A 'dispatcher' gene--described in the Dec. 12, 1997, issue of Science--seems to juggle assignments for many `sentry' genes in a model plant system and may ultimately help researchers design hardier, more disease-resistant food plants, a University of Delaware scientist says.
"This dispatcher, the NDR1 gene, apparently processes a great deal of disparate information from many sources upstream to coordinate the disease-resistance response in this model plant--Arabidopsis thaliana, a common mustard weed," explained Allan D. Shapiro, an assistant professor in UD's Department of Plant and Soil Sciences. "It's possible that other dispatcher genes, similar to NDR1, may mediate disease-resistance in crops, too." (Shapiro coauthored the Science paper, with lead author Karen Century, senior research team member Brian J. Staskawicz of the University of California at Berkeley and others.)
Researchers worldwide are scrambling to identify individual genes that protect crops from specific pathogens, Shapiro notes. Already, researchers have described several of these disease-fighting genes in corn, rice and tomatoes. But, he says, resistance genes are often "highly specific," targeting only a particular pathogen.
The NDR1 gene, however, seems to dispatch messages from many resistance genes, allowing Arabidopsis to defend itself against a broad range of attackers, Shapiro says. If NDR1 is mutated, he reports, the plant becomes susceptible to numerous strains of the bacterial pathogen, Pseudomonas syringae, and the fungal pathogen, Peronospora parasitica.
Exactly how NDR1 translates messages from different pathogens and resistance genes remains a mystery--for now. But ongoing research may point to the exact mechanisms at work. "The ultimate aim," Shapiro says, "is to engineer more disease-resistant plants by understanding how the process works in nature and then souping it up."
After identifying NDR1 as a critical dispatcher by isolating mutant plants lacking a functional copy of the gene, the research team then used the mutants to go after the gene itself. The approach, called "positional cloning," involves crossing the mutant plant to a different race of Arabidopsis, Shapiro says. The genetic code of the two races is different in numerous places throughout the Arabidopsis chromosomes. By following both the behavior of these differences and disease resistance in subsequent plant generations, the researchers were able to identify NDR1's location: a small region of Arabidopsis chromosome 3. Next, they identified pieces of DNA corresponding to this region of the chromosome. Finally, they identified a smaller piece of DNA containing NDR1 by introducing the DNA into the mutant plants. This extra DNA made the plants disease-resistant, Shapiro notes.
Based on the sequence of this piece of DNA, he says, NDR1 was classified as a novel gene--not yet identified by any other researchers. Although the sequence didn't reveal a precise function for NDR1, Shapiro says it provides valuable tools for use in future research.
"This is still basic science," Shapiro cautions. "But this work should help us to further delineate both the function of this particular gene, and the signal transduction pathways by which information flows from resistance genes."
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