Plant Stem Cell Niches Research Articles

Mechanical Stress Acts via Katanin to Amplify Differences in Growth Rate between Adjacent Cells in Arabidopsis

The presence of diffuse morphogen gradients in tissues supports a view in which growth is locally homogenous. Here we challenge this view: we used a high-resolution quantitative approach to reveal significant growth variability among neighboring cells in the shoot apical meristem, the plant stem cell niche. This variability was strongly decreased in a mutant impaired in the microtubule-severing protein katanin. Major shape defects in the mutant could be related to a local decrease in growth heterogeneity. We show that katanin is required for the cell's competence to respond to the mechanical forces generated by growth. This provides the basis for a model in which microtubule dynamics allow the cell to respond efficiently to mechanical forces. This in turn can amplify local growth-rate gradients, yielding more heterogeneous growth and supporting morphogenesis.

WUSCHEL protein movement mediates stem cell homeostasis in the Arabidopsis shoot apex

WUSCHEL (WUS) is a homeodomain transcription factor produced in cells of the niche/organizing center (OC) of shoot apical meristems. WUS specifies stem cell fate and also restricts its own levels by activating a negative regulator, CLAVATA3 (CLV3), in adjacent cells of the central zone (CZ). Here we show that the WUS protein, after being synthesized in cells of the OC, migrates into the CZ, where it activates CLV3 transcription by binding to its promoter elements. Using a computational model, we show that maintenance of the WUS gradient is essential to regulate stem cell number. Migration of a stem cell-inducing transcription factor into adjacent cells to activate a negative regulator, thereby restricting its own accumulation, is a theme that is unique to plant stem cell niches.

Sculpting the meristem: The roles of miRNAs in plant stem cells

The whole structure of higher plants is generated dynamically throughout the life cycle by the activity of stem cell niches at the apex of shoot and root. Hormone molecules and many transcription factors cooperate to balance the stem cell maintenance and differentiation. It is becoming increasingly clear that microRNA (miRNA) molecules are also participants in these processes. Here, we highlight the advances that have been made in regarding the roles of miRNAs in plant stem cell control. These advances provide a framework for our understanding of how signals are integrated to specify and position the stem cell niches in plants.

Transcriptional Control of a Plant Stem Cell Niche

Despite the independent evolution of multicellularity in plants and animals, the basic organization of their stem cell niches is remarkably similar. Here, we report the genome-wide regulatory potential of WUSCHEL, the key transcription factor for stem cell maintenance in the shoot apical meristem of the reference plant Arabidopsis thaliana. WUSCHEL acts by directly binding to at least two distinct DNA motifs in more than 100 target promoters and preferentially affects the expression of genes with roles in hormone signaling, metabolism, and development. Striking examples are the direct transcriptional repression of CLAVATA1, which is part of a negative feedback regulation of WUSCHEL, and the immediate regulation of transcriptional repressors of the TOPLESS family, which are involved in auxin signaling. Our results shed light on the complex transcriptional programs required for the maintenance of a dynamic and essential stem cell niche.

Hypersensitivity to DNA damage in plant stem cell niches

The growing apices of plants contain stem cells that continually produce tissues, which, in the shoot, include the germline. These stem cell populations remain active throughout the plant's life, which can last for centuries, and are particularly exposed to environmental hazards that cause DNA damage and mutations. It is not known whether plants have mechanisms to safeguard the genome specifically in these crucial cell populations. Here, we show that root and shoot stem cells and their early descendants are selectively killed by mild treatment with radiomimetic drugs, x-rays, or mutations that disrupt DNA repair by nonhomologous end-joining. Stem cell death required transduction of DNA damage signals by the ATAXIA-TELANGIECTASIA MUTATED (ATM) kinase and, specifically in the root, also the ATM/RAD3-RELATED (ATR) kinase. Consistent with the absence of p53 and the core apoptotic machinery in plants, death of the stem cells did not show apoptotic but autolytic features as seen in other cases of plant developmentally programmed cell death. We propose that plants have independently evolved selective death as a stringent mechanism to safeguard genome integrity in their stem cell populations.

Plant Stem Cell Niches: Standing the Test of Time

Similar to animal stem cells, plant stem cells require special niche microenvironments to continuously generate the tissues that constitute the plant body. Recent work using computer modeling and live imaging is helping to elucidate some of the mechanisms responsible for the specification and maintenance of stem cells in the root and shoot.

The dynamic plant stem cell niches

Stem cells exist in specific locations called niches, where extracellular signals maintain stem cell division and prevent differentiation. In plants, the best characterised niches are within the shoot and root meristems. Networks of regulatory genes and intercellular signals maintain meristem structure in spite of constant cell displacement by division. Recent works have improved our understanding of how these networks function at the cellular and molecular levels, particularly in the control of the stem cell population in the shoot meristem. The meristem regulatory genes have been found to function partly through localised control of widely used signals such as cytokinin and auxin. The retinoblastoma protein has also emerged as a key regulator of cell differentiation in the meristems.

KNOX Gene Function in Plant Stem Cell Niches

Homeobox genes encode transcriptional regulators that control development in multicellular eukaryotes. In plants, post-embryonic shoot growth relies on the activity of indeterminate cell populations termed shoot meristems, within which members of the class-1 KNOX sub-family of homeobox genes are expressed. KNOX genes are differentially required for meristem development and function to inhibit cell expansion and differentiation associated with organogenesis. Mechanisms must therefore be employed to prevent KNOX gene expression in developing lateral organs such as leaves. This review focuses on the expression patterns, meristematic functions and regulation of KNOX genes, and how the activities of these genes are integrated within the framework of pathways that control plant development.