Nov. 19, 1998 PULLMAN, Wash.--Given the nature of nature, it should come as no surprise that when viruses attack, plants don't just sit there and take it. They fight back.
James Carrington, WSU professor at the Institute of Biological Chemistry, studies plant-virus interactions from both the virus and the host ends. Recent work in his lab suggests that it is not just the plants that fight back. At least some plant viruses have figured out how to overcome gene silencing, one of the most elegant of the plant defense systems. Carrington and co-researcher Kristin Kasschau report on their work in the November 13 issue of the journal Cell.
Gene silencing differs from many other plant defense systems in that it is adaptive. The plant perceives information from the infecting virus in the form of the virus's genome. The plant then designs a counter attack that is determined by and specific for that genome. The potential diversity of the system appears to be unlimited, for its diversity is not pre-defined in the plant genome. Silencing is as adaptable as the human immune system, for it customizes its response to each individual virus.
The silencing response may not be limited to the plant cell the virus is actively infecting, but may also spread throughout the plant, especially into newly dividing cells at the plant's growing points. This enables plant cells far removed from the initial infection site to be prepared when viruses get to them.
"Given the vast adaptability of gene silencing, it's reasonable to suggest that it is a general anti-viral defense mechanism capable of acting against a broad range of viruses," says Carrington. It is also reasonable to suggest that some viruses have evolved to overcome gene silencing.
Carrington's investigation into how a virus overcomes plant defenses, particularly a defense as powerful as gene silencing, has culminated in about ten years of work on a protein called HC-Pro. HC-Pro is made by the tobacco etch virus (TEV) and has many functions in the viral infection process.
Successful TEV infections have several steps, as do most plant viral infections. First the virus must move from plant to plant. Then the virus must replicate within the cells that are first inoculated with it, move to adjacent cells within the plant, and move throughout the plant via the plant's vascular system. The fact that HC-Pro is necessary for most of these steps suggested to Carrington that it may function by suppressing the plant's defense response.
In order to test this hypothesis, Carrington and co-worker Kristin Kasschau produced genetically modified plants in which the gene silencing system was triggered against a foreign "reporter" gene. These plants were immune to viruses engineered to contain that reporter gene, but were not immune to otherwise identical viruses that did not contain the gene.
Carrington and Kasschau then crossed these plants with another group of plants that were genetically modified to contain a copy of the HC-Pro gene and the crosses were challenged with the test virus. Without exception, plants that contained the HC-Pro gene were unable to silence the reporter gene and therefore were susceptible to all of the test viruses.
"This means that viruses like TEV can promote their own infections by knocking out a potent defense response," says Carrington.
The combination of the plants' ability to silence incoming viruses and the capacity of some viruses to suppress this host response means that virus infections in plants are much more complicated than once thought, Carrington adds. "Immune systems that can recognize new viruses were thought to be a unique property of animals. Now we know that plants have a flexible, adaptive defense response that may be just as effective as the human immune system in limiting damages due to viruses. The finding that some viruses possess counter-defensive mechanisms that overcome the plant's silencing system indicates that viruses come prepared to do battle."
The research Carrington conducts now is focused on identifying the plant genes that are necessary for anti-viral responses such as silencing. Using the plant Arabidopsis thaliana as a model, Carrington hopes to find plant mutants that are unable to mount defense responses. These mutants will help him learn what genes are important in defining the plant's immune system, and possibly how the virus targets parts of this system to enhance its own infections.
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