Jumping Genes That Control Plant Traits

Lila O. Vodkin, research geneticist, Plant Molecular Genetics Laboratory, Beltsville Agricultural Research Center, Agricultural Research Service.

Recent advances in molecular genetics have generated excitement in agriculture and the plant sciences. One reason for this is the potential for commercial application, and another is the expanded opportunity to find out at the molecular level how genes function to control plant growth and development. These aims are not mutually exclusive but are likely to be closely interdependent. Advances in one area will stimulate progress in the other.

Mobile or transposable elements (sometimes popularly referred to as jumping genes) provide an opportunity to isolate and identify genes that would enhance crop quality and productivity. These mobile elements provide a direct link between plant characteristics (for example, disease resistance, plant height, organ shapes) and the DNA molecules that control the particular traits.

Gene Research

For thousands of years, genes have been manipulated empirically by plant and animal breeders who monitor their effects on specific characteristics or traits of the organism to improve productivity, quality, or performance. A basic understanding of how traits are transmitted from on( generation to the next was formed by Gregor Mendel in the 19th century.

His experiments and concepts showed that traits were controlled by units of heredity called genes. Extensions of this work led to formation of applied genetics and breeding programs.

The physical and chemical nature of genes remained unknown until the 1950's when James Watson and Francis Crick discovered that genes consist of a chemical known as DNA (deoxyribonucleic acid). DNA contains the information to control the synthesis of enzymes and other proteins that perform the basic metabolic processes of all cells. Each gene is a specific DNA sequence, and more than 100,000 different genes are found in a higher plant or animal species. This total set of genes for an organism (referred to as the nuclear genome) is organized into chromosomes within the cell nucleus. The process by which a multicellular organism develops from a single cell through an embryo stage and into an adult is ultimately controlled by a program contained in the genetic information of the cell and by an interaction of genes and gene products with environmental factors.

Flow of Genetic Information

DNA produces a short-lived molecule called messenger RNA (mRNA) that is similar in structure to DNA. It serves as a temporary message carrier through which information encoded in the DNA is transmitted. Using complex cell machinery, the RNA messages are translated into specific sequences of amino acids that produce the structure of enzymes and other proteins. These protein molecules do the actual work in forming cells and organs and in carrying out metabolic processes within them. They interact in complex sets of pathways to produce particular characteristic features, or traits, of the individual. An observable or measurable trait of an organism is referred to as its phenotype, while the physical makeup of its genetic material is known as its genotype.

To physically isolate a gene that codes for a particular protein, the researcher generally works backward along this path of genetic information from proteins to DNA. Knowledge at the protein level is central to the process. At best, enough of the protein of interest must be produced to allow its isolation and characterization. Using knowledge of the properties of the protein, the researcher can identify the mRNA and eventually isolate the gene that encodes the protein. Since the discovery of recombinant DNA technology in 1975, isolating and identifying genes has become technically much easier. Because few proteins are abundant enough for this process to work easily, a large gap still exists, however, in our ability to identify and physically isolate genes that control most plant traits.

Transposable elements provide one way of physically isolating genes that control complex plant traits because they provide a direct connection between the observable trait and the DNA molecule that controls it. Despite the fact that all the intermediate steps in the process (the messenger RNA, the proteins produced by the genes, and the pathways) may be unknown, the gene responsible for the trait can be identified and physically isolated using a procedure known as transposon tagging.

Transposon Tagging

While most genes occupy a fixed position in the chromosomes, transposable elements can change their locations. When an element moves, it inserts into or near other genes and often affects how the invaded gene expresses itself, leading to a mutant trait.

The classic work of Barbara McClintock and other maize geneticists during the 1940's documented the existence of transposable elements in corn. In recent years, several different types of mobile elements in corn have been isolated and cloned by recombinant DNA techniques. These elements can now be used to "tag" other genes. For example, assume the need to isolate a gene involved in controlling plant height. The dominant form of the gene is designated D, and a mutant form that produces dwarf plants is designated with a lower case d. The process begins with a genetic cross between these two parental lines, one of which harbors an active transposable element (designated TE) somewhere in its chromosomes.

After identifying and isolating useful genes, researcher develops cell tissue cultures to modify plant germplasm and, ultimately, help plant breeders design better crops.