Linking photoreceptor excitation to changes in plant architecture

Plants sense neighbor proximity as a decrease in the red to far-red ratio of incoming light, which triggers a series of developmental responses known collectively as the “shade avoidance syndrome” (SAS). In Arabidopsis, phytochrome B is the major sensor of shade light, but to date phytochrome excitation has not been linked directly to a growth response. We show that the bHLH transcription factor, PIF7, previously shown to interact with phytochrome B, accumulates in its dephosphorylated form in shade, allowing it to bind to the promoters of a subset of auxin biosynthetic genes and increase their expression. Since new auxin is required for changes in growth in the shade, our studies link directly the perception of a light quality signal to a rapid growth response.