Ultradian rhythms and clocks in plants and yeast

Studies of ultradian rhythms (< 1 day) in plants and in yeasts provide insights into the temporal hierarchy of living organisms. Primarily a reflection of intracellular control circuits, special rhythms are temperature-compensated that have evolved as timekeepers. The best understood ultradian clock is that in yeast; it provides a timeframe for the coherent behaviour of biochemical activities from metabolic and membrane-associated functions to the transcription, translation, assembly of organelles, as well as the replication and partitioning of the genome and all cellular constituents before cell division. Furthermore, the ultradian clock is intimately involved in cell-cell signalling and in the concerted behaviour of the population in a yeast culture or biofilm. Extension of these subcellular and cellular functions to the tissues of plants provides a timeframe within which, shoot and leaf movements, developmental changes and photoperiodic responses (including flowering) are correlated and coordinated. The system can be modelled as a multi-oscillator; one possibility is that a controlled chaotic attractor provides a tuneable output with a wide range of periods. Ultradian rhythms provide the ancestral and mechanistic basis for longer-period biological clocks. An understanding of the integrated physiology of the whole organism is required to fully comprehend circadian rhythmicity.