Early birds and night owls: natural variation of circadian traits in plants

Circadian clocks have long been hypothesized to tightly link cellular and physiological processes to the appropriate time within the twenty-four-hour cycle of the earth's daily rotation. According to this hypothesis, circadian rhythms with cycle lengths that differ significantly from twenty-four hours would be disadvantageous, as they would generate a desynchronization between the endogenous and exogenous cycles that would place stress upon an organism through the required daily resetting at dawn. However, recent work has demonstrated that endogenous circadian cycles that differ from twenty-four hours by two hours or more are prevalent within the green lineage. Herein, we review recent work on the prevalence of and adaptive advantages associated with natural variation in circadian cycles. Based on known photoperiodic sensing mechanisms we also describe a set of principles that allow the same changes in circadian period to cause different plant responses. This fine-tuning of clock output pathways provides a flexible mechanism enabling plants to use a wide range of life history strategies for plant adaptation to different environmental niches. Further studies are needed to determine how variations of the clock and other signals are integrated in different plants. These studies highlight the circadian clocks' position as a prime adaptation target for migration of plant species into new environmental ranges.