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, Ca2+ 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.