At the root of quiescence: function and regulation of the quiescent center

Abstract

The quiescent center (QC) of roots consists of a rarely dividing pool of stem cells within the root apical meristem (RAM). The QC maintains the surrounding more frequently dividing initials, together building the stem cell niche (SCN) of the RAM. The initials, after several rounds of divisions and differentiation, give rise to nearly all tissues necessary for root function. Hence, QC establishment, maintenance and function are key to produce the whole plant root system and are therefore at the foundation of plant growth and productivity. Although the concept of the QC is known since the 1950s, much of its molecular regulations and their intricate interconnections, especially in more complex root systems like cereal RAMs remain elusive. In Arabidopsis, molecular factors like phytohormones, small signaling peptides and their receptors, as well as key transcription factors (TFs) play important roles in a complex and intertwined regulatory network. In cereals, homologs of these factors are present, however, QC maintenance in these larger RAMs might also require a more complex control of QC cell regulation by a combination of asymmetric and symmetric divisions. Here, we summarise the current knowledge on QC maintenance in Arabidopsis and compare it with that of agriculturally relevant cereal crops.

Salicylic Acid in Root Growth and Development

“…The root stem cell niche (SCN) consists of the mitotically inactive SCN organizer and the quiescent center (QC) and is surrounded by stem cells, including cortex/endodermis initials (CEIs), stele cells initials (SCIs), epidermal/lateral root cap initials, and columella stem cells (CSCs) [211,212] (reviewed in [213]). Lateral root cap initials and CSCs form the root cap or distal meristem, and the rest of the stem cells, together with their dividing descendants, belong to the proximal meristem (reviewed in [214]).…”

Genetic basis controlling rice plant architecture and its modification for breeding

“…However, more than four mitotically inactive cells are identified in rice RAMs concomitantly with the broader QUISCENT-CENTER-SPECIFIC HOMEOBOX (QHB)/ OsWOX5 promoter activity (Ni et al 2014). Although the QC identity of these cells remains to be determined, a higher number of QC cells might be required for the maintenance of the larger RAMs in rice and other cereals (Strotmann and Stahl 2021). CRL1 functions in crown root maintenance through the control of QHB/OsWOX5 (Fig.…”

“…1A , 1C ) ( Leyser and Day 2002 ). In the RAMs, stem cells (also called “initial cells”) are arranged around a small group of cells, called the “quiescent center (QC)”, although the exact number of the QC cells in rice roots is still controversial (see the subsection “ Mechanism of radial patterning of crown root and its maintenance ”) ( Ni et al 2014 , Strotmann and Stahl 2021 ). The QC cells themselves usually do not divide but have an important role to promote the proliferation of the surrounding stem cells.…”

A mutation in THREONINE SYNTHASE 1 uncouples proliferation and transition domains of the root apical meristem: experimental evidence and in silico proposed mechanism

“…The single shr or scr and the double plt1 plt2 loss-of-function mutants are not capable of RAM maintenance and show RAM exhaustion and determinate root growth ( Aida et al, 2004 ; Galinha et al, 2007 ; Helariutta et al, 2000 ; Nakajima et al, 2001 ; Sabatini et al, 2003 ). The coordinated role of some of these transcription factors with hormones has been unraveled with the aid of mathematical models ( Cruz-Ramírez et al, 2012 ; Salvi et al, 2020b ), and new experimental data show that they act in concert with peptides and other factors to maintain the quiescence of the QC and RAM activity ( Bertolotti et al, 2021 ; Pardal and Heidstra, 2021 ; Santuari et al, 2016 ; Shimotohno et al, 2018 ; Strotmann and Stahl, 2021 ). Among the main factors involved in maintaining the quiescence of the QC cells are WUSCHEL RELATED HOMEOBOX 5 (WOX5) and BRASSINOSTEROIDS AT VASCULAR AND ORGANIZING CENTER (BRAVO) transcription factors ( Betegón-Putze et al, 2021 ; Forzani et al, 2014 ; Sarkar et al, 2007 ; Vilarrasa-Blasi et al, 2014 ).…”