植物根尖干细胞的表观遗传调控

干细胞是一类具有特化为不同细胞类型能力的多能性细胞,他为多细胞生物的器官发生、损伤修复和再生源源不断提供新细胞。干细胞的特化和维持需要复杂的基因调控网络来有序调控。此外,表观遗传调控在包括干细胞命运决定在内的许多生物学过程中发挥极其重要的作用。本文归纳了近年来对植物,主要是模式植物拟南芥（Arabidopsis thaliana（L.）Heynh.）根尖干细胞表观遗传调控方面的研究进展,重点论述了表观调控因子与控制干细胞的关键转录因子之间如何互作、调控植物根尖干细胞的自我更新和分化,并对今后研究的突破方向进行了展望。     

Emerging role of cytokinin as a regulator of cellular differentiation

Perhaps the most amazing feature of plants is their ability to grow and regenerate for years, sometimes even centuries. This fascinating characteristic is achieved thanks to the activity of stem cells, which reside in the shoot and root apical meristems. Stem cells function as a reserve of undifferentiated cells to replace organs and sustain postembryonic plant growth. To maintain meristem function, stem cells have to generate new cells at a rate similar to that of cells leaving the meristem and differentiating, thus achieving a balance between cell division and cell differentiation. Recent findings have improved our knowledge on the molecular mechanisms necessary to establish this balance and reveal a fundamental signaling role for the plant hormone cytokinin. Evidence has been provided to show that in the root meristem cytokinin acts in defined developmental domains to control cell differentiation rate, thus controlling root meristem size.     

Stem cells: The root of all cells

The plant basic body plan is laid down during embryogenesis. All post-embryonic development has its origin in the stem cells located in niches in the heart of the shoot and root meristems. Creating the root niche requires auxin dependent patterning cues that provide positional information in combination with parallel inputs to specify and maintain the root stem cell niche from embryogenesis onwards. Once established, the architecture of the root niche differs from that in the shoot but recent findings reveal a conserved module for stem cell control. Important for stem cell maintenance is the balance between cell division and differentiation. Dealing with the environment is the biggest challenge for plants and that includes complete regeneration of stem cell systems upon damage. Here we will address these issues as we follow the formation, function and maintenance of the root stem cell niche during development.     

Brassinosteroids control meristem size by promoting cell cycle progression in Arabidopsis roots

Brassinosteroids (BRs) play crucial roles in plant growth and development. Previous studies have shown that BRs promote cell elongation in vegetative organs in several plant species, but their contribution to meristem homeostasis remains unexplored. Our analyses report that both loss- and gain-of-function BR-related mutants in Arabidopsis thaliana have reduced meristem size, indicating that balanced BR signalling is needed for the optimal root growth. In the BR-insensitive bri1-116 mutant, the expression pattern of the cell division markers CYCB1;1, ICK2/KRP2 and KNOLLE revealed that a decreased mitotic activity accounts for the reduced meristem size; accordingly, this defect could be overcome by the overexpression of CYCD3;1. The activity of the quiescent centre (QC) was low in the short roots of bri1-116, as reported by cell type-specific markers and differentiation phenotypes of distal stem cells. Conversely, plants treated with the most active BR, brassinolide, or mutants with enhanced BR signalling, such as bes1-D, show a premature cell cycle exit that results in early differentiation of meristematic cells, which also negatively influence meristem size and overall root growth. In the stem cell niche, BRs promote the QC renewal and differentiation of distal stem cells. Together, our results provide evidence that BRs play a regulatory role in the control of cell-cycle progression and differentiation in the Arabidopsis root meristem.     

Control of root cap formation by MicroRNA-targeted auxin response factors in Arabidopsis

The plant root cap mediates the direction of root tip growth and protects internal cells. Root cap cells are continuously produced from distal stem cells, and the phytohormone auxin provides position information for root distal organization. Here, we identify the Arabidopsis thaliana auxin response factors ARF10 and ARF16, targeted by microRNA160 (miR160), as the controller of root cap cell formation. The Pro(35S):MIR160 plants, in which the expression of ARF10 and ARF16 is repressed, and the arf10-2 arf16-2 double mutants display the same root tip defect, with uncontrolled cell division and blocked cell differentiation in the root distal region and show a tumor-like root apex and loss of gravity-sensing. ARF10 and ARF16 play a role in restricting stem cell niche and promoting columella cell differentiation; although functionally redundant, the two ARFs are indispensable for root cap development, and the auxin signal cannot bypass them to initiate columella cell production. In root, auxin and miR160 regulate the expression of ARF10 and ARF16 genes independently, generating a pattern consistent with root cap development. We further demonstrate that miR160-uncoupled production of ARF16 exerts pleiotropic effects on plant phenotypes, and miR160 plays an essential role in regulating Arabidopsis development and growth.     

