The quiescent center and root regeneration

Insights into multilevel spatial regulation within the root stem cell niche

All differentiated root cells derive from stem cells spatially organized within the stem cell niche (SCN), a microenvironment located within the root tip. Here, we compiled recent advances in the understanding of how the SCN drives the establishment and maintenance of cell types. The quiescent center (QC) is widely recognized as the primary driver of cell fate determination, but it is recently considered a convergence center of multiple signals. Cell identity of the cortex endodermis initials is mainly driven by the regulatory feedback loops between transcription factors (TFs), acting as mobile signals between neighboring cells, including the QC. As exemplified in the vascular initials, the precise spatial expression of these regulatory TFs is connected with a dynamic hormonal interplay. Thus, stem cell maintenance and cell differentiation are regulated by a plethora of signals forming a complex, multilevel regulatory network. Integrating the transcriptional and post-translational regulations, protein-protein interactions, and mobile signals into models will be fundamental for the comprehensive understanding of SCN maintenance and differentiation.

Maintenance of stem cell activity in plant development and stress responses

Stem cells residing in plant apical meristems play an important role during postembryonic development. These stem cells are the wellspring from which tissues and organs of the plant emerge. The shoot apical meristem (SAM) governs the aboveground portions of a plant, while the root apical meristem (RAM) orchestrates the subterranean root system. In their sessile existence, plants are inextricably bound to their environment and must adapt to various abiotic stresses, including osmotic stress, drought, temperature fluctuations, salinity, ultraviolet radiation, and exposure to heavy metal ions. These environmental challenges exert profound effects on stem cells, potentially causing severe DNA damage and disrupting the equilibrium of reactive oxygen species (ROS) and Ca2+ signaling in these vital cells, jeopardizing their integrity and survival. In response to these challenges, plants have evolved mechanisms to ensure the preservation, restoration, and adaptation of the meristematic stem cell niche. This enduring response allows plants to thrive in their habitats over extended periods. Here, we presented a comprehensive overview of the cellular and molecular intricacies surrounding the initiation and maintenance of the meristematic stem cell niche. We also delved into the mechanisms employed by stem cells to withstand and respond to abiotic stressors.

The inhibition of maize (Zea mays L.) root stem cell regeneration by low oxygen is attenuated by Phytoglobin 1 (Pgb1) through changes in auxin and jasmonic acid

MAIN CONCLUSIONS

Over-expression of Phytoglobin1 increases the viability of maize root stem cells to low oxygen stress through changes in auxin and jasmonic acid responses. Hypoxia inhibits maize (Zea mays L.) root growth by deteriorating the quiescent center (QC) stem cells of the root apical meristem. Over-expression of the Phytoglobin1 ZmPgb1.1 alleviates these effects through the retention of the auxin flow along the root profile required for the specification of the QC stem cells. To identify QC-specific hypoxia responses and determine whether ZmPgb1.1 exercises a direct role on QC stem cells, we performed a QC functionality test. This was done by estimating the ability of QCs to regenerate a root in vitro in a hypoxic environment. Hypoxia decreased the functionality of the QCs by depressing the expression of several genes participating in the synthesis and response of auxin. This was accompanied by a decrease in DR5 signal, a suppression of PLETHORA and WOX5, two markers of QC cell identity, and a reduction in expression of genes participating in JA synthesis and signaling. Over-expression of ZmPgb1.1 was sufficient to mitigate all these responses. Through pharmacological alterations of auxin and JA, it is demonstrated that both hormones are required for QC functionality under hypoxia, and that JA acts downstream of auxin during QC regeneration. A model is proposed whereby the ZmPgb1.1 maintenance of auxin synthesis in hypoxic QCs is determinant for the retention of their functionality, with JA supporting the regeneration of roots from the QCs.

Comparisons between Plant and Animal Stem Cells Regarding Regeneration Potential and Application

Regeneration refers to the process by which organisms repair and replace lost tissues and organs. Regeneration is widespread in plants and animals; however, the regeneration capabilities of different species vary greatly. Stem cells form the basis for animal and plant regeneration. The essential developmental processes of animals and plants involve totipotent stem cells (fertilized eggs), which develop into pluripotent stem cells and unipotent stem cells. Stem cells and their metabolites are widely used in agriculture, animal husbandry, environmental protection, and regenerative medicine. In this review, we discuss the similarities and differences in animal and plant tissue regeneration, as well as the signaling pathways and key genes involved in the regulation of regeneration, to provide ideas for practical applications in agriculture and human organ regeneration and to expand the application of regeneration technology in the future.

