Research ArticlePlant biology

Stem-piped light activates phytochrome B to trigger light responses in Arabidopsis thaliana roots

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Sci. Signal.  01 Nov 2016:
Vol. 9, Issue 452, pp. ra106
DOI: 10.1126/scisignal.aaf6530

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Plant stems pipe light to roots

Light affects not only the development and physiology of the shoots (stems, leaves, and flowers) of plants but also the underground root system. Light triggers shoot cells to release signals that travel to the root and affect the development and physiology of the root system. Like shoot cells, root cells also have photoreceptors that can be activated by light, leading Lee et al. to investigate if light actually reaches these underground parts of the plant. Exposing Arabidopsis thaliana shoots to light while protecting the roots from light activated the photoreceptor phyB in the roots. In the root, phyB activated Hy5, a transcription factor that mediates cellular responses to light and was important for growth of the primary root and for root gravitropism, the proper downward orientation of roots. Arabidopsis stems efficiently conducted only certain wavelengths of light to the root tissues, and these conducted wavelengths activated phyB directly in the roots. These findings demonstrate that roots not only receive information about light conditions through signaling molecules that travel from the shoot to the root in response to light but also directly perceive light that is conducted through the plant tissues.

Abstract

The roles of photoreceptors and their associated signaling mechanisms have been extensively studied in plant photomorphogenesis with a major 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-function mutation 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.

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