Apical meristems: the plant's fountain of youth

During postembryonic development, all organs of a plant are ultimately derived from a few pluripotent stem cells found in specialized structures called apical meristems. Here we discuss our current knowledge about the regulation of plant stem cells and their environments with main emphasis on the shoot apical meristem of Arabidopsis thaliana. Recent studies suggest that stem cells are localized in specialized niches where signals from surrounding cells maintain their undifferentiated state. In the shoot meristem, initiation of stem cells during embryogenesis, regulation of stem-cell homeostasis and termination of stem-cell maintenance during flower development appear to primarily involve regulation of the stem-cell niche.     

The Arabidopsis OBERON1 and OBERON2 genes encode plant homeodomain finger proteins and are required for apical meristem maintenance

Maintenance of the stem cell population located at the apical meristems is essential for repetitive organ initiation during the development of higher plants. Here, we have characterized the roles of OBERON1 (OBE1) and its paralog OBERON2 (OBE2), which encode plant homeodomain finger proteins, in the maintenance and/or establishment of the meristems in Arabidopsis. Although the obe1 and obe2 single mutants were indistinguishable from wild-type plants, the obe1 obe2 double mutant displayed premature termination of the shoot meristem, suggesting that OBE1 and OBE2 function redundantly. Further analyses revealed that OBE1 and OBE2 allow the plant cells to acquire meristematic activity via the WUSCHEL-CLAVATA pathway, which is required for the maintenance of the stem cell population, and they function parallel to the SHOOT MERISTEMLESS gene, which is required for preventing cell differentiation in the shoot meristem. In addition, obe1 obe2 mutants failed to establish the root apical meristem, lacking both the initial cells and the quiescent center. In situ hybridization revealed that expression of PLETHORA and SCARECROW, which are required for stem cell specification and maintenance in the root meristem, was lost from obe1 obe2 mutant embryos. Taken together, these data suggest that the OBE1 and OBE2 genes are functionally redundant and crucial for the maintenance and/or establishment of both the shoot and root meristems.     

Shoot meristem formation and maintenance

The shoot apical meristem of higher plants is a self-maintaining stem cell system which gives rise to the entire above-ground part of a plant. In the past year, genetic and molecular studies have provided increasing insight into the processes of shoot meristem formation and maintenance, as well as into the relation between the apical meristem and its products.     

Gene NANA regulates cell proliferation in Arabidopsis thaliana shoot apical meristem without interaction with CLV1, CLV2, CLV3

A constancy of stem cell pool in shoot apical meristem of Arabidopsis thaliana is provided by a genetic regulation system with negative feedback loop based on the interaction of the gene WUS, which maintains indeterminate state of cells, with CLV genes, which restrict the level of WUS expression and stem cell pool size. clv mutations lead to an increase in the pool of stem cells in the apical and floral meristems and wus mutation leads to the opposite effect. Mutation na (nana), like wus mutation, causes premature termination of shoot apical meristem function, although it does not affect the activity of the flower meristem. To elucidate the role of NA in the control of shoot apical meristem functioning, the interaction of NA with CLV genes were investigated. Additive phenotype of double mutants na clv1, na clv2-1, and na clv3-2 indicates that the NA gene makes an independent contribution to the functioning of the shoot apical meristem. It is assumed that the NA gene controls apical meristem cell proliferation during the transition to the reproductive phase of plant development, acting much later and independently of the genes WUS-CLV.     

