How does white clover protect itself?

By Chelsie Vandaveer

July 17, 2003

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White clover (Trifolium repens Linnaeus) is a highly nutritious forage plant. Native to Europe, it has been introduced to temperate zones around the world. It frequently escapes cultivation and naturalizes into the surrounding wild habitats. Although white clover appears a defenseless plant, it protects itself by the production of two chemicals: linamarin and linamarase.

Linamarin is a cyanogenic glycoside, a molecule composed of two moieties or parts--a sugar molecule, glucose, attached to acetone cyanohydrin. Linamarase is a protein, specifically an enzyme; it breaks the beta-bond between the sugar and the acetone. As long as the linamarin molecule is intact, it is relatively harmless. Acetone cyanohydrin is an unstable molecule. Once the beta-bond is broken, the acetone and cyanide part company.

Plant of the Week 07/07/2003

Trifolium repens Linnaeus var. 'Good Luck'
Plant of the Week 07/07/2003

Cyanide wreaks havoc by interfering with cytochrome. When the cytochrome is neutralized, no energy comes from the mitochondria, the cells' powerhouses. The affected cells cease to function and the body dies. Cyanide is as lethal to plant cells as it is to animal cells. The linamarin and the linamarase are kept separate. The two are combined when something chews on the clover.

Simply because of dosage size, the clover's protection is particularly effective against small herbivores like slugs, snails, insects, and small mammals. Large animals--horses, cattle, and sheep--are poisoned when the clover comprises most of their food intake; the kidneys get rid of small amounts of cyanide.

But not all white clover plants are capable of cyanogenesis (releasing cyanide). To release the cyanide the plant has to make the linamarin and the linamarase. These are controlled by the genes the plant inherits. It must have inherited the dominant alleles for two different genes--one to make the cyanogenic glycoside and one to make the enzyme. ("Plant cyanogenic glycosides", János Vetter, Toxicon 38 (2000) 11-36, Elsevier Science Ltd. 1999)

If the linamarase is absent, the clover is not cyanogenic, but it is still toxic. Bacteria in the intestines of the herbivore can break the beta-bond releasing the acetone cyanohydrin. Clover breeding involves selecting plants with the recessive genes so the plants cannot make linamarin.

The University of Wyoming has a brief discussion of white clover and photographs of three types of clovers. To view the page, click on the link:

http://www.uwyo.edu/plants/forages/wclover.htm

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