Gene-spliced rice resists stripe virus.

In the Japanese language, the terms "rice" and "a good meal" are synonymous, reflecting the importance of the white grain to both the diet and the culture of Japan. Accordingly, the Japanese regard anything that threatens their rice crop as Public Enemy Number One.

A group of Japanese agricultural researchers has now made an advance that takes aim at one of the most serious rice scourges. Takahiko Hayakawa of the Plantech Research Institute in Yokohama and his colleagues have developed two genetically engineered rice varieties that resist infection by the rice stripe virus.

The rice stripe virus destroys millions of dollars' worth of rice harvests each year in Japan, Korea, China, Taiwan, and the former Soviet Union. An insect pest called the brown planthopper transmits the virus as it eats rice plant leaves. Rice stripe virus stunts the growth of rice seedlings, causes yellow stripes to develop on the leaves of mature plants, and reduces the amount of seed produced.

To foil the disease, Hayakawa's group turned to a modern-day adaptation of a classical agricultural technique: viral cross-protection. This strategy, which farmers worldwide have used for decades, involves inoculating young plants with relatively benign viruses in order to prepare them to fend off subsequent infection by more destructive viral strains.

In the mid-1980s, plant biotechnologists refined the technique by using genetic engineering to create plants whose cells make their own, benign bits of otherwise virulent invaders. Roger N. Beachy of Washington University in St. Louis and his colleagues demonstrated in 1986 that tobacco plants containing the gene that directs the production of the outer, or coat, protein of the tobacco mosaic virus can resist later infection by the virus.

Since then, the U.S. Department of Agriculture has approved 70 proposals by nearly two dozen research teams to grow outdoor test plots of plants containing genes for viral coat proteins. These field tests have included evaluations of tomatoes resistant to tomato mosaic virus, potatoes resistant to potato leaf-roll virus, and cantaloupe and squash resistant to cucumber mosaic virus (SN: 8/19/89, p.120). None of the gene-spliced vegetables has yet reached the market.

In the new report, published in the Oct. 15 PROCEEDING OF THE NATIONAL ACADEMY OF SCIENCES, Hayakawa's group describes the first successful use of the viral-coat-protein strategy in a cereal crop. Working with clumps of immature rice-plant cells taken from two Japanese varieties of rice, the researchers inserted a gene that directs the production of the rice stripe virus' coat protein. They found that the clumps grew into young plants that made the coat protein.

To test whether the coat protein would allow the genetically engineered plants to resist infection by the rice stripe virus, Hayakawa and his colleagues placed virus-infected brown planthoppers onto 31 rice plants containing the coat protein and onto 17 control rice plants lacking the protein. They found that while 80 percent of the controls developed viral symptoms, only 20 to 40 percent of the genetically engineered plants did.

Moreover, the plants that did not display the viral symptoms also lacked a second protein marker of rice stripe virus infection, says Plantech researcher Ko Shimamoto, a member of Hayakawa's team. Shimamoto says the group plans to grow outdoor test plots of the genetically engineered rice plants next year.

Sivramiah Shantharam, a microbiologist at the USDA's Division of Biotechnology, Biologics, and Environmental Protection in Hyattsville, Md., says Hayakawa's team has taken steps toward solving a significant agricultural problem. However, he adds, "the exact mechanism of protection is not really known.... The coat protein might interact with an incoming viral particle, preventing its replication, but we really don't know how it works."