A Fungus Stronger Than an Elephant

The leading crop killer and a growing threat to people with weakened immune systems, fungi--particularly some of the more vicious species--can drill into another organism's tissue with astonishing force. In today's issue of Science, researchers describe a new tool that measures this fungal power and which may help develop better fungicides.

Some fungi clutch their victims with stiff projections called appressoria, which are tipped with thin, daggerlike hypha. To launch their assault, these fungi pump substances such as glycerol into the appressoria, swelling them until the pressure forces the hypha into the host plant. Scientists have had to rely on indirect measurements, such as the concentration of glycerol or the pressure within host cells, to estimate the penetrative power.

"Now we have a way to see what these fungi are doing in real time," says Martin Bastmeyer, a cell biologist at the University of Konstanz in Germany. He and colleagues in Konstanz and Martin Luther University in Halle, Germany, planted Colletotrichum graminicola--a fungus that produces round dead spots on the leaves of corn plants--on a "waveguide," a 1-micrometer-thick silicone film sandwiched between layers of aluminum. They directed a laser beam onto the waveguide. As the fungus grew, the hypha pushed against the silicone layer, deforming it and changing the intensity of the reflected light. The researchers calculated an average invasive force of 17 micronewtons, which is consistent with measurements from previous research. Taking the hypha's 1- to 2-micrometer diameter into account, that force translates into 54.5 kilograms per square centimeter--more than 50 times greater than the force exerted by an adult elephant's foot.

"It's an amazing study," says Nicholas Money, a mycologist at Miami University in Oxford, Ohio. "Now we have real numbers to put on the forces that fungi produce." According to Money, such accuracy will be important in testing a future generation of fungicides that may work by preventing the fungi from penetrating host tissue.

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