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Lichens: An Unexpected Source of New Herbicides

A new study of lichens provides the basis for developing an entirely new area of research-- exploiting these unusual organisms as sources of natural herbicides.

A natural compound in lichens that may be a potential new herbicide has been found by Agricultural Research Service scientists at the Natural Products Utilization Research Unit, Oxford, Miss., and colleagues at the National Center for Natural Products Research, University of Mississippi-Oxford.

Lichens are a hybrid of two kingdoms--a fungus and an alga living symbiotically. Of the more than 20,000 known species, only a few have been analyzed and identified as containing biologically active compounds.

These compounds typically arise from the secondary metabolism of the lichen’s fungal part, and most are unique to lichens. Only a small minority--about 60 out of over 600 known lichen compounds--occur in other fungi or higher plants.

The bioactivity associated with these compounds has been generally ignored. However, the team found that one common lichen metabolite--usnic acid–indirectly inhibits photosynthesis.

Now the researchers have explained the phytotoxicity of usnic acid for the first time. 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. This bleaching activity was found to work in several plants including barley, lettuce and cucumber. It could also be made to occur in weed plants, as well.

Although several synthetic compounds inhibit this key enzyme, the scientists found that usnic acid was more than 10 times more effective than other compounds tested in the laboratory. Their finding is one of the first examples of a natural product inhibiting carotenoid biosynthesis in plants.

For more details, see the January 2001 issue of Agricultural Research magazine. It’s also available online.

ARS is the chief scientific research agency in the U.S. Department of Agriculture.

Scientific contact: Franck E. Dayan, ARS Natural Products Utilization Research Unit, Oxford, Miss., phone (662) 915-1039, fax (662) 915-1035, fdayan@ars.usda.gov.