The roles of photoreceptors and their associated signalingmechanisms have been extensively studied in plant photomorphogenesis with amajor focus on the photoresponses of the shoot system. Accumulating evidence indicates that light also influences root growth and development through the light-induced release of signaling molecules that travel from the shoot to the root.We explored whether aboveground light directly influences the root system of Arabidopsis thaliana. Light was efficiently conducted through the stems to the roots, where photoactivated phytochrome B (phyB) triggered expression of ELONGATED HYPOCOTYL 5 (HY5) and accumulation of HY5 protein, a transcription factor that promotes root growth in response to light. Stimulation of HY5 in response to illumination of only the shoot was reduced when root tissues carried a loss-of-functionmutation in PHYB, and HY5 mutant roots exhibited alterations in root growth and gravitropism in response to shoot illumination. These findings demonstrate that the underground roots directly sense stem-piped light to monitor the aboveground light environment during plant environmental adaptation.