We have used rice and Arabidopsis mutants to study how terpenoid hormones regulate growth and development in plants. Our work in Arabidopsis has shown that gibberellins play an important role in growth responses to environmental signals, such as light and temperature. In collaboration with other research groups, we uncovered new gibberellin deactivation mechanisms, including epoxidation by a cytochrome P450 monooxygenase in rice and methylation by methyltransferases in Arabidopsis. Previous studies using shoot branching mutants of pea, Arabidopsis and rice have suggested the involvement of a new hormonal signal in inhibiting shoot branching in these species. Recently, we set out to reveal the chemical identity of this new hormone using a series of rice branching mutants. We found that strigolactones, a group of terpenoid lactones that were initially discovered as germination stimulants of root parasitic plants, act as the novel shoot branching inhibitor or its biosynthetic precursors.