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LICHENS—An
Unlikely Source
of New Herbicides
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Chemist Dhammika Nanayakkara observes a crude
fraction from a lichen extract.
(K9228-1)
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In their search for new and more
environmentally friendly herbicides, scientists are leaving no proverbial stone
unturned. Now Agricultural Research
Service scientists in collaboration with the National Center for Natural
Products Research, University of Mississippi-Oxford, have found a natural
compound in lichens that may be a potential new herbicide.
"Lichens are unusual and intriguing organisms," says Franck E. Dayan,
a plant physiologist in the ARS Natural Products Utilization Research Unit at
Oxford, Mississippi. "They're a hybrid of two kingdoms—the
quintessential symbiosis of a fungus and an alga.
"As a successful alliance between these two organisms, lichens live as a
single organism. Both inhabit the same body, or thallus, with each doing what
it does best and thriving as a result of the natural cooperation." |
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Usnic acid, a natural lichen product, is under
study as a potential herbicide. Plant physiologist Franck Dayan
examines the molecular interactions between usnic acid and the
enzyme it inhibits.
(K9227-1)
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After their first meeting—if the fungus and alga are compatible—they
can merge. But only certain algae and specific fungi can get together to form a
lichen. When it's possible, the fungus creates the thallus to house both
organisms. Each fungus-alga union results in a unique type of thallus.
Systematists use this structure to help assign names to lichens and to make
identifications.
"The scientific names of lichens are based on the fungal parts, which are
very diverse taxonomically, with over 20,000 different species," says
former ARS plant physiologist Joanne G. Romagni. She worked with Dayan at the
Oxford lab and is now on the faculty of St. Thomas University in Houston,
Texas.
"In other words, many different kinds of fungi are lichenized over
time," she says. "However, relatively few species of algae are
associated with lichens—only 20 to 40 different species—and most of
these are common, single-celled green algae." |
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Closeup of the lichen Cladina rangiferina collected
by plant physiologist Joanne Romagni.
(K9230-1)
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According to Dayan, "Once the
two are joined, the alga will begin to use sunlight to make sugars as food for
itself and the fungus. The fungus provides protection from environmental
stress, such as excess light." And, unlike the alga, the fungus reproduces
sexually.
Dayan says that of the more than 20,000 known lichen species, only a few have
been analyzed and identified as containing biologically active compounds. These
natural compounds typically arise from the secondary metabolism of the fungal
part of the lichen. They are deposited on the surface of threadlike
hyphae—the tiny filaments that connect fungi to a food source or
host—rather than compartmentalized in the cells. This location makes them
easier to extract.
"Most of these compounds are unique to lichens," says Dayan,
"with only a small minority—about 60 out of over 600 known lichen
compounds—occurring in other fungi or higher plants. While the secondary
chemistry of lichen compounds is better documented than that of any other
plantlike group, the bioactivity associated with these compounds has been
generally ignored."
Although natural lichen products have been traditionally overlooked, the ARS
scientists and University of Mississippi natural products chemist Dhammika
Nanayakkara discovered that one common lichen metabolite—usnic
acid—inhibits carotenoid synthesis. This chemical compound has also been
found to have antihistaminic, antiviral, and antibacterial activities. For the
first time, the researchers can explain how usnic acid is toxic to plants
(phytotoxicity).
"It works by bleaching the first leaves a plant forms, causing a decrease
of both chlorophylls and carotenoids in treated plants. Usnic acid does this by
preventing photosynthesis through a key enzyme involved in pigment
biosynthesis," Nanayakkara says. "This bleaching activity was found
to work in several plants, including barley, lettuce, and cucumber. Maybe it
could be used to control the growth of weeds, as well."
Although several synthetic compounds inhibit this key pigment enzyme, the
scientists found that usnic acid was over 10 times more effective than others
tested. Their finding is one of the first examples of a natural product's
inhibiting this herbicide target site, which is so critical to the process of
carotenoid biosynthesis.
The lichen project provides the basis for developing an entirely new area of
research: exploiting lichens as sources of natural herbicides.—By
Hank Becker,
Agricultural Research Service Information Staff.
This research is part of New Uses, Quality, and Marketability of Plant and
Animal Products, an ARS National Program (#306) described on the World Wide Web
at http://www.nps.ars.usda.gov.
Franck E. Dayan is in the USDA-ARS
Natural
Products Utilization Research Unit, Room 2012, University of Mississippi,
Oxford, MS 38677-8048; phone (662) 915-1039, fax (662) 915-1035. |
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"LICHENS—An Unlikely Source of New
Herbicides" was published in the
January 2001
issue of Agricultural Research magazine.
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