Editors' ChoicePlant Physiology

Nitrogen assimilation gets a HY5

Sci. Signal.  15 Mar 2016:
Vol. 9, Issue 419, pp. ec59
DOI: 10.1126/scisignal.aaf6660

Carbon fixation (photosynthesis) occurs in plant leaves and shoots, and nitrogen assimilation occurs in the roots. These processes must be coordinated for optimal growth. Chen et al. found that the light-regulated bZIP transcription factor ELONGATED HYPOCOTYL5 (HY5) was required for the growth of Arabidopsis thaliana roots in response to illumination of the shoot. Illumination of the shoot, but not the root, promoted nitrate uptake through the transporter NRT2.1 in the root in a HY5-dependent manner. Grafting experiments with wild-type plants and hy5 mutants demonstrated that production of HY5 in the shoot promoted root growth and nitrate uptake. In hy5 mutants carrying a transgene encoding a fluorescent- or epitope-tagged form of HY5 (HY5-GFP or myc-HY5, respectively), both the tagged protein and its corresponding transcripts were present in the shoot, but only the tagged protein, not the corresponding transcripts, accumulated in the root. When the authors expressed a form of HY5-GFP fused to a very large protein through a linker containing a TEV protease recognition site, this large fusion protein did not accumulate in roots in response to shoot illumination. However, when this large fusion protein was coexpressed with TEV protease, HY5-GFP accumulated in roots in response to shoot illumination, indicating that the there is a size limit to the transport of proteins from shoots to roots. HY5 bound to the NRT2.1 promoter in vitro and in vivo. In grafting experiments, shoot-derived HY5-GFP stimulated the expression of NRT2.1 in both wild-type and hy5 mutant roots. Shoot-derived HY5-GFP also stimulated the expression of HY5 in wild-type roots, indicating the presence of a positive feedback mechanism for amplifying the shoot-derived HY5 signal. Additional experiments indicated that HY5 also promoted the production and efflux of sucrose, the primary form in which fixed carbon is transported from shoots to roots through the phloem, and that sucrose enhanced the binding of HY5 to the NRT2.1 promoter in the root in response to shoot illumination. Thus, shoot-derived HY5 and sucrose cooperate to ensure that plants take up enough nitrogen to meet photosynthetic and growth demands. Many mobile transcription factors move between neighboring plant cells through plasmodesmata. These findings demonstrate that regulatory proteins can also be transported between distant tissues in plants, implying that the phloem and xylem, like the circulatory system of animals, is important for more than just moving nutrients and water throughout the plant.

X. Chen, Q. Yao, X. Gao, C. Jiang, N. P. Harberd, X. Fu, Shoot-to-root mobile transcription factor HY5 coordinates plant carbon and nitrogen acquisition.Curr. Biol. 26, 640–646 (2016). [PubMed]