Hormones' effect on plant growth.

• Position: Auxin and cytokinin

Biochemists at the University of Missouri-Columbia are studying the plant hormones cytokinin and auxin and the genes that switch hormone production on and off at different times and places within the plant. They are seeking to understand the transcription mechanism by which plants express their genes. This, in turn, determines how auxin and cytokinin regulate plant growth and development.

Roy Morris uses intravenous needles to inject hormones into corn. He places cytokinin just below the ear at a critical period of growth, soon after pollen is released. This keeps it growing actively for longer and increases seed numbers by 30%. Grain high in cytokinin upgrades the harvest since the plants will contain more grain and less chaff and straw.

To solve the problem of applying minute doses of hormones at the right time and in the correct position, Tom Guilfoyle and Yi Li have isolated genes that produce enzymes for two different stages in the synthesis of the hormones. They have linked them to gene promoters that are site-specific, limiting the operations of the hormone-synthesis genes to certain locations in the plant, such as the ovary, seed, or fruit. Other gene promoters may be development-specific, promoting hormone synthesis at specific times in the plant's life cycle, such as during flowering. The genes cause local overproduction of hormones, often with dramatic effects on plant growth.

The promoter and hormone-synthesis genes are transferred to target plants using tumor-forming crown gall bacteria. This organism inserts its own DNA into the cells of a host...