Letter

Nature 453, 1094-1097 (19 June 2008) | doi:10.1038/nature06943; Received 7 February 2008; Accepted 25 March 2008; Published online 7 May 2008

Cytokinin and auxin interaction in root stem-cell specification during early embryogenesis

Bruno Müller1 & Jen Sheen1

  1. Department of Molecular Biology, Massachusetts General Hospital, Department of Genetics, Harvard Medical School, Boston, Massachusetts 02114, USA

Correspondence to: Bruno Müller1Jen Sheen1 Correspondence and requests for materials should be addressed to J.S. (Email: sheen@molbio.mgh.harvard.edu) or B.M. (Email: mueller@molbio.mgh.harvard.edu).

Plant stem-cell pools, the source for all organs, are first established during embryogenesis. It has been known for decades that cytokinin and auxin interact to control organ regeneration in cultured tissue1. Auxin has a critical role in root stem-cell specification in zygotic embryogenesis2, 3, but the early embryonic function of cytokinin is obscure4, 5, 6. Here, we introduce a synthetic reporter to visualize universally cytokinin output in vivo. Notably, the first embryonic signal is detected in the hypophysis, the founder cell of the root stem-cell system. Its apical daughter cell, the precursor of the quiescent centre, maintains phosphorelay activity, whereas the basal daughter cell represses signalling output. Auxin activity levels, however, exhibit the inverse profile. Furthermore, we show that auxin antagonizes cytokinin output in the basal cell lineage by direct transcriptional activation of ARABIDOPSIS RESPONSE REGULATOR genes, ARR7 and ARR15, feedback repressors of cytokinin signalling. Loss of ARR7 and ARR15 function or ectopic cytokinin signalling in the basal cell during early embryogenesis results in a defective root stem-cell system. These results provide a molecular model of transient and antagonistic interaction between auxin and cytokinin critical for specifying the first root stem-cell niche.

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