Soil Fungi Critical
to Organic Success
Micrograph of fungal arbuscules (treelike
structures) in the cells of leek roots.
|One casualty of America's
agricultural revolution were valuable native soil fungi that enabled crops to
grow well with less water, nutrients, and pesticides.
Increased agricultural productivity has been largely dependent on high levels
of chemical fertilizers and synthetic pesticides. There is a growing interest
in reducing this dependency by encouraging biologically based systems to
enhance productivity and product quality on farms.
That's exactly what ARS chemist Philip
E. Pfeffer and his co-workers hope to accomplish by helping farmers reestablish
the beneficial soil organisms called mycorrhizal fungi. Pfeffer is at the
Agricultural Research Service's Eastern Regional Research Center (ERRC) in
At Stoneleigh Estate in Villanova,
Pennsylvania, chemist Philip Pfeffer
(left) and microbiologist David Douds
inspect a tropical grass planted to
produce compost-based inoculum.
| Mycorrhizal fungi live within the
roots of most plants in a mutually beneficial relationship (symbiosis). They
help roots scavenge more nutrients and water from the soil in exchange for
sugar to make the molecules they need to live and grow. These fungi extend long
threads, called hyphae, outside the roots. The hyphae transport phosphorus and
other nutrients into plant roots. Mycorrhizae also enable plants to use water
more efficiently and resist pests.
Pfeffer and co-workers study the most common type of mycorrhizae, which are
called endomycorrhizae because the fungi live insiderather than
betweenroot cells. They are also called arbuscular mycorrhizae because of
the treelike structures (see photo above), or arbuscules, they build within the
cells. The branches transfer nutrients to the plant cells in exchange for sugar
for the fungi. The trunks of the arbuscules attach to the hyphae.
Microbiologist David Douds
inspects carrot root culture
system used to study in vitro
interactions of the host plant
and mycorrhizal fungi.
| Today, farmers who grow row crops,
like corn and soybeans, must rely on whatever soil fungi survived the decades
of high chemical application that began when American agricultural production
intensified in the 1950s.
Horticultural crop producers fare better because they can buy potting mixes
with beneficial fungi added. Home gardeners can buy the fungi as soil
inoculants from seed catalogs. But it's impractical for farmers to buy and
apply the large quantities of fungi they'd need for farm fields.
On-Farm Fungi Production
One member of the ARS team, David D. Douds, is supervising experiments to find
practical ways for farmers to grow and apply their own mycorrhizal fungi. At
the Rodale Institute Experimental Farm in Kutztown, Pennsylvaniaa
long-time proponent of organic farmingand at nearby Stoneleigh Estate,
Douds has tried growing the fungi in compost. That would enable farmers to
apply the fungi along with compost with no extra effort or cost.
Chemist Gerald Nagahashi
transfers roots into liquid
medium. Compounds released
into the medium by these
roots will be used for in
| Douds planted a tropical grass in
the compost after inoculating its roots with arbuscular mycorrhizal fungi. He
hoped the roots would harbor the fungi and spread them throughout the compost,
but the fungi didn't spread well enough.
"We think the compost was so rich in nutrients that the grass roots didn't
encourage the fungi to proliferate, because their help wasn't needed for
getting nutrients," he says. This spring, Douds and his colleagues will
try mixing the compost with a less nutrient-rich soil to see if that promotes
The researchers' goal is to have suppliers sell farmers colonized host plants
for plantingnot as a crop, but to start colonies of the fungi in soil,
compost, or a compost/soil mix. Then, after the fungi have had time to
multiply, farmers would apply the colonized soil in manure spreaders along with
"Instead of farmers having to buy and transport a whole field's worth of
inoculum, they could buy a small fraction packed in with host seedlings. Then,
they'd plant the mycorrhizal seedlings and increase the inoculum on their
own," Douds explains. Farmers would eventually have crop fields with
colonies of beneficial microbes rivaling those of yesteryear.
How Host Roots Communicate With Mycorrhizal Fungi
Another member of the ARS team, chemist Gerald Nagahashi, has found that plant
roots release signals to encourage or discourage proliferation during at least
two of the fungi's seven life stages. He found that in the first stage, when
the fungal spores start growing hyphae in the soil, roots exude compounds that
encourage prolific hyphal growth. This helps the fungus find the root, colonize
it, and produce the arbuscules.
"We used carrot roots for these studies because they're a good model and
can be grown easily in liquid culture," Nagahashi says. The scientists
have improved techniques for growing the fungi with carrot roots in petri
dishes and hope to do this someday without the roots.
Nagahashi also did an experiment using light and found it triggered more fungal
hyphal branching. "We predicted that exposing inoculum to light would
increase colonization of corn seedlings by the fungi, and that's exactly what
happened," he says.
The team is studying the basic physiology of the mycorrhizal fungi and their
interactions with plants so they can find a way to grow fungal colonies without
host plants. This would permit large-scale production of inoculum for field
"We also need to find out what nutrients and other conditions have to be
met so the fungi grow through all seven life stages and multiply," Pfeffer
says. "Using specially marked molecules and nuclear magnetic resonance
spectroscopy to analyze a mycorrhizal system in a petri dish, we've learned a
great deal about how carbonin the form of carbohydratesflows from
the plant to the fungi. We've also learned that at certain stages of the life
cyclesuch as during spore germinationthe fungi can take carbon
directly from the soil without getting it from plants, which encourages us.
"However, we need to find out why the spores are unable to utilize this
carbon to replenish their lipid stores, which are needed for completion of
their life cycle," says Pfeffer. "We'd like to be able to feed the
fungi glucose in the laboratory and have them reproduce in mass quantities in
fermentation vats, as we do with bacteria and other fungi."
In collaboration with Yair Shachar-Hill and Peter Lammers at New Mexico State
University, the ERRC team is also examining gene expression of these fungi at
each life-cycle stage. With this information the researchers hope to turn on
the mechanisms necessary for the fungi to complete their life cycle in the
absence of the host plant.By
Agricultural Research Service Information Staff.
This research is part of Soil Resource Management (#202) and Plant
Biological and Molecular Processes (#302), two ARS National Programs described
on the World Wide Web at http://www.nps.ars.usda.gov.
The ARS scientists mentioned in this article are at the ARS
Eastern Regional Research Center, 600
East Mermaid Lane, Wyndmoor, PA 19038; phone (215) 233-6400, fax (215)
233-6581, e-mail email@example.com
"Soil Fungi Critical to Organic Success"
was published in the May 2001 issue
of Agricultural Research magazine.