The time machine: feedback loops, post-transcriptional regulation, and environmental integration in the plant circadian oscillator

Abstract

Daily rhythms in physiology are obvious and widespread. While for millennia it was thought that these cycles represent passive responses to environmental cycles, we now recognize that many of them are governed by circadian oscillators. In plants, these cell-autonomous oscillators regulate daily processes such as photosynthesis, organ growth, and hormone production, as well as seasonal transitions like flowering. Furthermore, the circadian system gates plant responses to biotic and abiotic stresses, modulating susceptibility to pathogens and environmental extremes in a time-of-day-dependent manner. Variants of circadian clock genes have been repeatedly selected during crop domestication and improvement, highlighting the importance of the circadian system to plants and its relevance for agriculture. Here, I review the history of circadian studies in plants and summarize our current understanding of the molecular nature of the circadian oscillator. I also discuss how this complex network both responds to and is buffered against changes in the environment. Next, I examine how circadian oscillators differ between various tissues and how their activities are coordinated throughout the plant body. Finally, I discuss emerging directions, such as ways in which this understanding can be applied to crop improvement in the face of climate change.