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  • From: Aozotorp AT aol.com
  • To: market-farming AT franklin.oit.unc.edu
  • Subject: Soil Fungi Critical to Organic Success
  • Date: Fri, 25 May 2001 00:04:16 EDT


ttp://www.ars.usda.gov/is/AR/archive/may01/fungi0501.htm

Soil Fungi Critical to Organic Success

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 Wyndmoor, Pennsylvania.

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 inside-rather than
between-root 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.

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, Pennsylvania-a
long-time proponent of organic farming-and 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.

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
fungal proliferation.

The researchers' goal is to have suppliers sell farmers colonized host
plants for planting-not 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 their compost.

"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 application.

"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 carbon-in the form of carbohydrates-flows from the
plant to the fungi. We've also learned that at certain stages of the life
cycle-such as during spore germination-the 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."




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