Gibberellins: synthesis, compartmentation and physiological process

This paper has two objectives: (1) to examine the nature of regulation of gibberellin (GA) biosynthesis and metabolism with respect to GA biological activity, and (2) to identify cellular compartments which play a role in biosynthesis, metabolism and release of GAs. The importance of ent-kaurene synthetase and its regulation for the production of GAs is emphasized. Similarly, factors regulating the enzymes mediating oxidation steps to GA $\_{12}$ -aldehyde and hydroxylation steps to GAs are identified. Specificity of hydroxylation for high and low biological activity as well as conjugation reactions resulting in deactivation of GAs are related to the problem of regulation of GA concentration in cells. Compartmentation of GA synthesis and sequestration and release of GAs from organelles are discussed. The need for careful separation of organelles and membrane systems to assess properly their roles in GA synthesis and compartmentation is stressed. The role of plastids in biosynthesis and release of GAs is analysed. Red light is known to enhance GA levels in intact tissues and plastid preparations; whether the increase is due to red light promoted conversion from 'bound' to 'free' GAs or to altered permeability characteristics of the chloroplast membranes is not known. Metabolism of [ $^3$ H] GA $\_1$ in protoplasts and vacuoles is described in a progress report. Protoplasts are capable of metabolizing GA $\_1$ to GA $\_8$ , GA $\_8$ -glucoside and other metabolites, whereas isolated vacuoles appear to metabolize GA $\_1$ only to GA $_8$ .