The phytochromes are photosensitive substances which play an essential role in the various phases of plant development and growth. As photoreceptors they act as sensors that convey information to the plant about changes in ambient light. The two existing forms -active and inactive- can switch one to the other in response to such changes. Under red light they become active and trigger off a cascade of biochemical, in the plant cells to induce processes such as germination, flowering or stem elongation. Although it is well established that phytochromes function like light switches, the chain of biochemical reactions triggered when they are activated are still not well understood. Phytochromes were discovered in bacteria five years ago. However, their role even in these simple organisms has remained difficult to fathom. This has now been partially clarified by researchers from the IRD (Institut de Recherche pour le Dveloppement) and the CEA in conjunction with the CNRS. Their revelatory work has just been published in Nature. The study of bacteriophytochromes might give useful insights into the functional mechanisms of their counterparts in plants.

A research team jointly involving the IRD, the CEA and the CNRS has very recently found phytochromes in a strain of nitrogen-fixing bacterium, Bradyrhizobium (1), symbiont on certain tropical leguminous plants (the Aeschynomene). Techniques of molecular biology, biophysics and biochemistry revealed that the newly-discovered phytochrome has an essential role as regulator of the bacteriums photosystem synthesis. An identical function was shown in the photosynthetic bacterium Rhodopseudomonas palustris, phylogenetically very close to Bradyrhizobium (2).

The researchers experimented by subjecting Bradyrhizobium cells to different wavelengths of light, from the red to the infrared. It appeared that the bacterial photosynthetic apparatus was synthesized in its complete form only when the phytochrome was in its active
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Contact: Marie Lise Sabrie
sabrie@paris.ird.fr
33-01-48-03-75-36
Institut de Recherche Pour le Dveloppement
13-May-2002

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