Plants possess internal circadian oscillators, containing a series of gene expression feedback loops. These oscillators enable plants to coordinate their biological processes with the light- dark cycles of the Earth. Arabidopsis circadian oscillators are entrained by internal and external cues, including metabolites. Energy limitation lengthens the free-running circadian period of Arabidopsis, which can be restored by exogenous sugar application. The energy-sensing kinase SUCROSE NON-FERMENTING RELATED KINASE 1 (SnRK1) signals energy depletion to circadian oscillators, through the core oscillator gene PSEUDO- RESPONSE REGULATOR 7 (PRR7). Another energy-sensing kinase, TARGET OF RAPAMYCIN (TOR), was recently proposed to feedback energy levels to Arabidopsis circadian oscillators. As existing knowledge on the extent of circadian regulation by TOR is limited, this thesis aimed to increase understanding of TOR signalling in regulating Arabidopsis circadian oscillators.

Chemical inhibition of TOR, silencing TOR expression and knockout of TOR subunit REGULATORY-ASSOCIATED PROTEIN OF TOR 1B (RAPTOR1B) all impaired Arabidopsis circadian period shortening by sucrose under low light. Additionally, circadian phase was delayed in raptor1b mutants under low light diel cycles. An intermediate response to low light stress in double raptor1b prr7 mutants indicates these genes might regulate circadian oscillators independently when energy is scarce. Interactions between TOR signalling components and circadian oscillator proteins might suggest a direct role for TOR in regulating Arabidopsis circadian oscillators. Furthermore, knockout of downstream TOR signalling component YET ANOTHER KINASE 1 (YAK1) lengthened circadian period and delayed circadian phase under low light. These findings support the involvement of TOR signalling in the energy-dependent regulation of Arabidopsis circadian oscillators. They also imply that TOR and SnRK1 might regulate circadian oscillators independently under energy depletion.

This thesis also identified that decreased nitrogen availability shortened Arabidopsis circadian period under low light. Circadian period shortening by reduced nitrogen was found to depend on the energy status of the plant, suggesting carbon nitrogen balance might regulate Arabidopsis circadian oscillators.