In Gulliver's Garden: Tiny Tomatoes Speed Genetic Engineering of Plants

REHOVOT, Israel - December 10, 1997 - A tiny tomato, dubbed "Micro-Tom," may mean big news for genetic engineering. The Lilliputian plant, adapted for research by Dr. Avraham Levy of the Weizmann Institute of Science, is the key to a new method that may speed the process of unraveling the genetic code of plants, making it easier to identify and use commercially valuable genes.

Working together with Weizmann Institute Ph.D. student Rafi Meissner and Dr. Yoni Elkind of the Hebrew University of Jerusalem, Dr. Levy of the Plant Sciences Department has taken Micro-Tom, a humble plant bred for city dwellers with limited gardening space, and joined it with a unique combination of technologies in order to speed up mutagenesis - the creation of new mutant plant strains.

The method, for which a patent has been applied, is described in a paper published in the December issue of The Plant Journal and is featured on the journal cover.

While mutations are commonly used to identify the function of individual plant genes, Levy's Micro-Tom - which puts out fruit twice as fast as conventional tomatoes - cuts the time necessary to produce such mutations by half. It also drastically reduces the amount of greenhouse space necessary for cultivating new mutant plant strains, making it easier to work with large plant populations.

Levy's method also makes mutations easy to analyze. Prevailing techniques, which use chemicals or radiation to create a mutant plant, result in random mutations that are difficult to trace to a particular spot in the plant's genetic code. The new technique, on the other hand, marks the plant genome with easily-identified genetic "tags" that allow Levy to locate the exact spot where a mutation has taken place.

This traceability, together with the use of large plant populations, makes it feasible to identify the function of any plant gene: "If earlier techniques for creating mutations are something like playing the lottery," says Levy, "with this new method, we can buy all the tickets."

Higher plants, like tomatoes, have approximately 50,000 genes. In most cases, scientists have not yet linked them to specific traits - characteristics such as shape, taste or nutritional content that give a plant its unique identity.

In recent years, scientific advances have made it easier to identify genes and their function, creating the tantalizing possibility of a genetic "boutique" where plant breeders could browse among thousands of traits and select genetic material for customizing fruits and vegetables.

However, before such a boutique opens its doors, each trait in the "inventory" must be produced in an isolated, living plant - a process that requires working with huge plant populations. It is estimated that identifying every gene in the tomato genome would require examination of over 100,000 tomato plants.

Levy addresses this problem with a miniature breakthrough. The Micro-Tom, which stands as low as 5 - 10 centimeters when fully grown, makes large-scale analysis of the tomato genome feasible for the first time, by greatly reducing the time and expense involved in working with large plant populations. With the Micro-Tom, Levy can grow up to 1,000 plants per square meter as opposed to five plants per square meter in the case of normal tomatoes -a 99% reduction in greenhouse space.

The Micro-Tom's rapid growth cycle allows Levy and his team to cultivate four generations per year as opposed to the usual two.

Once the genes are isolated and their functions clarified using the miniature plant, commercially-desirable genes can be transferred into tomatoes of normal size.

Levy analyzes gene function in his new Micro-Tom with the help of a natural phenomenon discovered half a century ago by Nobel Prize laureate Barbara McClintock. McClintock was a pioneer in the study of "jumping genes," a type of genetic material found in maize which moves spontaneously between plants.

Jumping genes, also known as transposons, are the genetic "wild cards" that give ears of Indian corn their seemingly random distribution of color.

When Levy introduces specially-engineered transposons to his Micro-Tom tomatoes, they practice the biological equivalent of the one-two punch, "knocking out" naturally-occurring genes by inserting themselves into the genetic code.

These transposon-induced mutations cause specific tomato traits - be they color, size or sugar content - to be expressed abnormally, providing evidence of the genes' function under normal circumstances. A mutation that causes a tomato plant to put out yellow leaves, for example, indicates the presence of a gene that, in non-mutant plants, determines that leaves should be green.

Once mutation is effected in Micro-Tom, the plant's genetic material can be stabilized, preventing the transposons from "jumping" further to create unwanted mutations.

Levy's method also introduces a biological structure that allows scientists to determine whether a transposon has been successfully implanted into the target genome. This is achieved by the use of "reporter" genes - genetic material which activates a recognizable signal when the transposon has jumped into place.

Levy's transposons have been specially engineered to carry reporter genes that code for beta-glucuronidase - an enzyme that causes plants to turn blue when treated with a special stain. This engineered reporter is activated when the transposon jumps into a gene, and "unfurls" a blue flag to indicate that the mutation has been successful. Moreover, this blue effect is localized to a specific part of the tomato plant - the fruit, for example, or the leaves - allowing scientists to identify the part of the plant where the mutant gene is expressed.

And this "flag" is so big it's impossible to miss; Levy's engineered transposons are almost 5,000 bases long, making it easy for scientists to identify the exact spot on the tomato genome where mutation has taken place.

Levy's technique was designed for use on tomatoes, one of the most important crops for the fresh and processed food industries. But the method can be applied to any crop where farmers are interested in engineering new, more marketable strains.

With the completion of this study, the Micro-Tom is ripe for use in the commercial arena, and several companies have already expressed interest in Levy's new method. Meanwhile, Levy and his team are putting his Micro-Tom method to work, creating new mutations in order to identify useful tomato traits. Dr. Levy holds the David and Pauline Segal Career Development Chair.