Photoconversion and Nuclear Trafficking Cycles Determine Phytochrome A's Response Profile to Far-Red Light

SummaryPhytochrome A (phyA) is the only photoreceptor in plants, initiating responses in far-red light and, as such, essential for survival in canopy shade. Although the absorption and the ratio of active versus total phyA are maximal in red light, far-red light is the most efficient trigger of phyA-dependent responses. Using a joint experimental-theoretical approach, we unravel the mechanism underlying this shift of the phyA action peak from red to far-red light and show that it relies on specific molecular interactions rather than on intrinsic changes to phyA's spectral properties. According to our model, the dissociation rate of the phyA-FHY1/FHL nuclear import complex is a principle determinant of the phyA action peak. The findings suggest how higher plants acquired the ability to sense far-red light from an ancestral photoreceptor tuned to respond to red light.

Graphical AbstractFigure optionsDownload full-size imageDownload high-quality image (337 K)Download as PowerPoint slideHighlights► Constitutively active phyA does not accumulate in the nucleus ► Cycling between the active and inactive state is essential for phyA signaling ► PhyA degradation is important for shifting responsiveness toward FR ► Response to FR requires coupling between two antagonistic photoconversion cycles