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Responses of ferns to red light are mediated by an unconventional photoreceptor


Efficient photosynthesis is essential for plant survival. To optimize photosynthesis, plants have developed several photoresponses. Stems bend towards a light source (phototropism), chloroplasts move to a place of appropriate light intensity (chloroplast photorelocation) and stomata open to absorb carbon dioxide. These responses are mediated by the blue-light receptors phototropin 1 (phot1) and phototropin 2 (phot2) in Arabidopsis (refs 1–5). In some ferns, phototropism and chloroplast photorelocation are controlled by red light as well as blue light6. However, until now, the photoreceptor mediating these red-light responses has not been identified. The fern Adiantum capillus-veneris has an unconventional photoreceptor, phytochrome 3 (phy3), which is a chimaera of the red/far-red light receptor phytochrome and phototropin7. We identify here a function of phy3 for red-light-induced phototropism and for red-light-induced chloroplast photorelocation, by using mutational analysis and complementation. Because phy3 greatly enhances the sensitivity to white light in orienting leaves and chloroplasts, and PHY3 homologues exist among various fern species, this chimaeric photoreceptor may have had a central role in the divergence and proliferation of fern species under low-light canopy conditions.

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Figure 1: rap2 mutants lack red-light-induced phototropism and chloroplast relocation movement in the sporophyte generation.
Figure 2: Functional correlation of phy3 with the rap phenotype.
Figure 3: The phototropic response of rap2 leaves is less sensitive to dim white light than that of wild-type (WT) leaves.


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We thank W. R. Briggs for critical reading and discussions about the manuscript; H. Nishida for suggestions about the manuscript; M. Hasebe for H. punctata genomic DNA; Arabidopsis Biological Resource Center and R. Vierstra for psmRS-GFP plasmid; K. Kikuchi for technical advice; and Y. Takahashi, F. Toyama, K. Hara, E. Hayashida, E. Sugiyama and T. Yasuki for their technical assistance. This work was partly supported by Promotion of Basic Research Activities for Innovative Biosciences from Bio-oriented Technology Research Advancement Institution and by Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan to M.W. and by a grant from Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists to H.K., S.C. and Y.S.

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Correspondence to Masamitsu Wada.

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Kawai, H., Kanegae, T., Christensen, S. et al. Responses of ferns to red light are mediated by an unconventional photoreceptor. Nature 421, 287–290 (2003).

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