view of an arbuscular mycorrhizal fungus growing on a corn root. The round
bodies are spores, and the threadlike filaments are hyphae. Below,
microbiologist David Douds inspects a carrot root culture system used to study
in vitro interactions of the host plant and mycorrhizal fungi. Click the
images for more information about them.
Knowledge of Nitrogen Transfer between Plants and
Beneficial Fungi Expands By
Jim Core June 9,
New findings show that a beneficial soil fungus plays a large role in
nitrogen uptake and utilization in most plants.
In the current issue of the journal Nature, Agricultural
Research Service (ARS) chemist
E. Pfeffer and cooperators report that beneficial arbuscular mycorrhizal
(AM) fungi transfer substantial amounts of nitrogen to their plant hosts. A
lack of soil nitrogen often limits plant growth.
The studies were conducted by Pfeffer and
Douds at the ARS
Regional Research Center, Wyndmoor, Pa.; Michigan State University scientists headed by
Yair Shachar-Hill; and New Mexico State
University scientists headed by Peter J. Lammers and including graduate
student Manjula Govindarajulu.
AM is the most common type of symbiotic fungus that colonizes the
roots of most crop plants. The fungi receive glucose and possibly other organic
materials from the plant, while enhancing the plant's ability to take up
mineral nutrients, primarily phosphorus.
The scientists previously identified enzymes and genes involved in
nitrogen absorption and breakdown in AM fungi, but very little was known about
how nitrogen is moved from fungus to plant or in which form nitrogen moves
within the fungus.
The researchers discovered a novel metabolic pathway in which
inorganic nitrogen is taken up by the fungi and incorporated into an amino acid
called arginine. This amino acid remains in the fungus until it is broken down
and transferred to the plant.
The results show that the symbiotic relationship between mycorrhizal
fungi and plants may have a much more significant role in the worldwide
nitrogen cycle than previously believed. With this in mind, farmers may benefit
from promoting the proliferation of mycorrhizal fungi through diminished
fertilizer input, thereby making more efficient use of the nitrogen stores in
ARS is the U.S. Department of
Agriculture's chief scientific research agency.