Plants relay signals much as animals do.

Scientists first noticed ethylene's effects on plants at the turn of the century. when they realized that this gas, leaking from street lamps, caused trees to drop their leaves. They later discovered that ethylene is a plant hormone that can dramatically alter the shape of seedlings grown in the dark. By studying these odd seedlings, molecular geneticists have now uncovered hard-to-obtain details about how plant hormones work.

Ethylene sets off a chemical cascade inside plant cells that alters genetic activity, says Joseph J. Kieber of the University of Pennsylvania in Philadelphia.

He and his colleagues describe one chemical in this cascade -- a protein kinase enzyme -- in the Feb. 12 CELL. Remarkably. the enzyme's gene resembles genes for similar enzymes in animals.

"It's a real breakthrough," comments Elliot M. Meyerowitz, a molecular geneticist at the California Institute of Technology in Pasadena. "It's the first molecular identification of an intermediate [chemical] in a plant hormone signal transduction pathway."

Scientists seek to understand ethylene because it helps plants alter their growth and development in response to the environment. Emerging seedlings make ethylene so they can break through hard soil. Later in the plant's life, the rapid production of this substance may protect a torn leaf from infection. Finally. ethylene affects the rate at which fruit ripens or petals fade.

Since many companies seek to control fruit ripening or floral blooming, this report "is tremendously interesting from a practical and basic perspective," says Harry Klee, a plant molecular biologist at Monsanto Co. in St. Louis. Also, clues about ethylene may help clarify how nitric oxide, a simple gas and important messenger in animals (SN: 7/4/92, p. 10), works, he adds.

To learn about ethylene, the Pennsylvania group screened more than a million Arabidopsis seedlings, culling out short ones with curled-up tips. These had grown as if they had been exposed to too much ethylene. The researchers added ethylene inhibitors to the short seedlings and discarded the ones that then began to grow normally: They represented plants that simply overproduced ethylene. The remaining seedlings represented plants with mutations in the signal pathway

One mutation turns out to be in a gene that codes for a protein kinase, an enzyme that adds a phosphate to a protein, which then becomes the next signal in this chemical cascade. Without this protein-phosphate complex, cells act as if they were constantly being stimulated by ethylene, so the plant becomes stunted, says Joseph R. Ecker of the University of Pennsylvania.

"[This gene] turns out to be semifamiliar," says Meyerowitz. Yeast, worms, and fruit flies, as well as people, use similar protein kinases to relay chemical messages within cells. "It implies a commonality between plants and animals," he adds. -E. Pennisi