SCR Suppressor Mutants: Role in Hypocotyl Gravitropism and Root Growth in Arabidopsis thaliana

The SCARECROW (SCR) transcription factor plays a key role in plant growth and development. However, we know very little about the role of SCR regulated pathways in plant development. Here, we used the homozygous scr1 mutant Arabidopsis thaliana (Wassilewskija ecotype), which had a T-DNA insertion in the SCR coding region and lacks a detectable SCR transcript. This scr1 mutant has a determinate mode of root growth, shoot agravitropism and abnormal internal architecture in all organs examined. To screen for mutants that suppress the scr1 abnormal phenotypes, we exposed homozygous scr1 seeds to ethyl methane sulphonate (EMS) mutagen. Upon growth out of these mutagenized seeds, thirteen suppressor mutant-harboring strains were identified. All thirteen suppressor-harboring strains were homozygous for scr1 and lacked the SCR transcript. Ten scr hypocotyl gravitropic suppressor lines showed improved hypocotyl gravitropic response. These ten suppressors fall into six complementation groups suggesting six different gene loci. Similarly, three independent scr root length suppressor lines rescued only the root growth phenotype and fell into three complementation groups, suggesting the involvement of three different gene loci. These suppressors might identify novel functions of the SCR gene in plant development.

BRAVO self-confined expression through WOX5 in the Arabidopsis root stem-cell niche

In animals and plants, stem-cell niches are local microenvironments that are tightly regulated to preserve their unique identity while communicating with adjacent cells that will give rise to specialized cell types. In the primary root of Arabidopsis thaliana, two transcription factors, BRAVO and WOX5, among others, are expressed in the stem-cell niche. Intriguingly, BRAVO, a repressor of quiescent center divisions, confines its own gene expression to the stem-cell niche, as evidenced in a bravo mutant background. Here, we propose through mathematical modeling that BRAVO confines its own expression domain to the stem-cell niche by attenuating a WOX5-dependent diffusible activator of BRAVO. This negative feedback drives WOX5 activity to be spatially restricted as well. The results show that WOX5 diffusion and sequestration by binding to BRAVO are sufficient to drive the experimentally observed confined BRAVO expression at the stem-cell niche. We propose that the attenuation of a diffusible activator can be a general mechanism acting at other stem-cell niches to spatially confine genetic activity to a small region while maintaining signaling within them and with the surrounding cells.

Pluripotency acquisition in the middle cell layer of callus is required for organ regeneration

In plant tissue culture, callus forms from detached explants in response to a high-auxin-to-low-cytokinin ratio on callus-inducing medium. Callus is a group of pluripotent cells because it can regenerate either roots or shoots in response to a low level of auxin on root-inducing medium or a high-cytokinin-to-low-auxin ratio on shoot-inducing medium, respectively¹. However, our knowledge of the mechanism of pluripotency acquisition during callus formation is limited. On the basis of analyses at the single-cell level, we show that the tissue structure of Arabidopsis thaliana callus on callus-inducing medium is similar to that of the root primordium or root apical meristem, and the middle cell layer with quiescent centre-like transcriptional identity exhibits the ability to regenerate organs. In the middle cell layer, WUSCHEL-RELATED HOMEOBOX5 (WOX5) directly interacts with PLETHORA1 and 2 to promote TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS1 expression for endogenous auxin production. WOX5 also interacts with the B-type ARABIDOPSIS RESPONSE REGULATOR12 (ARR12) and represses A-type ARRs to break the negative feedback loop in cytokinin signalling. Overall, the promotion of auxin production and the enhancement of cytokinin sensitivity are both required for pluripotency acquisition in the middle cell layer of callus for organ regeneration.

The quiescent centre and root apical meristem: organization and function

This special issue is dedicated to the 100th anniversary of the birth of Frederick Albert Lionel Clowes, who discovered the quiescent centre (QC) of the root apical meristem (RAM). His discovery was a foundation for contemporary studies of the QC and RAM function, maintenance, and organization. RAM function is fundamental for cell production and root growth. This special issue bundles reviews on the main tendencies, hypotheses, and future directions, and identifies unknowns in the field.

The quiescent centre of the root apical meristem: conceptual developments from Clowes to modern times

In this review we discuss the concepts of the quiescent centre (QC) of the root apical meristem (RAM) and their change over time, from their formulation by F.A.L. Clowes to the present. This review is dedicated to the 100th anniversary of the birth of Clowes, and we present his short biography and a full bibliography of Clowes' work. Over time, the concept of the QC proved to be useful for the understanding of RAM organization and behaviour. We focus specifically on conceptual developments, from the organization of the QC to understanding its functions in RAM maintenance and activity, ranging from a model species, Arabidopsis thaliana, to crops. Concepts of initial cells, stem cells, and heterogeneity of the QC cells in the context of functional and structural stem cells are considered. We review the role of the QC in the context of cell flux in the RAM and the nature of quiescence of the QC cells. We discuss the origin of the QC and fluctuation of its size in ontogenesis and why the QC cells are more resistant to stress. Contemporary concepts of the organizer and stem cell niche are also considered. We also propose how the stem cell niche in the RAM can be defined in roots of a non-model species.