by Robert Sanders
Two teams of scientists at Berkeley and the US Department of Agriculture have reported a milestone in the search for genes that enable plants to fend off disease.
While perhaps hundreds of such disease-resistance genes exist, only a few have actually been found and cloned, and how they work is still a mystery.
The Berkeley and USDA teams have isolated and cloned two new genes that are unique in representing a closely knit family that confers resistance to a variety of plant diseases in a broad range of plants.
A third gene reported at a scientific meeting earlier this year appears to be a member of the same family.
The discovery of a family of genes that operate against a broad range of disease organisms brings closer the eventual insertion of genes like these into commercial crops to make them resistant to disease.
Disease-resistant plants would reduce the need for chemical pesticides.
"This is a big breakthrough in plant biology," says Brian J. Staskawicz, professor of plant biology and leader of one of the teams.
"These findings will allow us to determine how these genes work, and then manipulate them to engineer almost any plant for resistance."
The genes, found in tobacco, a mustard-like plant called Arabadopsis, and flax, encode for resistance to a virus, a bacteria and a fungus, respectively.
Such broad resistance within a single family of genes could signify that the genes will be easier to transfer into other plants, the scientists say.
"The amazing thing about these three genes is their similarity, considering how diverse the plants and pathogens are," says Barbara Baker, leader of the second team.
She is an adjunct assistant professor of plant biology at Berkeley and a researcher at the Plant Gene Expression Center in Albany, operated jointly by Berkeley and the USDA's Agricultural Research Service.
"This will help us understand the anatomy of disease resistance and bring us closer to inserting resistance genes into other species that may suffer from similar diseases."
Disease-resistant plants can be created through normal breeding, she says, but this can take decades. Standard crossbreeding works only between closely related plants anyway, so it would be impossible to breed resistance from one species into another.
Staskawicz, Andrew Bent, currently an assistant professor at the University of Illinois, Barbara N. Kunkel, now an assistant professor of biology at Washington University in St. Louis, and their colleagues reported their findings in the Sept. 23 issue of Science magazine. They cloned a gene (RPS2) from Arabidopsis thaliana that confers resistance to a common bacterial pathogen, Pseudomonas syringae.
A separate group at Harvard, led by Fred Ausubel, have cloned the same gene, which they report in a September issue of the journal Cell.
The second team, led by Baker, Berkeley graduate student Steve Whitham, and postdoctoral fellow Dinesh Kumar, also report their results in a September issue of Cell. They found and cloned a gene (the N gene) that makes tobacco resistant to tobacco mosaic virus (TMV), a common and destructive virus that attacks plants in the tobacco and tomato families.
The N gene is the first plant gene to be cloned that defends against a plant virus.
The third team of researchers, led by Jeffrey Ellis from the Commonwealth Scientific and Industrial Research Organization in Canberra, Australia, cloned a gene (the L-6 gene) from flax that confers resistance to a rust disease caused by the fungus Melampsora lini. They reported this development at a meeting in Scotland earlier this summer.
Baker now is trying to insert the tobacco gene into tomato plants to see if she can make tomatoes that are resistant to tobacco mosaic virus. Resistant tomatoes already exist, she says, but her experiments will tell her whether a gene from one plant will operate in a different species.
Staskawicz plans to insert his Arabidopsis gene into tomatoes also to test whether it functions in a different species.
Staskawicz and Baker both expect to encounter problems transferring disease-resistance genes from one plant to another.
In particular, it is unclear whether a gene that works in one plant will work in another, even against the identical viral, bacterial, or fungal strain.
According to Staskawicz, the genes themselves are involved somehow in the recognition of disease organisms. The genes may code for the protein receptor that recognizes foreign proteins sitting on the outer surface of these pathogens.
Once a plant recognizes a pathogen, it releases enzymes and free radicals that attack and kill the pathogen.
Plants also react with a "hypersensitive response" that kills off part of the infected plant as a defense mechanism to protect the remaining parts of the plant.
The finding that related genes work against a broad range of plant pathogens and in a broad range of plants suggests there may be a common trigger that sets off these responses in all plants, Staskawicz says.
"This is an exciting time for plant research. We're seeing a common motif in these genes that encode resistance to totally different types of pathogens," Staskawicz says. "This will allow us to manipulate them and determine how genes work, and even use this common motif to pull out other disease-resistance genes.
"This has been a bottleneck for many years."
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