Dr. Xuemin Zhang, Senior Scientist in Dr. Donald Nuss' laboratory at UMBI's Center for Biosystems Research (CBR), presented findings that may one day help us to control the Chestnut Blight fungus, a pathogen responsible for the deaths of Chestnut trees all over the East Coast of the United States. In a seminar presented at UMBI's Shady Grove campus on January 15, Dr. Zhang described research on the interactions that take place between the Chestnut Blight fungus, Cryphonectria parasitica, and a virus-known as a hypovirus-- which infects the fungus and prevents it from killing the Chestnut trees. This is a case of one pathogen-the hypovirus-controlling another pathogen-the Chestnut Blight fungus.

The research concerns a fascinating mechanism-RNA silencing-- that the fungus deploys to defend itself against the virus. Although RNA silencing as an antiviral defense response has been described in a variety of animals and plants, the studies by Dr. Zhang and other colleagues in Dr. Nuss' laboratory provided the first definitive evidence that RNA silencing also serves as an antiviral defense mechanism in fungi.

Studying the mechanism of RNA silencing in the Chestnut Blight fungus may shed light on the general mechanisms of viral defense in other organisms such as human beings. In addition, blocking the viral defense mechanisms of the Chestnut Blight fungus might provide a means to save Chestnut trees which once dominated East Coast forests. The majestic trees, which grow to heights of 100 feet and can live for 600 years, were devastated by an outbreak of the fungus which began in New York in 1904, and by 1950 had killed 90% of the Chestnut trees all over the East Coast.

The Nuss laboratory has described the role played by a key defensive gene in the fungi known as dicer-2, dcl-2, a dicer-like gene that is closely related to dicer genes in other fungi plants and animals that are involved in RNA silencing. The investigators have shown that when they knock out the dcl-2 gene, the fungus becomes highly susceptible to virus infection.

How does dcl-2 normally defend against the virus. The investigators have shown that when the fungus is exposed to the virus, virus-derived small interfering RNA (vsRNA) -short sequences of 21 nt RNA are produced-as a result of dcl-2's cleavage of viral double stranded RNA.

The investigators clearly showed that if dicer-2 activity is stopped by knocking out the gene, the production of the short sequences of RNA is curtailed, and the level of virus accumulation increases dramatically. The investigators also showed that RNA silencing can be restored by adding back the dicer-2 gene.

The investigators also discovered a new and surprising role for RNA silencing in viral RNA recombination. Their study suggested that the dcl-2 gene contributes to the production of defective interfering RNAs (DI RNA), the internal deletion forms of their parental RNA, which interferes with the replication of the parental viral RNA. They also showed that dcl-2 contributes to the instability of foreign gene sequences in viral RNA gene expression vectors, a major obstacle to the use of RNA virus vectors for practical applications such as gene therapy.

Future work will involve closer studies of RNA silencing-mediated viral RNA recombination, and experiments designed to better understand the precise mechanisms of RNA silencing by the fungal cells. In addition, the scientists will also see what happens when they suppress RNA silencing by other means.

These studies, supported by grants from the National Institutes of Health (NIH), should continue to provide important insights into the general mechanisms of RNA silencing and viral RNA recombination, while providing a possible means to control the Chestnut Blight fungus.