Hormonal Regulation of Stem Cell Proliferation at the Arabidopsis thaliana Root Stem Cell Niche

Abstract

The root stem cell niche (SCN) of Arabidopsis thaliana consists of the quiescent center (QC) cells and the surrounding initial stem cells that produce progeny to replenish all the tissues of the root. The QC cells divide rather slowly relative to the initials, yet most root tissues can be formed from these cells, depending on the requirements of the plant. Hormones are fundamental cues that link such needs with the cell proliferation and differentiation dynamics at the root SCN. Nonetheless, the crosstalk between hormone signaling and the mechanisms that regulate developmental adjustments is still not fully understood. Developmental transcriptional regulatory networks modulate hormone biosynthesis, metabolism, and signaling, and conversely, hormonal responses can affect the expression of transcription factors involved in the spatiotemporal patterning at the root SCN. Hence, a complex genetic-hormonal regulatory network underlies root patterning, growth, and plasticity in response to changing environmental conditions. In this review, we summarize the scientific literature regarding the role of hormones in the regulation of QC cell proliferation and discuss how hormonal signaling pathways may be integrated with the gene regulatory network that underlies cell fate in the root SCN. The conceptual framework we present aims to contribute to the understanding of the mechanisms by which hormonal pathways act as integrators of environmental cues to impact on SCN activity.

Keywords: gene regulatory networks; hormonal regulation; plant development; quiescent center; root stem cell niche; stem cell regulation.

FIGURE 1

(A) The root apical meristem is composed of the SCN, the proliferation domain, and the transition domain (Ivanov and Dubrovsky, 2013). The SCN houses the QC cells and the ICs [structural and funcional initials, respectively (Barlow, 1997)] which divide asymmetrically and produce cells of different root tissues. The QC cells produce most root tissues and are considered a reserve of multipotent stem cells. (B) QC divisions can be periclinal or anticlinal and produce columella initials or cortex/endodermis initials, respectively. Yellow asterisks mark the daughter cells that replace an initial cell in each case.

FIGURE 2

Expression and activity domains of the main genetic regulators of the QC cells fate. In (A), SHR, SCR, and WOX5, and in (B), the PLT family of transcription factors. (C) Protein complexes that bind to the regulatory regions of WOX5 promoter. (D) Schematic representation of some of the regulatory interactions underlying the division of the QC cells. (E) Network depicting the regulatory interactions underlying QC divisions discussed throughout this review. The blue lines indicate protein–protein interactions; dotted lines indicate the role of auxin influx transporters, and the dashed line between auxin and AAO indicates that this particular regulation was observed in maize roots.