April 24, 1997

Article at The American Reporter

Seedless Fruit on the Way, Say Geneticists

by Wilson da Silva
American Reporter Correspondent 

SYDNEY, Australia – Seeds in oranges may be natural, but most of consumers would rather do without them. Now molecular biologists in Australia and Japan believe they have figured out how to make seeds disappear altogether: they get the plants themselves to obliterate them. 

In what could be described as a case of floral auto-hysterectomy, the researchers have managed to convince tobacco plants to kill and then crush their own seeds at a very early stage of development. 

Dr. Anna Koltunow of the Division of Horticulture at Australia’s national science agency, the Commonwealth Scientific and Industrial Research Organization, and Dr. Fumio Takaiwa of Japan’s National Institute of Agrobiological Resources in Tsukuba, believe the genetic engineering technique they have developed could now be used to trigger the same effect in their originally-intended target: citrus plants. 

There are various new ways of removing plant organs genetically: recently, researchers have removed pollen from plants by targeting genes which, when “switched on”, trigger a cytotoxic effect that kills off pollen production. 

Using a similar strategy, the Australian-Japanese team targeted embryo and seed development. First, they identified a highly specific promoter, or genetic “on-off switch”, that locks in on the seed development. They then merged this with a cytotoxic plant gene found in a range of plants, but which does not normally act on seed development. The resulting chimeric gene – made up of two naturally occurring but normally separate genotypes grafted together – delivers a highly accurate death blow against seeds even as they are beginning to form. 

“It looks promising from the results we’ve got in tobacco,” the Adelaide-based Koltunow told The American Reporter. “All of the seeds in the plant stop developing, but the rest of the tobacco capsule grows. All that is left is a small, soft ‘seed trace’.” She declined to reveal the origin of the cytotoxic plant gene or the promoter, as these may be subject to patenting. 

The seed-crushing gene, dubbed SDLS-2, was developed by the team in an attempt to genetically engineer new varieties of seedless oranges, lemons and mandarins. Funded in part by Australia’s Horticultural Research and Development Corporation, it is an enhanced version of a gene under development since 1992. The first version killed seeds, but did not obliterate them. 

Another of the early problems was ensuring that the gene did not switch on too early, which in citrus plants was likely to cause fruit to fall off the tree too early. Through trial and error, Koltunow and Takaiwa established that the killer gene should only be triggered after the seeds had started developing but while they were still young -- in tobacco, when the seeds were less than half a millimeter long. 

Koltunow’s team is now inserting SDLS-2 into citrus plants, and are positive about the chances of success. But because these take so long to flower -- some as long as five years -- results are not expected immediately. In the meantime, they hope to get some early indications from tests on the short-lived weed “Arabidopsis,” whose seeds develop much the same way as citrus seeds. 

The work is to be submitted for publication in the Journal of Molecular Breeding later this year.