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Abstract

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.