Chapter I. General Introduction 1 1.1 Overview of biological rhythms and ultradian rhythms’ research 1 1.2 Objectives 3 Chapter II. Discovery and characteristics of an UR in excised Arabidopsis leaves 5 2.1 Introduction 5 2.2 Materials and Methods 6 2.3 Results 7 2.3.1 An ~3 h UR is discovered in promoter activity of some circadian clock-controlled genes in excised Arabidopsis leaves 7 2.3.2 The UR generation is driven by an oscillatory mechanism distinctive from circadian clock system 10 2.3.3 The UR is triggered in excised Arabidopsis leaves specifically 13 2.4 Discussion and conclusion 14 Chapter III. Strong positive correlation between the UR and de novo root regeneration (DNRR) process in excised Arabidopsis leaves 16 3.1 Introduction 16 3.2 Materials and methods 18 3.3 Results 20 3.3.1 Consistent patterns of the UR and DNRR in response to many internal and external factors 20 3.3.2 Widespread existence of ultradian oscillating genes with enrichment of hormonal pathways 26 3.4 Discussion and conclusion 33 Chapter IV. Mechanism of ROU1/EAR1 in the UR and DNRR regulation 35 4.1 Introduction 35 4.2 Materials and methods 35 4.3 Results 37 4.3.1 Mutants impaired in the UR generation delay rooting initiation and reduce rooting rate 37 4.3.2 EAR1, an ABA signalling component is a crucial UR regulator 38 4.3.3 ROU1/EAR1 positively regulates the UR generation and DNRR in an ABA-specific pathway which is decoupled from ABA-mediated inhibition of seed germination and root growth 42 4.3.4 Spatial localization of ROU1/EAR1 at the petiole and mechanism driving the UR in excised leaves 46 4.4 Discussion and conclusion 50 References 51 Summary in Korean 58 Acknowledgement 60 Curriculum Vitae 61