Humble crop goes high-tech.

Every day, about 500 million people dine on the starchy roots of the cassava plant, a tropical vegetable that tolerates acidic, infertile soil.

Cassava is ripe for improvement, however. It lacks protein, and its susceptibility to pests and diseases often costs farmers up to 80 percent of their crop (SN: 10/30/93, p. 277).

Numerous features of the plant make it difficult to improve through traditional breeding methods, so scientists have taken a nontraditional approach and developed techniques for genetically engineering cassava. Reports of these achievements appear in the June Nature Biotechnology.

Hong-Qing Li and his colleagues at the Swiss Federal Institute of Technology in Zurich have grown cassava plants that express two foreign genes. One, a marker gene, turns blue any cell it has penetrated, revealing to the scientists that their technique has worked. The other gene confers resistance to antibiotics.

The Swiss researchers inserted the two foreign genes into cassava with the help of Agrobacterium tumefaciens (SN: 1/17/87, p. 37). This unusual bacterium has the advantage of being able to insert some of its genes into plant cells.

The scientists used a transformed version of A. tumefaciens in which the transferable genes had been replaced with the foreign marker and antibiotic-resistant genes.

They then dipped leaves from a cassava embryo into a solution containing the bacteria. Over a few days, the bacteria transferred the foreign genes to cells in the leaves. The team exposed the leaves to an antibiotic, which killed off any remaining normal cells and left only the engineered ones. With the help of growth hormones, the group forced the leaves to form shoots and grow into mature cassava plants.

Christian Schopke of the Scripps Research Institute in La Jolla, Calif., and his colleagues are also growing new-fangled cassava, but they employ a different technique, biolistics.

This method involves bombarding the plants' embryogenic calli with tiny gene-coated gold bullets. These calli are composed of cells that can become embryos and eventually mature plants. The team used a special liquid culture to grow the cells into plants.

Nigel J. Taylor, also of Scripps, and his colleagues developed this method of growing cassava from embryogenic calli. His group and Schopke's describe their techniques in the June Nature Biotechnology.

Like the Swiss team, Schopke and his coworkers used an antibiotic- resistant gene and a marker gene in their study. In more recent experiments, however, they have also inserted genes to make the plant resistant to the cassava common mosaic virus, found in South America, and to the much more destructive African cassava mosaic virus, says Claude Fauquet of Scripps, a coauthor of the biolistics report.

Biolistics will probably work on a wider variety of cassava genotypes than the Agrobacterium method, predicts Fauquet. Using biolistics, researchers can also insert multiple genes into a single plant. They have added 15 foreign genes to rice, for example.

On the other hand, Agrobacterium is easier to use and requires no fancy equipment, the researchers note.