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Paris, september 7, 2006

The "sixth sense" of plants

Cryptochromes, proteins present in living organisms such as plants and animals, are receptor pigments for blue light which intervene in numerous processes (growth, development, etc.) linked to the circadian cycle (day/night). Researchers in the Cellular and Molecular Physiology of Plants Laboratory (Université Paris VI / CNRS), in collaboration with German teams at the Universities of Marburg and Frankfurt, have for the first time demonstrated their role in the perception of a magnetic field by plants, based on experiments using Arabidopsis thaliana plants. This work, published on September 6 2006 on springerlink, the website for the journal Planta, makes it possible to suggest a parallel between plants and migratory birds and constitutes a decisive advance towards elucidating the complex mechanism which underlies the geomagnetic orientation of birds.

Some birds, including migratory species, have the remarkable ability to utilize the terrestrial magnetic field to orient themselves. They "feel" the North-South orientation of this field and use it as a reference point to choose the direction in which to fly. This perception of the geomagnetic field is light-dependent. Indeed, the correct orientation of migratory birds is only possible in the presence of blue or green light, while other wavelengths of the spectrum perturb this phenomenon. The magnetic field is perceived in the eyes of birds, more precisely in the retina, a structure containing large numbers of nerve endings and photoreceptors. Cryptochromes, molecules present in the bird retina, may be involved in this mechanism and play a role in the response to the magnetic field, as they exhibit all the required physicochemical properties (absorption of blue or green light and formation of "radical pair"(1) structures). They thus allow certain birds, particularly of a migratory type, to orient themselves.

Because it was not possible to verify this hypothesis in migratory birds, the researchers turned to the behavior of a laboratory plant with similar physicochemical properties: Arabidopsis thaliana. It is indeed known that plants exhibit behavioral changes in response to cryptochrome activation by blue light: for example, inhibiting growth of the hypocotyl (a simple, early structure which mainly grows without cell division). In order to determine whether a magnetic field might influence cryptochrome function, the researchers placed Arabidopsis thaliana plants in the presence of blue light and a magnetic field of varying intensity. Their study showed that an increase in magnetic intensity(2) enhanced the inhibition of hypocotyl growth, but only in the presence of blue light; i.e. when cryptochromes were the photoreceptors being used by the plant. By contrast, no inhibitory effect was detected in the presence of red light, conditions under which plants utilize other photoreceptors, the phytochromes. In addition, mutant plants devoid of cryptochromes were insensitive to variations in the magnetic field.

These studies thus demonstrate for the first time that in plants, cryptochrome functions are sensitive to magnetic intensity, and suggest that the mechanism of magnetic perception in plants (and, by extension, in migratory birds) uses the same photosensitive molecules. Because cryptochromes are molecules which have been highly conserved during species evolution, it can be supposed that even if they do not use them, all living organisms are endowed with a sixth sense, that of perceiving magnetic fields.

Sixth sense

© CNRS 2006 Margaret Ahmad

Figure 1 – 5-day old plantules of Arabidopsis thaliana. They received the same quantity of light, but were subjected to different magnetic fields. The plantule with the smaller stem (hypocotyl) is that which received the weaker magnetic field: the effect of the magnetic field intensified the signal perceived by the plant from light and inhibited its growth.

Notes:

1) A molecule which can respond to the magnetic field.
2) From 33-44 µT to 500 µT

References:

Magnetic intensity affects cryptochrome–dependent responses in Arabidopsis thaliana Margaret Ahmad - Paul Galland -Thorsten Ritz - Roswitha Wiltschko - Wolfgang Wiltschko
Planta, week of September 4, 2006, View web site

Contact information:

Researcher
Dr Margaret Ahmad
T 01 44 27 29 16
ahmad@ccr.jussieu.fr

Press
Laetitia Louis
T 01 44 96 51 37
Laetitia.Louis@cnrs-dir.fr

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