Jumping genes make genetic leaps.
When McClintock began her work, heredity was known to be controlled by genes residing on chromosomes. She surprised the science community by showing that these genes could move--that they were not fixed in linear positions, as had been previously thought.
It was McClintock's jumping genes--technically called transposable elements--that helped ARS geneticist Paul H. Sisco tag a new gene in corn, Zea mays L.
"This gene, glossy-15, is responsible for producing wax and leaf hairs in corn plants," Sisco says. "And maybe even more important, we've found that it also plays a role in the transition of the plant from the juvenile to adult phase."
Little is known about the juvenile-to-adult vegetative transition phase in plants, so this research could be vitally important--not just for improving corn, but for enhancing the agronomic performance of other crops as well.
Wax-coated corn leaves protect the plant during drought, slowing down moisture loss. Wax can also shield a plant from insects, disease, and ultraviolet irradiation damage from the sun.
In addition to conserving moisture, waxy leaves repel water so that there can be an adequate exchange of gases between the air and the leaf surface. Leaf hairs secrete compounds that can protect corn from pests and disease.
"Glossy-15 is the first gene associated with juvenility to be cloned in higher plants," Sisco points out. "We're excited because this research shows the impact that basic science can have on the practical needs of agriculture."
In some plant species, woody types in particular, the adult phase is the most valuable. The number-one problem in forestry, for example, is getting adult wood, Sisco says.
The new gene won't produce lumber ready for the building trade from an immature oak tree. But it could help speed up the process.
For instance, if glossy-15 were found in trees species, it could be knocked out. Since this gene prolongs the juvenile stage, eliminating it would shorten the time it now takes for a tree to mature. And although the glossy-15 corn gene doesn't make the plant produce corn any faster, it is the first step in learning more about the maturation process.
Sisco says, "Speeding up this transition period may lead to a more efficient way to produce some commodities."
Aided by graduate student Stephen Moose, Sisco cloned the new gene a year ago at the ARS Plant Science Research Laboratory located at North Carolina State University. Raleigh.
Moose says," While searching scientific literature on corn genetic research, I kept coming across references to these corn plants that showed wax strips on the third leaf of each plant. Then, when we looked at some mutant corn plants in the greenhouse and realized that several plants showed the same characteristic, I knew I'd found my research mission."
The mutants had been discovered in Sisco's field plantings by William Brown, an agricultural research technician. Seed was collected and grown in the greenhouse for further study.
Intent on finding the gene responsible for the mutations, Moose and Sisco talked with retired ARS geneticist George F. Sprague, the world-renowned scientist who originally discovered the glossy gene back in the 1930's [see sidebar]. Sprague was a colleague of McClintock's and shared her passion for corn genetics. There are actually 17 glossy genes, but in Sprague's time--and up until the research by Moose and Sisco--it was thought that these genes were responsible just for wax production.
Glossy-15 appears to affect only the leaf surface, or epidermis, of a plant, Moose says. This is significant because plants secrete compounds through their leaf hairs, or trichomes.
The importance of leaf hairs can be seen in recent research on a wild relative of tobacco done by George Buta and ARS colleagues.
Buta, a chemist with the ARS Horticultural Crops Quality Laboratory in Beltsville. Maryland, identified as a group of sugar esters the natural compounds found on the surface of leaves of Nicotiana gossei.
These compounds have proven to be environmentally safe for insecticidal use against crop-damaging pests. They are produced by the plant's leaf hairs. The problem is that the plant produces only a very small amount of these compounds.
Could the new corn gene be inserted into the tobacco plant and instructed to produce more plant hairs, and thereby, more of the sugar esters? Maybe, Sisco says.
"Some plants already protect themselves by leaf hair secretions that are toxic to insects," he continues. Certain alkaloids produced in the plant's epidermis are known to deter insect feeding. This could be due to toxicity or to the bitter taste of the compounds.
Glossy-15 opens up new possibilities for genetically engineering pest-and disease-resistance into corn and other plants. Plant resistance is a major hope of growers, since numerous pesticides have been either discontinued or taken off the market because of potential environmental or health risks.
Impetus for Finding the New Gene
McClintock's discovery of transposable elements was based on her work with the genetics that controlled the purple and bronze color in Indian corn.
She found that when there was a change in corn kernel color, there was a corresponding change in the place where chromosomes tended to break. She hypothesized that this breakage was caused by a "jumping" gene.
A particular gene caused the kernels to be purple. But when the chromosomes broke apart, a different genetic element showed up next to the gene for purple color. So the offspring of that particular kernel produced golden-brown or bronze-colored kernels.
This color change also involved a third gene, which apparently activated the jumping gene. The different-colored kernels were produced by the color gene that did the work; a controlling element, or jumping gene, that told the color gene what to do; and an activator that started the work.
"This is the way we proved that a jumping gene was in glossy-15," Sisco says. "When the transposable element is present in the gene, it turns off the gene function, producing sections on the corn plant that have no wax. When the element hops out during leaf development, the gene turns back on, producing waxy sections on the plant."
Paul H. Sisco is in the USDA-ARS Plant Science Research Unit, North Carolina State University, Box 7620, Raleigh, NC 27695-7620; phone (919) 515-2705, fax (919) 515-7959.
An Ancient Crop, Updated
A staple in the diets of the Aztecs, Incas, and other native American peoples, corn as a crop is about 8,000 years old. Today, it is important as a raw material in producing industrial products such as ethanol. A major feed and food crop, both domestic and foreign, corn can only grow as a cultivated crop. It can't survive in the wild. The many superior corn hybrids that we have today may be attributed largely to one man: George F. Sprague.
"In the late 1920's when I started my research, farmers depended on open pollination, saving their own corn seed for next year's crop. This means that yields were relatively low--I'd say about 30 bushels per acre," muses Sprague. "Nowadays, U.S. average yields exceed 100 bushels."
Before retiring in 1973 as head of ARS Corn and Sorghum Investigations at Beltsville, Maryland, Sprague conducted research on corn genetics that is considered to be among the greatest plant breeding achievements of the 20th century.
He helped develop the scientific principles that provide the foundation for corn breeding and genetics research. Famous for using basic science to increase corn's productivity, Sprague applied his knowledge of corn's genetics to develop varieties that helped the farmer produce more.
"In the early days of corn research, we developed cooperative agreements with Corn Belt states to work on hybridization. Federal researchers were set up in universities in Iowa, Illinois, Indiana, Kansas, Missouri, and Ohio," he says.
The first hybrid corns were introduced in the early 1930's, and by 1940, most of the major corn-producing states were using only hybrids. Sprague says, "By the 1960's, 100 percent of U.S. corn acreage was planted with hybrids."
Stiff Stalk Synthetic, one of the most important corn germplasm sources, was introduced by Sprague. Lines from this source are the ones most widely used in producing commercial corn hybrids today.
In the 1930's, before molecular biology or genetic engineering was recognized as a credible scientific discipline, a young George Sprague discovered a glossy gene in corn. From his classical breeding experiments, he knew that this gene was involved in producing wax in the corn plant. But it was not until 1992 that Sprague, then 91 years old, saw the gene cloned and used in genetic testing by another ARS researcher and a student at North Carolina State University.
After 48 years with the Agricultural Research Service and a lifetime of research on corn, George Sprague's work is an excellent example of the integration of basic and applied science.
Until recently a Distinguished Professor of Genetics and Plant Breeding, Department of Agronomy, University of Illinois, Urbana, George Sprague now lives in Eugene, Oregon.
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|Title Annotation:||includes related article; corn genes|
|Date:||Jan 1, 1994|
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