Hormonal control of the shoot stem-cell niche
Zhong Zhao^1,2*, Stig U. Andersen³, Karin Ljung⁴, Karel Dolezal^{4, 5}, Andrej Miotk^{2, 3}, Sebastian J. Schultheiss^{3, 6}& Jan U. Lohmann^{2, 3}

¹School of Life Sciences, University of Science and Technology of China。
² Department of Stem Cell Biology, University of Heidelberg, D-69120 Heidelberg, Germany. 。
³Max Planck Institute for Developmental Biology, D-72076 Tuebingen, Germany.
⁴Department of Forest Genetics and Plant Physiology, Umea Plant Science Centre, Swedish University of Agricultural Sciences, S-901 83 Umea, Sweden.
⁵Laboratory of Growth Regulators, Palacky University and Institute of Experimental Botany Academy of Sciences of the Czech Republic, 783 71 Olomouc-Holice, Czech Republic.
⁶Friedrich Miescher Laboratory of the Max Planck Society, D-72076 Tuebingen, Germany.
*Corresponding Author: Tel: (+86551) 3600640; E-mail: zhzhao@ustc.edu.cn

Abstract
The classic phytohormonescytokinin and auxin play essential roles in the maintenance of stem-cell systems embedded in shoot and root meristems, and exhibit complex functional interactions. Here we show that the activity of both hormones directly converges on the promoters of two A-type ARABIDOPSIS RESPONSE REGULATOR (ARR) genes, ARR7 and ARR15, which are negative regulators of cytokininsignalling and have important meristematic functions. Whereas ARR7 and ARR15 expression in the shoot apical meristem (SAM) is induced by cytokinin, auxin has a negative effect, which is, at least in part, mediated by the AUXIN RESPONSE FACTOR5/ MONOPTEROS (MP) transcription factor. Our results provide a mechanistic framework for hormonal control of the apical stem-cell niche and demonstrate how root and shoot stem-cell systems differ in their response to phytohormones.

Key words: stem-cell niche, ARRs, MONOPTEROS, phytohormones, cross talk.