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Mycoviruses as Fungicides?

By Alex B. Berezow

Most living things get viral infections. Obviously, animals and plants are very susceptible. But so are bacteria, and even viruses can get viral infections! So, it probably shouldn't come as a surprise that fungi can be infected with viruses, too.

Unlike most other viruses, fungal viruses (known as mycoviruses) were not thought to have an extracellular phase in their life cycle. Most viruses typically commandeer a cell's machinery to convert it into a virus factory, pumping out new viruses into the environment which then search for their next victim. Mycoviruses, on the contrary, were thought to work only intracellularly, spreading the infection into the environment only when the fungus releases spores. (The viruses would be inside the spores.) Besides, fungi have a tough outer cell wall, and it was believed that mycoviruses couldn't get in that way.

All of that appears wrong, though. A new open-access PNAS paper describes how isolated mycovirus particles can infect healthy fungi. Researchers isolated the mycovirus SsHADV-1 and applied it to a healthy strain of its host, Sclerotinia sclerotiorum, a fungus which can infect and destroy many agriculturally important crops. The virus successfully infected the fungus and visibly altered its morphology. (See figure.)

Importantly, not only is the morphology changed, but so is the fungus' virulence. When spread on the surface of Arabidopsis leaves, SsHADV-1 is able to protect the plant from infection and death by S. sclerotiorum. (See figure.)

The upper-left corner shows the negative control. This plant did not have any virus particles added to it, and the fungus killed the plant. All of the other panels show varying concentrations of viral particles, and the lowest concentration of 0.1 mg/mL conferred at least partial protection against the fungus.

Thus, the authors conclude that viruses such as SsHADV-1 could be used as fungicides.

Source: Xiao Yu, Bo Li, Yanping Fu, Jiatao Xie, Jiasen Cheng, Said A. Ghabrial, Guoqing Li, Xianhong Yi, and Daohong Jiang. "Extracellular transmission of a DNA mycovirus and its use as a natural fungicide." PNAS. Published online before print: January 7, 2013. doi: 10.1073/pnas.1213755110

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