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

Thumbnail Image
Open Access Location
Journal Title
Journal ISSN
Volume Title
Massey University
The Author
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.
Arabidopsis thaliana, Genetics, Circadian rhythms, Photosynthesis regulation, Plants, Metabolism, Regulation