FUNCTIONAL ANALYSES OF TISSUE AND ORGAN SPECIFICITY AT THE CORE OF THE ARABIDOPSIS CIRCADIAN CLOCK

Abstract

The circadian clock is a timing mechanism that generates 24-hour biological rhythms. The importance of the circadian clock function is evident in almost all organisms examined to date, from bacteria to humans. Since plants as sessile organisms, the circadian function is particularly relevant for adaptation and survival. Understanding how the plant circadian system is organized in the context of cells, tissues and organs raises as one of the fundamental questions to fully understand plant physiology and metabolism. However, a major challenge in plant biology is to decipher how individual clocks are interconnected to sustain rhythms in the whole plant. In this PhD thesis, we show that the Arabidopsis thaliana shoot apex is composed of an ensemble of coupled clocks that influence rhythms in roots. A series of different protocols developed in this study revealed a disparity of circadian oscillations in excised organs, with hypocotyls, roots and leaves displaying reduced circadian precision and robustness. In contrast, analyses of shoot apexes showed highly synchronized and precise rhythms. The use of different clock mutants and reporter lines as well as analyses of the global circadian transcriptional landscape at the shoot apex indicated that such synchrony and precision is not likely due to a molecular circadian network that is specific for the shoot apex. Instead, in vivo live-imaging of rhythmic single cells, desynchronization of dispersed protoplasts and mathematical analysis using barycentric coordinates for high-dimensional space demonstrated that circadian precision relies on a tight circadian coupling (or communication) among the shoot apex clock cells. The increased rhythmic synchrony conferred robustness against genetic and pharmacological perturbations and particular capabilities for phase readjustments during "jet-lag" experiments. Rhythms in roots were altered by shoot apex ablation and micrografting, suggesting that signals from the shoot apex are able to synchronize distal organs. Similar to the circadian organization in mammals, our studies demonstrate that shoot apexes play a dominant role within the hierarchical circadian system in plants.

Date of Award	26 Sept 2017
Original language	English
Supervisor	Paloma Mas Martinez (Director)