Scientists have discovered another gene that will control the growth of plants.

The gene may allow growers to set the height of their grass, trees and other plants, eliminating the need to constantly prune and manicure gardens.

The gene makes a protein that breaks down a steroid hormone in plant stems.

Researchers from the Salk Institute for Biological Studies in San Diego, US, used their new-found knowledge to grow "dwarf" versions of familiar laboratory plants.

A tobacco plant that would normally grow about two metres (six feet) tall was engineered to mature at about 30 centimetres (12 inches).

"It [the hormone] very much parallels the steroids in football players. Plants buff up on it,'' Professor Joanne Chory, the Salk study's senior researcher, told agency reporters. "If you do something ... so it isn't expressed, you get these little dwarfy guys.''

Stem elongation

The dwarf versions are identical to the standard plants in every way but size, she said. The new "dwarf'' gene is called BAS-1. It regulates the amount of growth hormone - the steroid brassinolide - within specific plant tissues.

"It appears that brassinolide is made through the plant and then growth is controlled by selectively inactivating it,'' said Professor Chory, whose work is reported in the journal Proceedings of the National Academy of Sciences. "BAS-1 performs this step in stems, and so switching on BAS-1 will halt stem elongation.''

Professor Chory and her colleagues found the BAS-1 gene by searching for genes that would reverse the effects of a mutation in another gene that leads to overly long stems.

BAS-1 is the first gene identified that interacts with both the steroid hormone and light-detecting pathways, two of the major systems that control plant growth.

The Salk scientists said although the gene only controls the growth of plant stems, the discovery is an important addition to our knowledge of what controls an entire plant's growth.

"Presumably, additional genes will be found that regulate steroid-induced growth in other parts of the plant, such as leaves and petals,'' Professor Chory said. "By tinkering with the entire set, it should eventually be possible to influence every aspect of plant growth and appearance.''