Editors' ChoicePlant biology

Roots respond when the iron is not

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Science Signaling  26 Apr 2016:
Vol. 9, Issue 425, pp. ec96
DOI: 10.1126/scisignal.aaf9342

Iron deficiency stunts plant growth. In Arabidopsis thaliana, the transcription factor FIT heterodimerizes with bHLH38 or bHLH39 to promote the expression of genes involved in iron acquisition. The hormone gibberellic acid (GA) promotes growth by causing the degradation of nuclear DELLA proteins, which repress growth by controlling the activity of several bHLH family transcription factors. Wild et al. found that Arabidopsis mutants lacking all five DELLA homologs showed less root growth inhibition in response to iron deficiency than did wild-type plants. Iron deficiency reduced the amount of GA produced by Arabidopsis seedlings and caused the accumulation of the DELLA protein RGA in the root meristem. All five Arabidopsis DELLAs interacted with FIT, bHLH38, and bHLH39 in yeast two-hybrid assays and when coexpressed in Nicotania benthamania. RGA interacted with the DNA binding domain of FIT, reduced DNA binding by FIT-bHLH38 and FIT-bHLH39 heterodimers, and reduced FIT-dependent expression of a reporter gene. Although iron deficiency stabilized RGA in the root meristem, iron-deficient growth conditions reduced the abundance of RGA and increased the abundance of FIT in root epidermal cells, which are the cells that mediate iron uptake. Ectopic expression of a mutant form of the DELLA protein GAI—which is not degraded in response to GA signaling—in the root epidermis reduced the expression of FIT target genes in response to iron deficiency and reduced iron accumulation but had no effect on root elongation in response to iron deficiency. Although the mechanism by which iron deficiency destabilized RGA in the root epidermis was not determined, these results demonstrate cell-specific differences in the response of DELLAs to iron deficiency. Commentary by von Wirén and Bennett addresses why the inhibition of root elongation may be important for the response to iron deficiency and how these findings may be applied to enhance the micronutrient content of food crops.

M. Wild, J.-M. Davière, T. Regnault, L. Sakvarelidze-Achard, E. Carrera, I. L. Diaz, A. Cayrel, G. Dubeaux, G. Vert, P. Achard, Tissue-specific regulation of gibberellin signaling fine-tunes Arabidopsis iron-deficiency responses. Dev. Cell 37, 190–200 (2016). [PubMed]

N. von Wirén, M. J. Bennett, Crosstalk between gibberellin signaling and iron uptake in plants: An Achilles’ heel for modern cereal varieties? Dev. Cell 37, 110–111 (2016). [PubMed]

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