Coordinated transcriptional regulation between a reactive oxygen species-responsive gene network and the circadian clock in Arabidopsis thaliana : a thesis presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Plant Biology at Massey University, Plant Biology, New Zealand.
Most organisms have evolved endogenous biological clocks as internal timekeepers to
fine-tune physiological processes to the external environment. Energetic cycles such as
photosynthesis and glycolytic cycles are physiological processes that have been shown
to be under clock control. This work sought to understand the mechanism of the
synchrony between the circadian oscillator and products of energetic cycles. The fact
that plants rely on photosynthesis for survival,and that photosynthesis relies on the
sun, this would have meant that oxygen levels would have fluctuated across the day. A
common by-product of oxygen metabolism and photosynthesis is the Reactive Oxygen
Species(ROS). Evidence has proposed ROS as regulators of cellular signaling and plant
development. However, if ROS levels are left unmanaged, it may cause oxidative stress
in organisms, which could damage cellular components and disrupt normal
mechanisms of cellular signaling. Therefore, it is advantageous for plants to be able to
anticipate such periodic burst in ROS. My research investigates the role of the
circadian clock in regulating ROS homeostasis in the model plant Arabidopsis thaliana.
I found that ROS production and scavenging wax and wane in a periodic manner under
diurnal and circadian conditions. Not only that, at the transcriptional level, ROS7
responsive genes exhibited time-of-day specific phases under diurnal and circadian
conditions,suggesting the role of the circadian clock in ROS signaling. Mutations in the
core-clock regulator, CIRCADIAN3 CLOCK3 ASSOCIATED3 1 (CCA1), affect both the
transcriptional regulation of ROS genes and ROS homeostasis. Furthermore, mis-
expressions of other clock genes such as EARLY3 FLOWERING3 33 (ELF3), LUX3
ARRHYTHMO3 (LUX) and TIMING3 OF3 CAB3 EXPRESSION3 13 (TOC1) also have profound
effects on ROS signaling and homeostasis, thus suggesting a global clock effect on ROS
networks. Taken together, CCA1 is proposed as a master regulator of ROS signaling
where the response to oxidative stress is dependent on the time of CCA1 expression.
Plants exhibit the strongest response at dawn, the time when CCA1 peaks. Moreover,
CCA1 can associate to the Evening Element or CCA17Binding Site on promoters of ROS
genes in vivo to coordinate transcription. A common feature of circadian clocks is the
presence of multiple interlocked transcriptional feedback loops. It is shown here that the oscillator incorporates ROS as a component of the loop where ROS signals could
feed back to affect circadian behavior by changing CCA1 and TOC1 transcription. The
clock regulates a plethora of output pathways; particularly the transcription of an
output gene FLAVIN3BINDING3KELCH3REPEAT3FHBOX31(FKF1) is affected by ROS signals.
Temporal coordination of ROS signaling by CCA1 and the reciprocal control of circadian
behavior by ROS revealed a mechanistic link of which plants match their physiology to
the environment to confer fitness.