Gibberellin (GA) is one group of plant hormones that has a chemical structure of gibberellane skeleton (Figure 13.1). GA is involved in almost all phases of plant growth and development. It promotes germination, elongation growth, owering, and fruit development. Although several books and reviews have been published on the physiological role of GA, relatively limited references are available for roots, since GA does not strongly promote root elongation of many plants. GA was identied as a fungal toxin to cause unusual shoot elongation of rice plants, and it was further recognized as a plant hormone to regulate specically shoot growth. However, by using GA-decient mutants and inhibitors of GA biosynthesis, it is now evident that GA controls root growth at a low concentration range. Furthermore, molecular mechanism of GA signal transduction has recently been unveiled, and the sequential events occurring in shoots were found to take place also in roots. A short history and the research literatures of GA discovery were described in the previous edition (Tanimoto 2002).