Functional analyses of fern photoreceptor PHY3

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Phytochrome 3 (PHY3) is a red-light receptor found in some ferns, which preferentially grow under weak light conditions. Based on its predicted domain organization, fern PHY3 looks like a chimeric protein consists of both red・and blue-light photoreceptors. N-terminal half of PHY3 contains a chromophore-binding domain of phytochrome and its C-terminal half shows a remarkable similarity to phototropin, containing two LOV domains and a Ser/Thr kinase domain. To study functions of PHY3 polypeptide, we introduced CaMV 35S promoter driven Adiantum PHY3 gene into Arabidopsis phot1-5 phot2-1 plant and observed the phototropic responses. Here we report that PHY3 acts as a dual-channel photoreceptor with both the red-light sensing system of phytochrome and the blue-light sensing system of phototropin. Although these sensing-systems usually reside in independent photoreceptors, these two different light-sensing systems cooperate and generate a synergistic effect within PHY3 molecule, and consequently confer the enhanced light-sensitivity on PHY3.