Photosynthesis is optimized by orienting leaves and chloroplasts towards the light source, responses that are usually mediated by the blue-light receptors phototropin 1 (phot1) and phototropin 2 (phot2). In some ferns phototropism and chloroplast photorelocation are also controlled by red light, but no photoreceptor mediating these red-light responses had been identified. In the January 16 Nature, Hiroko Kawai and colleagues at the Tokyo Metropolitan University, Japan, show that an unconventional photoreceptor mediates ferns responses to red light (Nature, 421:287-290, January 16, 2003).

Kawai et al. examined Adiantum capillus-veneris (including red-light aphototropic mutants) by mutational analysis and complementation. They observed that this fern expressed an unconventional photoreceptor, phytochrome 3 (phy3) — a chimera of the red/far-red light receptor phytochrome and phototropin. In addition, they showed that the phy3 receptor induced red-light phototropism and red-light chloroplast photorelocation.

"Because phy3 greatly enhances the sensitivity to white light in orienting leaves...

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