. More intriguing is that it may even be tied to the defense system in animals.

Researchers have found similarities between many plant disease resistance genes and the genes that enable the well-studied fruit fly Drosophila to fight off fungal attack. The signals in the resistance pathway may therefore have their evolutionary roots in the early development of multicellular organisms.

The genes recently found in Arabidopsis can now be used to find their counterparts in crop plants. Century, for example, is working on disease resistance in rice.