Mycoviruses may reduce the virulence of plant-pathogenic fungi: A potential way of biocontrol

Fungi are pathogenic to plants as well as to humans and animals. The plant-pathogenic fungi may destroy the crops and lead to significant alterations in ecosystems. The chestnut blight fungus Cryphonectria parasitica (formerly Endothia parasitica) and the Dutch elm disease fungi Ophiostoma ulmi and Ophiostoma novo-ulmi are examples of the plant-pathogenic fungi.

Orange sunken canker on the main stem of an American chestnut caused by C. parasitica Image source: www.sfws.auburn.edu/enebak/4h/stem/stem.html

The current conventional measures for control of fungal infections in plants include the use of chemical fungicides and agronomic practices. However, emergence of resistance to the fungicides remains as a potential hazard and the issue is further complicated due to environmental concerns.

Mycoviruses persistently infect their fungal hosts and usually do not cause any discernable phenotypic changes. However and exceptionally, some mycoviruses may alter the virulence of the infecting fungi. This phenomenon of 'mycovirus-mediated attenuation of fungal virulence' is called hypovirulence. Due to the limitations of the current control measures, the potential utility of mycoviruses in biocontrol of plant fungal diseases has drawn attention.

The mycovirus infection is transmitted to a virulent fungal strain via intracellular routes, by cytoplasmic exchange during fusion of hyphae (=anastomosis) or during formation of the spores.

The members of Hypoviridae family infect and cause hypovirulence in C. parasitica. An infectious cDNA-based reverse genetics system which has been developed for Hypoviridae family has provided information on virus-fungus-plant interactions and proved to be a useful tool for engineering mycoviruses for biocontrol.

Hypovirulence is being used for the control of chestnut blight in European chestnut orchards and has proven to be successful in reducing the severity and extent of the chestnut blight epidemic as a result of the natural spread through the European chestnut forests. On the other hand, while the artificial introduction of the hypovirulent C. parasitica has been unsuccessful in effective biocontrol in North American ecosystems, naturally occuring hypovirulence proved to contribute to chestnut tree survival in Michigan, a region that is outside the natural range of the American chestnut. The factors that have limited the effectiveness of hypovirulence in North American ecosystems are probably the inability of the introduced hypoviruses to spread through the natural C. parasitica populations in forests, the barriers to cytoplasmic hypovirus transmission, and the phenotypic characteristics of the hypoviruses used for biocontrol.

Further studies on engineering hypovirulence-associated mycoviruses may provide broad applications of biocontrol for fungal diseases of fruit trees and crop plants.

Related reading

• Nuss DL. Hypovirulence: Mycoviruses at the fungal-plant interface. Nature Reviews 2005; 3: 632-642.