Plant Cell Biology: the Roots of Quiescence

RESEARCH HIGHLIGHTS

Confocal microscopy image PLANT CELL BIOLOGY of a root tip of Arabidopsis thaliana, showing quiescent centre cells (green fluorescence) surrounded The roots of quiescence by stem cells (within the white frame). Image courtesy of L. De Veylder, The growth and development of roots CCS52A2 is known to restrain QC cell Ghent University, Belgium. depend on the function of the root division, whereas the authors observed meristem, which has a well-defined low expression of ERF115 in QC cells. cellular organization: highly proliferative However, ERF115 mRNA levels stem cells surround a quiescent centre increased in conditions that promote (QC), comprised of a few cells that QC cell division, such as a modest only rarely divide. The QC is crucial for temperature rise or treatment maintaining the meristem as it controls with brassinosteroids, confirming the proliferation and differentiation of that it marks dividing QC cells. surrounding stem cells, and slow QC Importantly, a modified (dominant- self-renewal is central to these functions. negative) version of ERF115, unable De Veylder and colleagues now identify to activate transcription, prevented the ERF115 transcriptional activator as hyperproliferation of QC cells induced a rate-limiting factor of QC cell division. by brassinosteroids, indicating that ERF115 belongs to the ETHYLENE brassinosteroid-dependent QC division RESPONSE FACTOR family, members of at least partly depends on ERF115. This work suggests that QC which control the transcription of genes PHYTOSULFOKINE PRECURSOR 5 proliferation depends on the levels CCS52A2 involved in growth and development. (PSK5) — which encodes a peptide of ERF115, on which the APC/C ERF115 was identified in a screen hormone that controls root growth and brassinosteroids have antagonistic for proteins that bind to an activating and cell proliferation — was identified effects. In addition, the authors subunit of the APC/C (anaphase as a target of ERF115. Consistently, suggest that ERF115-dependent QC promoting complex; also known as both overexpression and stabilization proliferation replenishes surrounding the cyclosome) — CCS52A2 (CELL of ERF115 (through mutation of stem cells upon damage, thereby CYCLE SWITCH PROTEIN 52 A2) — APC/‌CCCS52A2) led to increased PSK5 contributing to the longevity of the that promotes QC cell proliferation transcript levels. Furthermore, in the stem cell niche. when ectopically expressed in absence of a functional receptor for PSK Kim Baumann

Arabidopsis thaliana. APC/C targets peptides, overexpression of ERF115 was ORIGINAL RESEARCH PAPER Heyman, J. et al. cell cycle proteins for degradation, and unable to induce QC proliferation, which ERF115 controls root quiescent center cell division biochemical data confirmed that ERF115 indicats that ERF115-dependent QC and stem cell replenishment. Science http//dx.doi. org/10.1126/science.1240667 (2013) is an APC/CCCS52A2 target. proliferation depends on PSK signalling.

NATURE REVIEWS | MOLECULAR CELL BIOLOGY VOLUME 14 | DECEMBER 2013 | 1