Trends in Cell Biology

Trends in Cell Biology

Volume 17, Issue 8, August 2007, Pages 403-410
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Review
Connecting the paths in plant stem cell regulation

https://doi.org/10.1016/j.tcb.2007.06.002Get rights and content

Stem cell niches are specialized microenvironments where pluripotent cells are maintained to provide undifferentiated cells for the formation of new tissues and organs. The balance between stem cell maintenance within the niche and differentiation of cells that exit it is regulated by local cell–cell communication, together with external cues. Recent findings have shown connections between key developmental pathways and added significant insights into the central principles of stem cell maintenance in plant meristems. These insights include the convergence of important stem cell transcriptional regulators with cytokinin signaling in the shoot meristem, the biochemical dissection of peptide signaling in the shoot niche and the identification of conserved regulators in shoot and root niches.

Section snippets

Introduction: the stem cell concept in plant meristems

Plants grow and form new organs throughout their life cycle, while integrating developmental and environmental cues. All postembryonically formed organs and tissues are derived from pluripotent stem cells (see Glossary) that reside in the growing tips of the plant, the shoot meristem and root meristem (several recent reviews consider various aspects of apical meristem function in detail 1, 2, 3, 4, 5). The stem cells are situated in a microenvironment, the stem cell niche, where differentiation

Maintaining shoot stem cells: transcription factors meet cytokinin

In the shoot meristem of the model plant Arabidopsis, approximately three stem cells are located in each of the three layers at the tip of the meristem [6] (Figure 1). Expression of the homeodomain transcription factor WUSCHEL (WUS) abutting the stem cells in a group of approximately ten cells, termed the organizing center (OC), is required to maintain the stem cells undifferentiated, indicating a signaling pathway from the OC to the stem cells [7]. In addition to staying undifferentiated, the

Stem cells and CLV3 signaling

Loss of CLV3 function results in a gradual enlargement of the WUS domain and, in turn, also of the CLV3 expression domain and the whole meristem. However, whether the increase in meristem size of clv3 mutants is primarily caused by the effects of CLV3 on gene expression or on cell division has remained unclear. Using time-lapse imaging, a recent study [34] showed that within 24–48 hours of downregulation of CLV3 mRNA levels in the stem cells, the CLV3 promoter becomes ectopically activated in

Dissecting CLV signaling

Components of the CLV signaling cascade that regulate stem cell specification and differentiation have proved difficult to identify. A recent elegant study showed that the POLTERGEIST (POL) and POL-like 1 (PLL1) PP2C phosphatases function as CLV1 signaling intermediates and are required for the correct maintenance of shoot stem cells [37]. Both POL and PLL1 function redundantly to maintain WUS transcription in the OC and are negatively regulated by CLV signaling. Therefore, the currently

How similar are the shoot and root apical stem cell niches?

Despite their common function in maintaining stem cells, it has remained unclear whether different stem cell niches utilize related regulatory pathways, and how they have evolved. At a cellular level, the root and the shoot meristem stem cell niches in Arabidopsis are organized differently and operate in distinct ways. In the root meristem, the stem cells surround a small group of infrequently dividing cells, the quiescent center (QC), and give rise to distal (columella), lateral (lateral root

Related organizers regulate shoot and root niches

Recent results [48] show that signaling from both organizers displays the most conspicuous molecular similarities. The WUS homolog WOX5 is expressed specifically in the QC of the root, similarly to WUS expression in the OC of the shoot. wox5 mutants fail to maintain the abutting columella stem cells as undifferentiated, indicating that WOX5 is required for a signal from the QC that represses differentiation in the columella stem cells. Conversely, ectopic expression of WOX5 represses the normal

Future directions

Major technical advances promise insight into stem cell niche mechanics in the future. Live imaging and spatial reconstruction shifts the analysis of gene expression and cell behavior from the static observation of two dimensions, such as those obtained from tissue sections, to four dimensions. Although in the shoot meristem these studies are currently mostly restricted to outer layers, new technologies are awaiting adaptation, such as deep-layer microscopy technologies [63]. By contrast, the

Acknowledgements

We thank members of the T.L. laboratory and three anonymous reviewers for helpful comments, and apologize to colleagues whose work we did not mention owing to space constraints. Work in the T.L. laboratory is supported by grants from the Deutsche Forschungsgemeinschaft. M.R.T. was supported by an EMBO long-term fellowship.

Glossary

Callus
a mass of proliferating cells derived from adult tissues of growing plants and maintained in tissue culture.
Cytokinin
derivatives of N6-isopentenyladenine that carry either an unmodified side chain, or a hydroxylated side chain, as in the case of trans-zeatin (tZ) or cis-zeatin (cZ).
Meristem
derived from the Greek word meristos (divided). A formative plant tissue, usually made up of small cells capable of dividing and giving rise to similar cells or to cells that differentiate to produce

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