Texas A&M Biologists Are Developing Genetically Modified Rice Resistant To Insects And Microbes

For many years, spraying insecticides on rice crops has been the best way to protect rice crops from insects. Scientists are now creating new strains of rice plants that would contain insect-killing proteins, so no insecticide would be needed.

"We are interested in killing insects that eat rice plants, like the rice water weevil," says Timothy C. Hall, distinguished professor of biology. "This insect feeds on the leaves and lays its eggs on them. When the eggs hatch, larvae grow and spend about a month chewing the roots."

To protect rice plants from water weevils, Hall and his colleagues insert insect-killing genes in the seeds of rice plants. Ideally, when the plants grow, these genes produce insect-killing proteins in the plant roots, preventing water weevils from eating the roots.

However, Hall and his group discovered that many of the insect-killing genes are not expressed or expressed only in the early stages of the growth of the plants. These genes -- also called transgenes -- are turned off once they are inserted in plants.

"This lack of expression -- also called gene silencing -- is a way for plant cells to protect themselves from invasion," Hall says. "When we put the gene in, it is seen by the plant cells as an invasive event, so there are various ways in which the cells turn off the expression of the foreign gene."

Hall and his colleagues are making new genes that would escape the gene silencing mechanisms.

"We want to create what we call stealth genes, basically genes that can get underneath the radar of the host cell protection," Hall says. "That involves understanding what the actual mechanisms of gene silencing are."

The silencing mechanisms are strategies by which plant cells inactivate and eliminate foreign genes.

Many silencing mechanisms have been encountered by Hall and his colleagues. Their results and those of others have been recently reviewed in the journal Plant Molecular Biology.

An important silencing mechanism consists of methylation, a process by which the introduced gene is modified, so it can be distinguished from the original genes of the cell.

Other silencing mechanisms include the degradation or rearrangement of the transgenes once they enter the cell, prior to their stable integration into the chromosomes.

Hall and his group are also designing fluorescent markers to track down the position of the transgene in the plant cell genome.

"We hope to be able to tell exactly where our genes are inserted and whether it really does make a difference where the transgene is inserted," Hall says.

Hall adds that these studies should be improved when the sequencing of the rice genome is completed in the next few months.

Hall and his colleagues are now developing and testing five different gene constructs that may be able to escape the gene silencing mechanisms, and produce insect-killing proteins. One gene construct is developed in collaboration with the Louisiana Agricultural Experiment Station, while the four others are elaborated in collaboration with Aventis.

"By changing various regions in the transgenes, we hope to be able to make the rice resistant to the water weevil," Hall says. "But we still need to learn a lot about the many ways in which gene silencing is achieved."