Root-to-shoot primordium conversion on Sesbania rostrata Brem. stem explants

The morphogenetic responses of cultured stem explants of Sesbaniarostrata Brem. from various positions along the stem axis wereanalysed after treatment with four growth regulators (BAP, NAA,kinetin, and GAJ. Internodal explants formed adventitious shootbuds when cultured on a Murashige and Skoog basal medium withoutadded growth regulators. Histological studies of regenerated shoot buds revealed thatapproximately 30% of the buds resulted from the conversion ofa preformed root primordium (characteristic of this species)into a shoot bud without a callogenesis phase. Each bud whichoriginated from a single root primordium grew into a leafy shoot.Preformed root primordia of stem explants of Sesbania rostratamay constitute an excellent model for physiological researchon plant differentiation. Key words: Organogenesis, adventitious bud, preformed root primordium, conversion, Sesbania rostrata     

Signals that regulate stem cell activity during plant development

Plant stem cells are used continuously to generate new structures during the entire life-span of the organism. In the adult plant, stem cells are found in specialized structures called meristems. The meristems contain the stem cell niche together with rapidly dividing daughter cells that will ultimately differentiate into specific cell types. Some of the master genes that orchestrate the establishment and maintenance of the stem cell niche have now been identified in both the root and the shoot. Recent results show that these genes also determine the fate of the stem cells and that feedback signals from differentiated cells are involved in stem cell specification. These advances have provided a framework to understand how short-range and long-range signals are integrated to specify and position the stem cell niche in the meristems, and how the differentiation potential of plant stem cells is controlled.     

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.     

Intraplant Nuclear DNA Content Variation in Diploid Nuclei of Maize (Zea mays L.)

Diploid nuclei from stem, mesocotyl, nodal root and root tiptissue of two maize hybrids were examined with respect to theirDNA content. The nuclei were isolated and stained with DAPIand passed through a flow cytometer-cell sorter. The titrationcurve for each tissue was determined. Significant variationwas observed among nuclei of different tissue types. Stem androot tips had the highest diploid nuclear DNA amounts while2-week-old mesocotyl had the lowest diploid nuclear DNA amount.These results provide evidence that during plant developmentand differentiation, the amount of DNA within a diploid nucleuschanges through loss of specific DNA sequences. This study alsodemonstrates the sensitivity of flow cytometry in detectingsmall intraplant variation in nuclear DNA. Key words: Flow cytometry, fluorochrome DAPI, DNA content, tissue differentiation, plant development     

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。     

Pattern analysis of stem cell differentiation during <Emphasis Type="Italic">in vitro Arabidopsis</Emphasis> organogenesis

Plant somatic cells have the capability to switch their cell fates from differentiated to undifferentiated status under proper culture conditions, which is designated as totipotency. As a result, plant cells can easily regenerate new tissues or organs from a wide variety of explants. However, the mechanism by which plant cells have such remarkable regeneration ability is still largely unknown. In this study, we used a set of meristem-specific marker genes to analyze the patterns of stem cell differentiation in the processes of somatic embryogenesis as well as shoot or root organogenesis in vitro. Our studies furnish preliminary and important information on the patterns of the de novo stem cell differentiation during various types of in vitro organogenesis.     

Stem cells in the root and the problem of stem cells in plants

Plant cells are capable of reversible transition from the proliferating to the stem state. This transition is determined by a system of cell-cell interactions and interelationships between plant parts. Stem cells defined as the cells preserving the capacity to divisions and differentiation for a long time arise repeatedly during development of the root and shoot primordial, rather than are clones of a population of stem cells laid down at a certain stage of embryogenesis. The quiescent center cells, rather than the surrounding actively dividing cells, best correspond to the characteristics of stem cells according to Loeffler and Potten. The factors that determine the quiescent center formation and maintenance in the root have been analyzed. The available data suggest that among these factors, indoleacetic acid transport and cap influence are of paramount significance. The cap formation precedes the quiescent center formation both during the root development and in the course of meristem regeneration after the root decapitation. The capacity of stem cell formation by the meristem suggests that not only meristem arises from the stem cells, but also that stem cells are formed from actively dividing cells. Repeated formation of stem cells allows long-term preservation of the capacity of plants for open morphogenesis and vegetative propagation.     

