A continuous flow of the plant hormone, auxin, is produced by the growing tip of plant shoot and travels down the plant stem into the roots. The flow creates a chemical gradient inside the plant that regulates the growth of side shoots and roots and so determines the overall shape and structure of the plant. It has been discovered that the 'Rht' gene controls an on/off switch for the growth of plant cells. 'Rht' genes produce 'DELLA' proteins that repress (switch off) the activity of genes that need to be active (switched on) for cells to grow. The plant hormone gibberellin destabalises DELLA proteins, reducing their quantity and repressive effect on cell growth and so (switches on) the activity of the genes. Both auxin and gibberellin stimulate the breakdown of these 'repressor' molecules and so work in partnership to stimulate the growth of cells. Dwarf mutant forms of this gene were responsible for the so-called 'green revolution' where an increase in grain productivity is obtained through plants putting more resources into developing (seeds/fruit) grain that longer stems (straw). The mutant 'Rht' gene makes the plants partially insensitive to gibberellin so the plant cells do not elongate as normal. Ray Mathias, John Innes Centre E-mail: sce.mail@bbscr.ac.uk www.AlphaGalileo.org