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.