Stem cells in the root and the problem of stem cells in plants

Plant cells are capable of reversible transition from the proliferating to the stem state. This transition is determined by a system of cell-cell interactions and interrelationships between plant parts. Stem cells defined as the cells preserving the capacity to divisions and differentiation for a long time arise repeatedly during development of the root and shoot primordial, rather than are clones of a population of stem cells laid down at a certain stage of embryogenesis. The quiescent center cells, rather than the surrounding actively dividing cells, best correspond to the characteristics of stem cells according to Loeffler and Potten. The factors that determine the quiescent center formation and maintenance in the root have been analyzed. The available data suggest that among these factors, indoleacetic acid transport and cap influence are of paramount significance. The cap formation precedes the quiescent center formation both during the root development and in the course of meristem regeneration after the root decapitation. The capacity of tem cell formation by the meristem suggests that not only meristem arises from the stem cells, but also that stem cells are formed from actively dividing cells. Repeated formation of stem cells allows long-term preservation of the capacity of plants for open morphogenesis and vegetative propagation.     

In Vitro Culture of Different Explants from Hubei Photoperiod Sensitive Genic Male-Sterile Rice

Young panicles, immature embryos, stem nodes, stem tips, leaf segments, root tips and anthers from Hubei Photoperiod Sensitive Genie Male-Sterile Rice (Oryza sativa subsp, japonica) “Nongken 58” (abbr. 58s) and fertile “Nongken 58” (abbr. 58f) were examined for callus induction, plant regeneration and direct plantlet formation on differentiation medium. 58s and 58f had equal ability in all explants cultures except anther cultures. The induction frequency of the anther callus and the regeneration frequency of the green plant in 58s were much lower than those in 58f, and such differences were not affected markedly by the change of fertility of 58s donor plants. Young panicle, immature embryo, stem node, stem tips showed direct plantlet formation when cultured on differentiation medium containing NAA and kinetin. Different explants produced various types of responses. Young panicles could produce callus and then regenerate plantlets. Evidences from histological observation showed that the plant regeneration in direct plaatlet formation of young panicles were mainly organogenetic, bowever, somatic embryogenesis was also possible.     

In vitro root differentiation and essential-oil accumulation in Angelica archangelica

Summary We studied the relationship between root differentiation and the accumulation of essential oils in Angelica archangelica in in vitro cultures and in the intact plant. Root regeneration was obtained using stem and leaf explants subjected to treatment with the auxins indole-3-butyric acid, indole-3-acetic acid and α-naphthaleneacetic acid. In both stem and leaf explants, treatment with indole-3-butyric acid induced the highest rhizogenic response in terms of both percentage of explants with roots and number of roots per explant. Independently of hormonal treatment, stem explants produced a higher average number of roots per explant. Root meristemoids were already visible at day 7 of culture in the treatments with indole-3-butyric acid and indole-3-acetic acid; they were formed directly by cambial-cell division. In vitro-regenerated roots retained primary root structure and differentiated only two primary ephemeral ducts in the pericycle; no accumulation of essential oils was detected. Same-size roots taken from the intact plant showed secondary structure and essential-oil accumulation. The results of this study suggest that the synthesis and accumulation of essential oils in Angelica archangelica is closely linked to the differentiation of secondary secretory ducts.     

Plant stem cell niches

Stem cells are required to support the indeterminate growth style of plants. Meristems are a plants stem cell niches that foster stem cell survival and the production of descendants destined for differentiation. In shoot meristems, stem cell fate is decided at the populational level. The size of the stem cell domain at the meristem tip depends on signals that are exchanged with cells of the organizing centre underneath. In root meristems, individual stem cells are controlled by direct interaction with cells of the quiescent centre that lie in the immediate neighbourhood. Analysis of the interactions and signaling processes in the stem cell niches has delivered some insights into the molecules that are involved and revealed that the two major niches for plant stem cells are more similar than anticipated.