Bioactive gibberellins (GAs) are plant
growth hormones, controlling diverse
processes such as seed germination,
stem elongation, leaf expansion, and
flower and fruit development. GA-mediated
growth events can be modulated by changing
the level of bioactive GAs and/or altering
the tissue responsiveness to GAs.
To understand the developmental and
environmental regulation of GA biosynthesis,
we and others have cloned several Arabidopsis
genes encoding enzymes that catalyze
early and late steps in GA biosynthetic
pathway. Their expression patterns
suggest that the first gene GA1 may
serve as a gate keeper to control the
flow of metabolites into the GA pathway,
whereas the late genes are important
for fine-tuning the amount of active
GAs in specific tissues.
To identify new components in the
GA signaling pathway, we took a genetic
approach and isolated the RGA gene
which encodes a negative regulator
of GA response in Arabidopsis. The
predicted amino acid sequence of the
RGA protein reveals that RGA belongs
to the newly identified GRAS (formerly
VHIID) regulatory family, whose members
include the radial root organizing
gene SCARECROW and another GA signal
transduction repressor GAI.
Several structural features in the
RGA protein suggest that it may be
a transcriptional regulator. In support
of this, we showed that a green fluorescent
protein (GFP)-RGA fusion protein is
localized to the nucleus in transgenic
Arabidopsis plants, and this fusion
protein can rescue the rga mutation.
Confocal microscopy and immunoblot
analyses demonstrated that exogenous
GA treatment resulted in a reduced
level of the GFP-RGA protein. This
suggests that modulating RGA protein
level by the GA signal is one of the
mechanisms that allows expression of
genes repressed by RGA. Recently, we
demonstrated that GA-induced proteolysis
of RGA is mediated by the SCFSLY1 E3
ubiquitin-26S proteasome pathway.
We are currently investigating this
GA-induced protein degradation pathway
and the function of the RGA protein
in GA response. Students participate
in all aspects of the research projects.
Christy Fleet, a former CMB student,
studied the regulation of GA biosynthesis
and GA response pathways. Hou-Sung
Jun, a former UPG student, focused
on the function and localization of
GA signaling repressor RGA. Ludmila
Tyler, a CMB student, is studying the
mechanism of GA signaling in Arabidopsis.
