Our projects aim to understand the molecular mechanisms of
circadian rhythms in Arabidopsis and the role of
circadian regulation in plant physiology. Circadian rhythms
reflect the output of an intracellular circadian oscillator (or 'biological clock'). A
long-standing interest of ours is in the mechanism of this oscillator, which we're
investigating using clock mutants and circadian-regulated genes.
Firefly luciferase (luc) activity in a transgenic Arabidopsis plant. The
"normal" reference image on the right was taken with external lighting. The
luminescence image on the left is the glow emitted by luciferase activity within the plant
(on a computer-generated colour scale, with red indicating bright luminescence and blue
low luminescence - none is visible to the naked eye).
The spatial and temporal pattern of luminescence reflects the expression pattern of the
CAB gene promoter that controls luciferase in this transgenic plant. The highest
levels are in young leaves, with little in the seedling stem (hypocotyl, extends to lower
Click here for a graph showing the circadian rhythm in
luminescence from a wild-type CAB:luc seedling, and from a mutant (toc1-1).
The mutant clock runs faster than normal. This was the first clock mutant identified in
plants (Millar et al., Science, 1995). The Kay lab's cloning of the mutated gene
has recently been reported (Strayer et al., Science, 2000). A growing
collection of clock
genes is revealing more
of the important components of the plant clock.
Our experimental methods combine real-time video imaging of transcription
and signal transduction in transgenic plants carrying bioluminescent reporter genes (luciferases), with Arabidopsis thaliana mutants
and molecular approaches, including microarray measurements
of RNA levels. Our theoretical methods centre on
differential equation models, parameter estimation and data analysis.
A control network within each cell. The plant cell integrates timing signals
with a variety of other stimuli, to ensure appropriate responses to the environment
throughout the day/night cycle. For example, many clock-regulated processes are also
controlled by light signals from the environment, via plant photoreceptors. Our gating
project studies how the cell balances light and timing information by rhythmically
regulating its response to light. This mechanism of "cross-talk" between
signalling pathways, known as circadian gating, is common to many organisms. In plants, it
probably occurs within single cells. We are studying how the clock controls the expression
and function of phytochrome photoreceptors. We showed that the gating
mechanism affects light input to the clock - in fact, gating is essential for the
clock to run under long days.
Modelling the clock. This collaborative project uses mathematical analysis and
simulation to understand the plant clock mechanism, and by
comparison to other clock models, to uncover the design principles underlying circadian clocks in all
organisms. We now have a rich exchange between theory and experiment: improving
our experimental designs through simulated experiments and validating mathematical models
with real data.
A systems-oriented database
of rhythm data.
Software for systems
Clocks under natural selection. Circadian clocks in all organisms are reset by
environmental signals like the light and temperature changes at dawn and dusk. However,
it's also important for clocks to keep accurate time under a range of environmental
conditions. We're using quantitative genetics (QTL analysis) to compare the clocks of
Northern European and subtropical Arabidopsis strains and find clock genes that differ
between the strains. These variant genes may have been retained during recent evolution of
the clock in the very different native habitats of the strains. We identified new
clock-associated genes in this way,
and also studied which aspects of the clock have been
modified to allow Arabidopsis to adapt to new environments
(Edwards et al. 2005, Genetics). Similar modifications could
also extend the range of agriculturally important plants, such as Brassica oleracea.
Autonomy or cooperation? Many or all plant cells contain a clock and
photoreceptors. Some animal cell clocks are linked to each other or to a central pacemaker
in the brain. Our cell-autonomy project aimed to determine whether plant cells compare
time and lighting information with their neighbours and whether plants have a central
circadian pacemaker. We have shown that each plant organ contains at least one circadian
system that is independent in the intact plant: plants have no pacemaker
(Thain et al. 2000, Current Biology). Rhythms in
epidermal cells can spontaneously desynchronise from rhythms in the underlying leaf mesophyll
cells, indicating that clocks are not only independent but can also differ among
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