Brassinazole resistant 1 (BZR1)-dependent brassinosteroid signalling pathway leads to ectopic activation of quiescent cell division and suppresses columella stem cell differentiation.

Hak-Soo Lee,Ju Won Kim,Soo-Hwan Kim,Yew Lee,Yong-Sic Hwang,Giang Pham,Stanley J Roux,Ji-Hye Song,Yoon Kim

doi:10.1093/jxb/erv316

Abstract

Previous publications have shown that BRI1 EMS suppressor 1 (BES1), a positive regulator of the brassinosteroid (BR) signalling pathway, enhances cell divisions in the quiescent centre (QC) and stimulates columella stem cell differentiation. Here, it is demonstrated that BZR1, a BES1 homologue, also promotes cell divisions in the QC, but it suppresses columella stem cell differentiation, opposite to the action of BES1. In addition, BR and its BZR1-mediated signalling pathway are shown to alter the expression/subcellular distribution of pin-formed (PINs), which may result in changes in auxin movement. BR promotes intense nuclear accumulation of BZR1 in the root tip area, and the binding of BZR1 to the promoters of several root development-regulating genes, modulating their expression in the root stem cell niche area. These BZR1-mediated signalling cascades may account for both the ectopic activation of QC cell divisions as well as the suppression of the columella stem cell differentiation. They could also inhibit auxin-dependent distal stem cell differentiation by antagonizing the auxin/WOX5-dependent pathway. In conclusion, BZR1-/BES1-mediated BR signalling pathways show differential effects on the maintenance of root apical meristem activities: they stimulate ectopic QC division while they show opposite effects on the differentiation of distal columella stem cells in a BR concentration- and BZR1-/BES1-dependent manner.

Highlights

The apical regions of plant roots are composed of mitotically cortical-endodermal initials, stele initials, and columella iniinactive quiescent centre (QC) cells and the surrounding mer- tials), which form a ‘stem cell niche’ (SCN)
Wild-type Arabidopsis thaliana (Columbia-0, Col-0), its ethylene biosynthesis mutant eto1-11 and BR-related mutants, and wild-type Arabidopsis thaliana (Enkheim-2, En-2) and its mutant bes1-D plants were used for QC, columella stem cell initials (CSCs), and columella cell (CC) phenotype analysis and root-regulating gene expression analysis. pBZR1::BZR1-YFP and p35S::BES1GFP plants were used for BZR1 subcellular localization study and ChIP-qPCR assays
BZR1 was evident within 15 min when BL was applied to 7-dold seedlings. This implies that the reported brassinosteroid insensitive 1 (BRI1) EMS suppressor 1 (BES1)- and the BZR1-mediated BR signalling pathway may actively be involved in QC division and the distal CSC differentiation

Summary

Introduction

The apical regions of plant roots are composed of mitotically cortical-endodermal initials, stele initials, and columella iniinactive quiescent centre (QC) cells and the surrounding mer- tials), which form a ‘stem cell niche’ (SCN) (Supplementary istemic stem cell initials (epidermal-lateral root cap initials, Figure S1 available at JXB online; Bennett and Scheres, 4836 | Lee et al.2010). The root is composed of a distal structure [lateral root cap and columella layers derived from the columella stem cell initials (CSCs)], the SCN, proximal meristem, transition zone, elongation zone, and differentiation zone (Perilli and Sabatini, 2010; Lee et al, 2013). This cylinder-like concentric root structure is established by a balance between cell division in initials, stem cells, and meristematic cells of the proximal/distal meristems, and differentiation of these cells into diverse cell types in the elongation/ differentiation zone. When asymmetric division of a stem cell initial occurs, the daughter cell that has contact with the QC remains as an initial cell whereas the other daughter cell—which is separate from the QC—divides to form transitamplifying cells at the boundary of the proximal meristem (Scheres, 2007; Perilli et al, 2012)

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