Organ specificity in the plant circadian clock

Circadian clocks are endogenous oscillators that control many physiological processes and confer functional and adaptive advantages in various organisms. These molecular oscillators comprise several interlocked feedback loops at the gene expression level. In plants, the circadian clock was recently shown to be organ specific. The root clock seemed to involve only a morning loop whereas the shoot clock also includes an evening loop in a more complex structure. My work aimed at refining the differences and similarities between the shoot and root clocks, using a combination of experimental and theoretical approaches. I developed an imaging method to obtain more data from the shoot and root clocks over time in various conditions. Some previous results were confirmed: the free running periods (FRPs) are longer in roots compared to shoots under constant light (LL). In addition, the amplitude of clock gene expression rhythms is lower in roots compared to shoots. However, the expression of several evening genes is circadian in roots, contrary to previous conclusions. This was confirmed with qPCR, and was observed in both light- and dark-grown roots. Yet light affects clock gene expression in roots, so an automatic covering system was designed to keep the roots in darkness and obtain data in more physiological conditions. Clock genes behaved differently in shoots and light-grown roots that were in the same environmental conditions, and may be differentially affected by blue and red light. However shoot and root clocks were more similar under constant darkness (DD). My imaging and RT-qPCR data, together with new microarray results and preliminary studies on clock mutants suggest that shoot and root circadian systems may have a similar structure but different input pathways. Entrainment is a fundamental property of circadian systems, which can be reset by cues such as light/dark (LD) cycles. I demonstrated that light can directly entrain the root clock in decapitated plants. The root clock could be entrained by a broad range of T cycles using low light intensity. In addition, rhythms were preferably entrained by low light than by any putative signal from shoots in experiments using conflicting LD cycles of different strengths. My results indicate that direct entrainment by LD cycles could be the main mechanism that synchronise the shoot and root clocks at constant temperature. This is physiologically relevant because dark-grown roots can perceive light channelled by the exposed tissues, in a fibre optic way. I also showed for the first time that clock and output genes could be rapidly entrained by temperature cycles in roots. Several mathematical models of the shoot circadian clock were used to try and fit the root clock data by optimising some parameters. The best set of parameters gave a good qualitative fit to root data under LD, LL and DD. It reproduced the long FRP observed in roots under LL and captured the entrainment under LD with lower amplitude in roots. The parameters that were changed for these simulations were all related to light input, which supports the idea of similar clock structures in shoots and roots but with different input pathways. Together my results confirmed that the plant circadian clock is organ specific and suggest that it is organ autonomous.