BLOOMINGTON, Ind. -- Studying self-replicating genetic units, called plasmids, found in one of the world's widest-ranging pathogenic soil bacteria -- the crown-gall-disease-causing microorganism Agrobacterium tumefaciens -- Indiana University biologists are showing how freeloading, mutant derivatives of these plasmids benefit while the virulent, disease-causing plasmids do the heavy-lifting of initiating infection in plant hosts. The research confirms that the ability of bacteria to cause disease comes at a significant cost that is only counterbalanced by the benefits they experience from infected host organisms.
A. tumefaciens is widely studied for its remarkable biology not only because it causes disease in over 140 genera of broadleaf plants, including fruit trees, grapes, roses and walnut trees, but also because it is considered one of the most important tools for plant biotechnology: It is the only organism known to routinely engage in inter-kingdom lateral gene transfer. A. tumefaciens infects host plants by transferring a portion of its own DNA into plant cells, and this integrated bacterial DNA is expressed in the plant cells, leading diseased plants to develop tumors and produce resources that benefit the pathogen.
"We've identified two types of costs the plant pathogen A. tumefaciens pays for traits conferred by genes carried on plasmids," said lead author Thomas G. Platt, a postdoctoral researcher in the IU Bloomington College of Arts and Sciences' Department of Biology. "There is a relatively low cost of maintaining the tumor-inducing virulence plasmid, but there is also a dramatically large cost of expressing the genes that are required to infect plants."
Plants with crown gall disease can also benefit a second type of plasmid that can be found in A. tumefaciens: Nonpathogenic plasmids that lack the genes required to infect plants, yet are still able to benefit from the breakdown of nu
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