Murderous fungi feed their insect victims to plants

Meaty snacks: they're not just for Venus flytraps. Almost any plant can be a carnivore with the help of fungi. Species of the ubiquitous Metarhizium genus kill insects and carry the nutrients from the corpses into the roots of plants.

"There's a neat little battle going on in the soil," says Michael Bidochka of Brock University in St Catharines, Ontario, Canada. Plants, insects and microbes battle over the nitrogen that they need.

Most plants can't take nitrogen directly out of the air or soil, so they rely on the fungi and bacteria that live in their roots to capture it from, for instance, decaying organic matter in the soil.

Among the most ubiquitous of these fungi are Metarhizium, which live on every continent and colonise most types of plants.

In addition to their role in helping plants capture nitrogen, the fungi also infect and kill many insect species. People in developing countries have even used them as a cheap way to kill locusts. The fungi kill by releasing enzymes that eat their way through an insect's outer shell. Once through the hard carapace, the fungi slowly take over the host and kill it from the inside.

Passing to the other side

Bidochka's team wondered whether there was a link between the insect killing and the plant feeding. They injected a labelled form of nitrogen into wax moth larvae (Galleria mellonella) and then infected them with fungi.

The researchers then buried the larvae in the soil with either beans (Phaseolus vulgaris) or switchgrass (Panicum virgatum) plants. Between the infected insects and the plants' roots, they placed a screen with pores too small for plant roots to penetrate but large enough for the fungi to traverse.

After 14 days, the insects were dead, and the researchers found their labelled nitrogen in the plants' tissues. It made up an impressive 28 per cent of the nitrogen in the beans and 32 per cent in the switchgrass.

Insects that hadn't been infected by Metarhizium didn't transfer any of their nitrogen to the plants after they died.

"It completes the circle," says Raymond St Leger, an entomologist at the University of Maryland in College Park. Insects eat plants, but plants get their revenge eventually.

What's next, he says, is to find out how widespread the phenomenon is in nature, and whether plants living in natural environments are as dependent on insects as a nitrogen source as the plants in the lab seemed to be.

Journal reference: Science, DOI: 10.1126/science.1222289

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