1. Section of Plant Biology, College of Biological Sciences, University of California, Davis, One Shields Avenue, Davis, California 95616, USA
2. Center for Integrative Genomics, University of Lausanne, Genopode Building, CH-1015 Lausanne, Switzerland
3. Present address: Swiss Institute of Bioinformatics, 1 rue Michel Servet, CH-1211 Geneva 4, Switzerland.

Correspondence to: Julin N. Maloof¹ Correspondence and requests for materials should be addressed to J.N.M. (Email: jnmaloof@ucdavis.edu).

Abstract

Most organisms use circadian oscillators to coordinate physiological and developmental processes such as growth with predictable daily environmental changes like sunrise and sunset. The importance of such coordination is highlighted by studies showing that circadian dysfunction causes reduced fitness in bacteria¹ and plants², as well as sleep and psychological disorders in humans³. Plant cell growth requires energy and water—factors that oscillate owing to diurnal environmental changes. Indeed, two important factors controlling stem growth are the internal circadian oscillator^{4, 5, 6} and external light levels⁷. However, most circadian studies have been performed in constant conditions, precluding mechanistic study of interactions between the clock and diurnal variation in the environment. Studies of stem elongation in diurnal conditions have revealed complex growth patterns, but no mechanism has been described^{8, 9, 10}. Here we show that the growth phase of Arabidopsis seedlings in diurnal light conditions is shifted 8–12 h relative to plants in continuous light, and we describe a mechanism underlying this environmental response. We find that the clock regulates transcript levels of two basic helix–loop–helix genes, phytochrome-interacting factor 4 (PIF4) and PIF5, whereas light regulates their protein abundance. These genes function as positive growth regulators; the coincidence of high transcript levels (by the clock) and protein accumulation (in the dark) allows them to promote plant growth at the end of the night. Thus, these two genes integrate clock and light signalling, and their coordinated regulation explains the observed diurnal growth rhythms. This interaction may serve as a paradigm for understanding how endogenous and environmental signals cooperate to control other processes.

1. Section of Plant Biology, College of Biological Sciences, University of California, Davis, One Shields Avenue, Davis, California 95616, USA
2. Center for Integrative Genomics, University of Lausanne, Genopode Building, CH-1015 Lausanne, Switzerland
3. Present address: Swiss Institute of Bioinformatics, 1 rue Michel Servet, CH-1211 Geneva 4, Switzerland.

Correspondence to: Julin N. Maloof¹ Correspondence and requests for materials should be addressed to J.N.M. (Email: jnmaloof@ucdavis.edu).

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