Abstract:
ABSTRACT
The ability to detect and respond to environmental signals is fundamental in coordinating floral
induction in plants to favourable conditions. An important flowering time cue is day length and
it is proposed that light signals are perceived and measured by an interaction between
photoreceptors and an internal pacemaker, the circadian clock. The control of flowering has
been best characterised in the model plant Arabidopsis thaliana L. Heynh (Arabidopsis). The
GIGANTEA (GI) gene has a complex role in both the promotion of flowering in response to
photoperiod and the regulation of the circadian clock. The expression of GI is under circadian
control and is affected by day length, light quality and temperature changes. The GI protein is
also circadian regulated and is actively degraded in the dark.
The biochemical function of GI is unknown and one method to elucidate the role of this protein
is to identify protein interactors. The aim of this thesis project was to characterise proteins that
interacted with GI. Previously, the BELL-LIKE HOMEODOMAIN 3 (BLH3) protein was
identified as a putative GI protein interactor. As part of this thesis work, yeast 2-hybrid and in
vitro pull down assays were utilised to confirm the interaction between GI and BLH3. Sequence
and phylogenetic analyses were used to further examine the BELL family of proteins. The
BELL-LIKE HOMEODOMAIN 10 (BLH10) protein was found to be closely related to BLH3
and also interacted with GI. Reverse 2-hybrid assays were used to determine the regions or
domains within the GI, BLH3 and BLH10 proteins required to mediate protein interactions.
Expression assays established that the BLH3 and BLH10 transcripts were present throughout
plant tissues and times of development. Further analyses revealed that BLH3 and BLH10 are not
directly regulated by the circadian clock. The results of GFP expression assays demonstrated
that the BLH3 protein is localised to the nucleus in plant cells. Transgenic blh3 and blh10
mutant plants were identified and analysed for flowering and light response phenotypes. BLH3
and BLH10 do not function with GI in the photoperiodic pathway to control flowering, yet the
blh3 and blh10 mutants do have a flowering phenotype in short day conditions. Like gi, the blh3
and blh10 mutants exhibited exaggerated hypocotyl elongation in response to red and low light
conditions. These results are suggestive of a role for BLH3, BLH10 and GI in flowering and deetiolation
responses to specific light conditions in plants.
