Plant-Friendly Fungus

A soil fungus, Metarhizium, which is a widespread insect pathogen and killer, has been found to purvey insect-derived nitrogen to plant hosts.

Fluorescently labeled Metarhizium fungus (green) is associated with the roots of a plant. Inset: Dead Galleria mellonella larva was infected with Metarhizium.

Michael J. Bidochka

To satisfy their need for nitrogen, most plants rely on nitrogen-fixing bacteria, which convert atmospheric nitrogen into a form plants can use. Other sources are decomposing organic matter (thanks mostly to bacteria) and even lightning, which can fix nitrogen. Carnivorous plants, such as the Venus flytrap, snare nitrogen-rich insects. Now researchers report on another strategy: enlisting a fungal friend.

Metarhizium, a globally abundant soil fungus, is a widespread insect pathogen and killer that also can be found residing in a mutualistic fashion, as an “endophyte,” inside the roots of plant hosts. Scott W. Behie of Brock University in Ontario, Canada, and two colleagues tested Metarhizium’s potential to purvey insect-derived nitrogen to plant hosts.

The team first injected larvae of the waxmoth Galleria mellonella, a soil-borne insect, with 15N—a nitrogen isotope scarce in nature and commonly used as a tracer. They wanted to see how much insect-derived nitrogen ended up in the leaves of two plant species: Phaseolus vulgaris, the quick-growing haricot bean, and Panicum virgatum, a slower-growing perennial bunchgrass. The researchers sterilized and germinated seeds from both plants, then planted seedlings in containers with sterile soil. In some of the containers they placed live waxmoth larvae, either infected with Metarhizium spores, infected with Aspergillus flavus—another fungal insect pathogen, but not an endophytic one—or uninfected. A fine mesh that only fungal threads could penetrate separated the larvae from plant roots.

The infected larvae quickly died. After two weeks, most of the nitrogen in bean and bunchgrass leaves had come from Metarhizium-infected insects, significantly more than in plants exposed to A. flavus infected larvae, uninfected larvae, or to no larvae at all. By week four, insect-nitrogen levels fell further in the fast-metabolizing beans but increased in bunchgrass, both still well ahead of the non-Metarhizium preparations. (Science)