A quantitative and dynamic model for plant stem cell regulation

Plants maintain pools of totipotent stem cells throughout their entire life. These stem cells are embedded within specialized tissues called meristems, which form the growing points of the organism. The shoot apical meristem of the reference plant Arabidopsis thaliana is subdivided into several distinct domains, which execute diverse biological functions, such as tissue organization, cell-proliferation and differentiation. The number of cells required for growth and organ formation changes over the course of a plants life, while the structure of the meristem remains remarkably constant. Thus, regulatory systems must be in place, which allow for an adaptation of cell proliferation within the shoot apical meristem, while maintaining the organization at the tissue level. To advance our understanding of this dynamic tissue behavior, we measured domain sizes as well as cell division rates of the shoot apical meristem under various environmental conditions, which cause adaptations in meristem size. Based on our results we developed a mathematical model to explain the observed changes by a cell pool size dependent regulation of cell proliferation and differentiation, which is able to correctly predict CLV3 and WUS over-expression phenotypes. While the model shows stem cell homeostasis under constant growth conditions, it predicts a variation in stem cell number under changing conditions. Consistent with our experimental data this behavior is correlated with variations in cell proliferation. Therefore, we investigate different signaling mechanisms, which could stabilize stem cell number despite variations in cell proliferation. Our results shed light onto the dynamic constraints of stem cell pool maintenance in the shoot apical meristem of Arabidopsis in different environmental conditions and developmental states.     

Stem Cell Signalling Networks in Plants

The essential nature of meristematic tissues is addressed with reference to conceptual frameworks that have been developed to explain the behaviour of animal stem cells. Comparisons are made between different types of plant meristems with the objective of highlighting common themes that might illuminate underlying mechanisms. A more in depth comparison of the root and shoot apical meristems is made which suggests a common mechanism for maintaining stem cells. The relevance of organogenesis to stem cell maintenance is discussed, along with the nature of underlying mechanisms which help ensure that stem cell production is balanced with the depletion of cells through differentiation. Mechanisms that integrate stem cell behaviour in the whole plant are considered, with a focus on the roles of auxin and cytokinin. The review concludes with a brief discussion of epigenetic mechanisms that act to stabilise and maintain stem cell populations.     

Plant meristems: <Emphasis Type="Italic">CLAVATA3/ESR</Emphasis>-related signaling in the shoot apical meristem and the root apical meristem

The plant meristems, shoot apical meristem (SAM) and root apical meristem (RAM), are unique structures made up of a self-renewing population of undifferentiated pluripotent stem cells. The SAM produces all aerial parts of postembryonic organs, and the RAM promotes the continuous growth of roots. Even though the structures of the SAM and RAM differ, the signaling components required for stem cell maintenance seem to be relatively conserved. Both meristems utilize cell-to-cell communication to maintain proper meristematic activities and meristem organization and to coordinate new organ formation. In SAM, an essential regulatory mechanism for meristem organization is a regulatory loop between WUSCHEL (WUS) and CLAVATA (CLV), which functions in a non-cell-autonomous manner. This intercellular signaling network coordinates the development of the organization center, organ boundaries and distant organs. The CLAVATA3/ESR (CLE)-related genes produce signal peptides, which act non-cell-autonomously in the meristem regulation in SAM. In RAM, it has been suggested that a similar mechanism can regulate meristem maintenance, but these functions are largely unknown. Here, we overview the WUS–CLV signaling network for stem cell maintenance in SAM and a related mechanism in RAM maintenance. We also discuss conservation of the regulatory system for stem cells in various plant species. S. Sawa is the recipient of the BSJ Award for Young Scientist, 2007.     

The Role of Gravity in Apical Dominance : Effects of Clinostating on Shoot Inversion-Induced Ethylene Production, Shoot Elongation, and Lateral Bud Growth

Shoot inversion-induced release of apical dominance in Pharbitis nil is inhibited by rotating the plant at 0.42 revolutions per minute in a vertical plane perpendicular to the axis of rotation of a horizontal clinostat. Clinostating prevented lateral bud outgrowth, apparently by negating the restriction of the shoot elongation via reduction of ethylene production in the inverted shoot. Radial stem expansion was also decreased. Data from experiments with intact tissue and isolated segments indicated that shoot-inversion stimulates ethylene production by increasing the activity of 1-aminocyclopropane-1-carboxylic acid synthase. The results support the hypothesis that shoot inversion-induced release of apical dominance in Pharbitis nil is due to gravity stress and is mediated by ethylene-induced retardation of the elongation of the inverted shoot.     

