Gibberellins: regulating genes and germination.

The range of processes regulated by gibberellins (GAs) covers all aspects of the life history of the plant from seed germination to vegetative growth and flowering. In seeds there has been an intensive search, using the techniques of both biochemistry and cell biology, for the regulatory molecules linking GA perception to gene regulation and the events of germination. Although a GA receptor has yet to be identified, the site of perception has been localized to the plasma membrane. Calmodulin, Ca and cGMP have also been identified as elements of the GA signal transduction pathway. These regulators parallel many of the signalling elements identified in the transduction of other signals such as phytochrome and ABA. Studies of GA-regulated gene expression, principally of the α-amylases of cereal aleurone, have identified core GA-responsive promoter elements, such as the gibberellin response element (GARE), box-1 and pyrimidine boxes, as well as elements that may lend specificity to GA-regulated expression, such as the Opaque-2-similar element (O2S), and TRE and CRE motifs. One of the most striking features of all of these studies of the molecular basis of GA action is the interaction of GA-dependent regulatory elements with those of other factors such as ABA. GA-response elements also appear to be conserved between disparate GA-response systems. For example, Myb transcription factors appear to regulate a multitude of GA-induced genes in cereal aleurone as well as to alter GA responses when expressed in Arabidopsis. Thus the study of GA signal transduction and response systems is highlighting the conservation of regulatory elements used by plants. These common factors, used by distinct signal transduction systems, provide a molecular basis for the integration of the GA signal with other growth regulators that is the hallmark of plant growth and development. CONTENTS Summary 363 I. Introduction 364 II. Responses of cereal aleurone to GA 364 III. GA and genes 370 IV. Conclusions and perspectives 378 Acknowledgments 379 References 379.