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Cryptochromes are blue light‐absorbing photoreceptors found in plants and animals with many important signalling functions. These include control of plant growth, development, and the entrainment of the circadian clock. Plant cryptochromes have recently been implicated in adaptations to temperature variation, including temperature compensation of the circadian clock. However, the effect of temperature directly on the photochemical properties of the cryptochrome photoreceptor remains unknown. Here we show that the response to light of purifiedArabidopsisCry1 and Cry2 proteins was significantly altered by temperature. Spectral analysis at 15°C showed a pronounced decrease in flavin reoxidation rates from the biologically active, light‐induced (FADH°) signalling state of cryptochrome to the inactive (FADox) resting redox state as compared to ambient (25°C) temperature. This result indicates that at low temperatures, the concentration of the biologically active FADH° redox form of Cry is increased,leading to the counterintuitive prediction that there should be an increased biological activity of Cry at lower temperatures. This was confirmed using Cry1 cryptochrome C‐terminal phosphorylation as a direct biological assay for Cry activationin vivo. We conclude that enhanced cryptochrome functionin vivoat low temperature is consistent with modulation by temperature of the cryptochrome photocycle.

 

PEMF (Pulsed Electromagnetic Field) stimulation has been used for therapeutic purposes for over 50 years including in the treatment of memory loss, depression, alleviation of pain, bone and wound healing, and treatment of certain cancers. However, the underlying cellular mechanisms mediating these effects have remained poorly understood. In particular, because magnetic field pulses will induce electric currents in the stimulated tissue, it is unclear whether the observed effects are due to the magnetic or electric component of the stimulation. Recently, it has been shown that PEMFs stimulate the formation of ROS (reactive oxygen species) in human cell cultures by a mechanism that requires cryptochrome, a putative magnetosensor. Here we show by qPCR analysis of ROS-regulated gene expression that simply removing cell cultures from the Earth’s geomagnetic field by placing them in a Low-Level Field condition induces similar effects on ROS signaling as does exposure of cells to PEMF. This effect can be explained by the so-called Radical Pair mechanism, which provides a quantum physical means by which the rates and product yields (e.g. ROS) of biochemical redox reactions may be modulated by magnetic fields. Since transient cancelling of the Earth’s magnetic field can in principle be achieved by PEMF exposure, we propose that the therapeutic effects of PEMFs may be explained by the ensuing modulation of ROS synthesis. Our results could lead to significant improvements in the design and therapeutic applications of PEMF devices. 

Cryptochromes are highly conserved blue light-absorbing flavoproteins which function as photoreceptors during plant development and in the entrainment of the circadian clock in animals. They have been linked to perception of electromagnetic fields in many organisms including plants, flies, and humans. The mechanism of magnetic field perception by cryptochromes is suggested to occur by the so-called radical pair mechanism, whereby the electron spins of radical pairs formed in the course of cryptochrome activation can be manipulated by external magnetic fields. However, the identity of the magnetosensitive step and of the magnetically sensitive radical pairs remains a matter of debate. Here we investigate the effect of a static magnetic field of 500 μT (10× earth's magnetic field) which was applied in the course of a series of iterated 5 min blue light/10 min dark pulses. Under the identical pulsed light conditions, cryptochrome responses were enhanced by a magnetic field even when exposure was provided exclusively in the 10 min dark intervals. However, when the magnetic stimulus was given exclusively during the 5 min light interval, no magnetic sensitivity could be detected. This result eliminates the possibility that magnetic field sensitivity could occur during forward electron transfer to the flavin in the course of the cryptochrome photocycle. By contrast, radical pair formation during cryptochrome flavin reoxidation would occur independently of light, and continue for minutes after the cessation of illumination. Our results therefore provide evidence that a magnetically sensitive reaction is entwined with dark-state processes following the cryptochrome photoreduction step.