3.5.4S

MOLECULAR APPROACHES TO IMPROVE THE USE OF VIRUSES AS BIOCONTROL AGENTS OF PLANT PATHOGENIC FUNGI


D RIGLING, P HOEGGER and U HEINIGER

Swiss Federal Institute of Forest, Snow and Landscape Research, CH-8903 Birmensdorf, Switzerland

Background and Objectives
Double-stranded RNAs (dsRNA) as genomes of mycoviruses have been detected in many plant pathogens. While most mycoviruses have no or only minor effects on their hosts, others cause severe disease or debilitation. The increasing number of examples, where fungal virulence is influenced by dsRNAs, indicates the potential for using these genetic elements as biocontrol agents. The chestnut blight fungus Cryphonectria parasitica serves as a model system to study biocontrol of plant diseases with mycoviruses. An overview of the molecular approaches in this system will be presented.

Results and Conclusions
Biocontrol of chestnut blight has been successful in many European regions, either naturally or after field application [1]. With a few exceptions, however, biocontrol was not effective in the US. The success of biocontrol depends on the ability of Cryphonectria hypoviruses (CHV) to infect a large portion of the fungal population. Spread of CHV is limited horizontally by a vegetative incompatibility system and vertically by exclusion of the virus during sexual reproduction of the fungus and additionally, by suppressed asexual sporulation of CHV infected strains. To overcome these limitations, C. parasitica has been genetically engineered by integrating an infectious CDNA copy of the virus into the fungal genome [2]. Field studies are in progress in the US to test the dissemination and persistence of the engineered hypoviruient strains. The potential to extend the natural host range of mycoviruses was demonstrated with CHV, which was introduced as synthetic transcripts into related fungal species.
To implement an effective biocontrol, a sound understanding of the biology and ecology of the disease and the biocontrol agent is needed. In this context, a variety of molecular markers served to study the population biology of C. parasitica [3]. Furthermore, molecular markers are useful for following the movement of viruses and fungal hosts after biocontrol treatments. We have developed RT-RCR/RFLP markers which allowed us to determine the temporal and spatial dynamics of CHV in the field. These markers also have potential for assessing the ecological fitness and efficiency of different viral strains in the same natural environment. Studies on the model system C. parasitica have outlined possible molecular approaches to improve or optimise biocontrol performance. To summarise, these strategies depend on (1) the development of an infectious CDNA clone of the virus, (2) the ability to transform the fungal pathogen, and (3) the availability of molecular genetic markers for both the pathogen and the biocontrol virus.

References
1. Heiniger U, Rigling D, 1994. Annu. Rev. Phytopathol. 32, 581-599
2. Nuss DL, 1992. Microbiol. Rev. 56, 561-576
3. Milgroom M, 1995. Can. J. Bot. 73, S311-S319