Bouquet of mustard for new genetics.

Animal geneticists owe their success to mice and the fruit fly Drosophila; microbial geneticists are indebted to the bacterium E. coli. Now molecular biologists propose that a small weed will prove to be a similar boon to plant geneticists.

Plant genetics has lagged behind other fields in the recent rapid progress in understanding mechanisms of inheritance and in applying that knowledge to genetic engineering. The explanation often given for this lag is the amount and complexity of the genetic material, called the genome. Plants have large amounts of repetitive DNA -- sequences of unknown function present in thousands of copies and scattered throughout the chromosomes. In addition, plants often have extra copies of their entire set of chromosomes.

A plant in the mustard family may allow geneticists to work with a much simpler genome. This harmless weed, Arabidopsis thaliana, which grows to about 5 inches tall, contains about 1 percent the amount of DNA in wheat, and less than 0.5 percent the amount of repetitive sequence. In addition, it is well suited for research: Its life cycle is only five weeks; one plant can produce thousands of seeds; and dozens of plants can be grown in a 2-inch-diameter pot. More than 75 genetic mutations have already been described and assembled into a map of the plant's five chromosomes.

Initial work on the molecular genetics of Arabidopsis is described in the Sept. 20 SCIENCE. Elliot M. Meyerowitz and Robert E. Pruitt of Caltech in Pasadena report that the individual genes of Arabidopsis are similar to those of other flowering plants. They predict that genes of interest can be easily located in the small genome of Arabidopsis and then used to pick out the corresponding genes in more complex plants of economic interest. Proteins encoded by large gene families in other species are encoded in Arabidopsis by a single gene or a few genes. For example, there are three genes in Arabidopsis for the light-harvesting chlorophyll protein. In contrast, in petunias this protein has 16 or more genes. Preliminary work in other laboratories suggests that it will be possible to do genetic engineering on Arabidopsis, using the gene carrier (the Ti plasmid) now employed for more complex plants.

At least a dozen laboratories are working on, or have plans to work on, the molecular genetics of Arabidopsis, Meyerowitz says. He and Pruitt conclude, "Our hope is that Arabidopsis will soon join the other organisms for which a combined genetic and molecular approach has led to both fundamental and practical scientific advances."