Interaction of the response regulator ARR4 with phytochrome B in modulating red light signaling

The Arabidopsis thaliana response regulator 4, expressed in response to phy tochrome B action, specifically interacts with the extreme amino-terminus o f the photoreceptor. The response regulator 4 stabilizes the active Pfr for m of phytochrome B in yeast and in planta, thus elevates the level of the a ctive photoreceptor in vivo. Accordingly, transgenic Arabidopsis plants ove rexpressing the response regulator 4 display hypersensitivity to red tight but not to light of other wavelengths. We propose that the response regulat or 4 acts as an output element of a two-component system that modulates red light signaling on the level of the phytochrome B photoreceptor.