12/05/91
CONTACT: Stanford University News Service (650) 723-2558
STANFORD -- Multi-colored Indian corn, which decorates many homes during autumn, may hold keys to mutations of other plants.
Discoveries in Indian corn by Stanford University molecular biologist Virginia Walbot may help scientists explore mutations, including how environmental stress - such as increased ultraviolet light from the thinning ozone layer - might trigger changes in the world's food crops.
Centuries of selective corn breeding by Native Americans not only produced diverse colors and patterns, it also enhanced a remarkable tool for genetics: transposable elements.
A transposable element, or transposon, is a segment of DNA that moves around the chromosomes of a cell, "jumping" in and out of genes. This activity rearranges genetic information, causing rapid mutations such as those that create the array of colors in corn kernels.
Examining bronze and purple pigment in Indian corn, Walbot recently isolated an unusual transposable element, Mu9, that appears to act as the master element, the "boss," of a family of fast-jumping transposons known as the Mutator elements of corn. Walbot, Stanford postdoctoral fellow Jane Hershberger and research assistant Christine Warren report on the discovery in the Nov. 15 issue of the Proceedings of the National Academy of Sciences.
With Mu9, the Mutator transposons produce mutations 100 times faster than other transposons inplants, and 1,000 times faster than mutations occur in organisms with no active transposons.
Mu9 is an aggressive piece of DNA that breaks the rules of genetic segregation and inheritance. Normally, each offspring receives just half of each parent's genes. However, one copy of Mu9 can turn into dozens just before the corn begins its reproductive cycle. Thus, all the corn's offspring receive at least one copy of Mu9.
While other transposable elements act at random times in the host organism's development, Mu9 acts only at a very specific time, late in the development cycle, Walbot said. Somehow it receives a message from the host plant causing it to function when it does.
Walbot also has shown that ultraviolet light can activate Mu9, resulting in plants that are stunted and sick. Her discovery sheds light on why some organisms survive and others are destroyed by environmental stress.
This ability of transposons to respond to events outside the genes, Walbot said, raises the intriguing idea that they may be a mechanism of rapid genetic change, a means of "accelerated evolution."
Transposons may be passive in most organisms until activated by environmental stress, because too many mutations could be harmful. But one or a few mutations that helped the organism adapt to the new environment could keep the species going.
Like most plants, ordinary corn apparently carries transposons only in a cryptic, or passive, state. However, as the ozone layer thins and increased levels of ultraviolet light penetrate, Walbot said, it is possible that transposons could be activated in the nation's 70-million acre corn crop, causing damaging mutations. Ultraviolet light might harm other organisms in the same way.
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This release was written by Bernice Wuethrich, a Stanford News Service science-writing intern.
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