Green genes blasted into chloroplasts.

Green genes blasted into chloroplasts

After years of failed efforts, scientists working with flowering plants have inserted foreign genes into chloroplasts -- the tiny, chlorophyll-packed sacs that green plants use to convert sunlight into usable energy. Researchers say the new-found ability to genetically manipulate these solar-powered substations opens the door to a host of improvements in crops.

"It's a big hurdle, a very significant step forward," says Wilhelm Gruissem, a botanist at the University of California, Berkeley. "Many people are going to want to use this technique."

Most plant genes reside within the cell nucleus, but a few genes critical for photosynthesis remain cloistered in the 100 to 300 chloroplasts scattered through the rest of the cell. Molecular biologists have become reasonably adept at inserting and deleting nuclear genes, and in 1988 scientists succeeded in altering the genetic sequence of chloroplasts in primitive algae bearing only one chloroplast per cell. But until now, nobody had genetically altered the more complex chloroplasts in higher plants.

The new work, described in the November PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES (Vol.87, No.21), was performed by Zora Svab, Peter Hajdukiewicz and Pal Maliga of Rutgers University in Piscataway, N.J. Working with tobacco seedlings grown in culture plates, the team sought to insert two new genes into the chloroplasts. The genes enable plant cells to resist antibiotics that would otherwise inhibit photosynthesis.

The researchers blasted the seedlings with tiny tungsten pellets coated with multiple copies of the antibiotic-resistance genes. When grown on a special growth medium containing antibiotics, most of the seedlings turned white, indicating that they had not incorporated the resistance genes and that their photosynthetic machinery had succumbed to the drugs. But a few plants grew green, and DNA tests confirmed that their chloroplasts included the protective genes. The progency of those plants inherited the new genes.

The insertion technique remains somewhat inefficient. One gene gets incorporated with every 50 blasts--only 1 percent the success rate routinely achieved with nuclear genes. But Maliga and others expect refinements of the method to yield a cornucopia of agricultural applications. By altering the blueprints for genes involved in photosynthesis, "we may make crop plants more efficient under environment conditions like drought or low temperatures, which adversely affect photosynthetic reactions," he says.

Moreover, since chloroplast DNA never gets incorporated into pollen grains, Maliga suggests the technique could help ensure that newly inserted plant genes--such as those that make crops resistant to herbicides -- don't get passed to surrounding weeds. "If you want to put a new gene in a plant, the chloroplast is a good target," he says. "It's an isolated compartment that's generating lots of energy."

Maliga envisions scientists someday blasting chloroplasts with genes that direct nitrogen fixation, reducing the need for fertilizers by allowing plants to derive nutrients directly from atmospheric nitrogen. "Right now that's science fiction," he concedes. But as evidenced by the little green leaves in his culture plates, "the chloroplast has now become a realistic target for gene manipulation."