A single chromoprotein with triple chromophores acts as both a phytochrome and a phototropin

Plants sense their environmental light conditions by using three photoreceptors that absorb in the UV, blue/near UV, and red/far-red spectral ranges. These photoreceptors have specific chromophore components corresponding to their absorption spectra. Phytochrome, a red/far-red light receptor, has phytochromobilin as its chromophore, whereas the blue/near UV photoreceptors cryptochrome and phototropin have a pair of flavin derivatives. Plants use these various photoreceptors to assess the surrounding light environment. Phytochrome 3 (PHY3) is a red light receptor found in some ferns, which preferentially grow under weak light. PHY3 is composed of a phytochrome chromophore-binding domain in its N-terminal portion and an almost full-length phototropin in its C-terminal half. This unusual domain organization implies that two different light-sensing systems coexist in this single photoreceptor, although these light-sensing systems usually reside in independent photoreceptors. Here, we show that PHY3 acts as a dual-channel photoreceptor that possesses both the red light-sensing system of phytochrome and the blue light-sensing system of phototropin. Furthermore, red- and blue-light signals perceived by PHY3 are processed synergistically within this single chromoprotein. These unusual properties might confer an enhanced light sensitivity on PHY3, allowing ferns to grow under a low-light canopy.