Cells within the nodal region of carnation shoots exhibit a high potential for adventitious shoot formation

Adventitious shoot formation was studied with leaf, stem and axillary bud explants of carnation (Dianthus caryophyllus L.). The shoot regeneration procedures were applicable for a wide range of cultivars and shoot regeneration percentages were high for all explant types. Using axillary bud explants, shoot regeneration efficiency was independent of the size of the bud and of its original position in the plant. In contrast, shoot regeneration from stem and leaf explants was strongly dependent on their original position on the plant. The most distal explants (just below the apex) showed the highest level of shoot regeneration. The adventitious shoot primordia developed at the periphery of the stem segment and at the base of leaf explants. In axillary bud, stem and leaf explants, shoot regeneration originated from node cells, located at the transition area between leaf and stem tissue. Moreover, a gradient in shoot regeneration response was observed, increasing towards the apical meristem.Abbreviations BA benzyladenine - NAA naphthaleneacetic acid     

Headcase Promotes Cell Survival and Niche Maintenance in the Drosophila Testis

At the apical tip of the Drosophila testis, germline and somatic stem cells surround a cluster of somatic cells called the hub. Hub cells produce a self-renewal factor, Unpaired (Upd), that activates the JAK-STAT pathway in adjacent stem cells to regulate stem cell behavior. Therefore, apical hub cells are a critical component of the stem cell niche in the testis. In the course of a screen to identify factors involved in regulating hub maintenance, we identified headcase (hdc). Hub cells depleted for hdc undergo programmed cell death, suggesting that anti-apoptotic pathways play an important role in maintenance of the niche. Using hdc as paradigm, we describe here the first comprehensive analysis on the effects of a progressive niche reduction on the testis stem cell pool. Surprisingly, single hub cells remain capable of supporting numerous stem cells, indicating that although the size and number of niche support cells influence stem cell maintenance, the testis stem cell niche appears to be remarkably robust in the its ability to support stem cells after severe damage.     

生长素与植物根尖干细胞巢的维持

干细胞巢的维持与后代细胞的分化是多细胞高等生物个体发育的基础。生长素对植物茎尖和根尖分生组织的形态建成, 尤其是对位于植物这2个末端的分生组织中心的干细胞巢的活性维持起着至关重要的作用。该文综述了近几年在植物根尖干细胞发育领域的研究进展, 主要阐释了PLT蛋白途径、SCR-SHR蛋白途径以及环境因子多信号调控模块维持植物根尖分生组织中干细胞巢稳定的机制, 揭示了生长素可以通过就近合成、极性运输以及信号转导3种方式参与这些信号模块的调控, 从而维持生长素在根尖静止中心细胞附近干细胞巢的浓度梯度, 精确地平衡植物干细胞巢中细胞的增殖与分化。     

Stem Cell Maintenance and Abiotic Stress Response in Shoot Apical Meristem for Developmental Plasticity

The shoot apical meristem (SAM) serves as a non-drying reservoir of pluripotent stem cells to supply new daughter cells forming above-ground tissues and organs such as leaves, stems, flowers and fruits throughout the life cycle of plants. Accordingly, the homeostasis control of stem cell division and differentiation must be an essential core mechanism for harmonic growth and development of plants as multicellular higher eukaryotes. Unlike animals, plants are sessile organisms and thus constantly face environmental factors, including abiotic stresses. Therefore, post-embryonic development derived from stem cells in the SAM likely interacts with surrounding abiotic stresses for plant adaptation and plastic development. For this reason, this review provides the most recent findings regarding comprehensive signaling networks involved in stem cell maintenance in the SAM, and then describes how stem cell signaling is related with abiotic stress response through involvement of phytohormones and reactive oxygen species in the SAM.     

Signaling pathways maintaining stem cells at the plant shoot apex

The above ground organs of plants are generated by the shoot apical meristem. Cellular characteristics and molecular markers indicate that the shoot meristem is patterned into domains with different functions, with stem cells residing in the outer three cell layers of the central zone of the meristem. The boundaries of the domains are determined by positional signals. Here we will discuss our current understanding of the signaling network involved in determining stem cell fate and in setting the boundaries of the stem cell niche at the plant shoot apex.