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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 light. 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 phototropin. 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.

In plant photomorphogenesis, it is well accepted that the perception of red͞far-red and blue light is mediated by distinct photoreceptor families, i.e., the phytochromes and blue-light photoreceptors, respectively. Here we describe the discovery of a photoreceptor gene from the fern Adiantum that encodes a protein with features of both phytochrome and NPH1, the putative blue-light receptor for secondpositive phototropism in seed plants. The fusion of a functional photosensory domain of phytochrome with a nearly full-length NPH1 homolog suggests that this polypeptide could mediate both red͞far-red and blue-light responses in Adiantum normally ascribed to distinct photoreceptors.Plants alter their growth and development in response to the light environment through a process known as photomorphogenesis. Several families of photoreceptors contribute to the whole-plant response to light from the UV-B to near-infrared region (1). Red͞far-red light is perceived by phytochrome, a biliprotein of approximately 120 kDa encoded by a multigene family both in seed plants and in cryptogams (2, 3). Blue-light perception is primarily mediated by cryptochromes (CRYs; refs. 4 and 5) and probably by the product of NPH1 locus (6). Based on the phenotypes of the nph1 (nonphototropic hypocotyl) mutant (7) and cry1cry2 double mutant (8), NPH1 has been implicated as the major photoreceptor responsible for blue light-dependent phototropic curvature in seed plants. Indeed, all of the strong nph1 mutant alleles are defective in both first-and second-positive curvature (6), whereas cry1cry2 double mutants are only impaired in first-positive phototropic curvature (8). Owing to the presence of putative flavin-binding sites on the NPH1 polypeptide (7) and epistasis analyses (9), NPH1 appears to be the primary photoreceptor mediating second-positive phototropism in plants.The influence of phytochrome and blue-light photoreceptors on each other's activity is well documented (10). Genetic analyses have clearly demonstrated an interaction between phytochrome and CRY1 signaling pathways (11). Moreover, a direct interaction between phytochrome and the CRY photoreceptors was recently documented (12). In seed plants, blue light-mediated phototropism has been shown to be affected by red-and far red-light treatments (13). Based on these and other studies, it appears likely that the signal-transduction pathways for red-and blue-light photoreceptor families share at least one common component (see ref. 14 for review).The co-action of phytochrome and blue-light photoreceptors has been examined at the cellular level in fern gametophytes, notably for the genus Adiantum (15, 16). In Adiantum, phytochrome-dependent spore germination is suppressed by blue-light irradiation (17). Phytochrome also prolongs blue light-induced cell-cycle progression in filamentous protonemal cells of Adiantum (18). By contrast, phytochrome and bluelight receptors act cooperatively to mediate phototropism of Adiantum protonemata (19) and to affect chloroplast photorelocation...