No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Cell-cell interactions: Taking cues from the neighbors
Abstract
Three lines of inquiry indicate that inductive interactions play a major role in the acquisition of cell identity during plant development.
... These tools have reduced the time necessary for hands-on work and allowed for studies at previously unattainable single-cell scales, enabling fundamental biological discoveries 23 . However, existing high-throughput single-cell analysis technologies are not well suited for studying cell interactions for which the ability to dynamically join, separate or reconfigure cells is important 30,31 . As a result, important questions in cellular behaviour and, in particular, single-cell systems, such as evaluating communication between multiple cells, remain beyond the reach of existing tools 32 . ...
Lab-on-a-chip devices leverage microfluidic technologies to enable chemical and biological processes at small scales. However, existing microfluidic channel networks are typically designed for the implementation of a single function or a well-defined protocol and do not allow the flexibility and real-time experimental decision-making essential to many scientific applications. In this Perspective, we highlight that reconfigurability and programmability of microfluidic platforms can support new functionalities that are beyond the reach of current lab-on-a-chip systems. We describe the ideal fully reconfigurable microfluidic device that can change its shape and function dynamically, which would allow researchers to tune a microscale experiment with the capacity to make real-time decisions. We review existing technologies that can dynamically control microscale flows, suggest additional physical mechanisms that could be leveraged towards the goal of reconfigurable microfluidics and highlight the importance of these efforts for the broad scientific community.
Lebendige Systeme müssen als in beständiger Entwicklung befindliche Systeme aufgefasst werden.Diese Feststellung gilt für die Einzelzelle ebenso wie für das vielzellige System. Kurz zusammengefasst besteht Entwicklung aus Wachstum, Differenzierung, Musterbildung und Morphogenese. Die Entwicklung der Pflanze wird einerseits durch ihre Gene dirigiert, andererseits aber auch in oft drastischer Weise durch die Umwelt modifiziert. Pflanzen sind auch bezüglich ihrer Entwicklung umweltoffene Systeme und unterscheiden sich in dieser Hinsicht grundsätzlich von den Tieren.
Lebendige Systeme müssen als in beständiger Entwicklung befindliche Systeme aufgefasst werden.Diese Feststellung gilt für
die Einzelzelle ebenso wie für das vielzellige System. Kurz zusammengefasst besteht Entwicklung aus Wachstum, Differenzierung, Musterbildung und Morphogenese. Die Entwicklung der Pflanze wird einerseits durch ihre Gene dirigiert, andererseits aber auch in oft drastischer Weise durch
die Umwelt modifiziert. Pflanzen sind auch bezüglich ihrer Entwicklung umweltoffene
Systeme und unterscheiden sich in dieser Hinsicht grundsätzlich von den Tieren. Das Zusammenwirken von Erbgut und Umwelt bei der
pflanzlichen Entwicklung ist ein wichtiges Thema der Entwicklungsphysiologie. Daneben steht die bei Pflanze und Tier gleichermaßen grundlegende Frage nach der Steuerung des Entwicklungsgeschehens von
den Keimzellen bis zum fortpflanzungsfähigen Organismus durch die Gene. Alle Zellen der Pflanzen stammen über mitotische Teilungen
von der befruchteten Eizelle (Zygote) ab. Obwohl sie über die gleiche genetische Information verfügen, entstehen im Verlauf
der Entwicklung distinkt verschiedene Zelltypen,welche sich zu geordneten höheren Einheiten (Gewebe, Organe) zusammenfügen.
Die Aufklärung der hierbei wirksamen Entwicklungsprogramme und ihrer Umsetzung bei der Merkmalsausprägung steht im Vordergrund
des Interesses der entwicklungsbiologischen Forschung, bei der heute vor allem eine Kombination genetischer, biochemischer
und histologischer Methoden eingesetzt wird.Obwohl hier noch viele Fragen offen sind, zeichnen sich doch erste Konturen einer
komplexen Hierarchie von Genen ab, welche teilweise, abhängig von organismuseigenen Faktoren und Signalen aus der Umwelt, das Entwicklungsschicksal der
einzelnen Zellen festlegen und diese zu einem harmonisch gegliederten, funktionell und strukturell organisierten Ganzen integrieren.
Cell fate is determined when the commitment of cells to a particular fate is autonomously maintained, irrespective of their environment. In Drosophila, fate determination is maintained through the action of the Polycomb-group and trithorax-group genes, which are required so that states of homeotic gene activity are inherited through cell division. It is shown here that the CURLY LEAF gene of Arabidopsis is necessary for stable repression of a floral homeotic gene and encodes a protein with homology to the product of the Polycomb-group gene Enhancer of zeste. We suggest that Polycomb-group genes have a similar role in fate determination in plants and animals.
The plant life cycle alternates between a diploid and a
haploid generation, the spore-producing sporophyte and
the gamete-producing gametophyte. Unlike in animals,
where meiotic products differentiate directly into gametes,
the haploid spores of plants divide mitotically to
form a multicellular haploid organism. The differentiation
of gametes occurs later in the development of the gametophyte.
The embryonic origin of the Arabidopsis root and hypocotyl region has been investigated using histological techniques and clonal analysis. Our data reveal the pattern of cell division in the embryo giving rise to the various initials within the root promeristem. A small region of the root at its connection with the hypocotyl appears not to be derived from the promeristem initials. This region contains two cortical cell layer and [3H]thymidine incorporation data suggest that it lacks postembryonic cell divisions. Sectors marked by transposon excision from the beta-glucuronidase marker gene are used to investigate cell lineages giving rise to root and hypocotyl. The position of end points from sectors with embryonic origin show little variation and hence reveal preferred positions in the seedling for cells derived from different regions of the embryo. The radial extent of complete root sectors is consistent with the radial arrangement of root meristem initials at the heart stage of embryogenesis inferred from histological analysis. Using the clonal data, a fate map is constructed depicting the destiny of heart stage embryonic cell tiers, in the seedling root and hypocotyl. The variability in the sector end points indicates that distinct cell lineages are not restricted for root or hypocotyl fate. In contrast, derivatives of the hypophyseal cell do appear to be restricted to the columella and central cell region of the root.
The Arabidopsis gene SUPERMAN (SUP) is necessary for the proper spatial development of reproductive floral tissues. Recessive mutations cause extra stamens to form interior to the normal third whorl stamens, at the expense of fourth whorl carpel development. The mutant phenotype is associated with the ectopic expression of the B function genes, AP3 and PI, in the altered floral region, closer to the centre of the flower than in the wild type, and ap3 sup and pi sup double mutants exhibit a phenotype similar to ap3 and pi single mutants. These findings led to SUP being interpreted as an upstream negative regulator of the B function organ-identity genes, acting in the fourth whorl, to establish a boundary between stamen and carpel whorls. Here we show, using molecular cloning and analysis, that it is expressed in the third whorl and acts to maintain this boundary in developing flowers. The putative SUPERMAN protein contains one zinc-finger and a region resembling a basic leucine zipper motif, suggesting a function in transcriptional regulation.
Postembryonic development in plants is achieved by apical meristems. Surgical studies and clonal analysis have revealed indirectly that cells in shoot meristems have no predictable destiny and that position is likely to play a role in the acquisition of cell identity. In contrast to animal systems, there has been no direct evidence for inductive signalling in plants until now. Here we present evidence for such signalling using laser ablation of cells in the root meristem of Arabidopsis thaliana. Although these cells show rigid clonal relationships, we now demonstrate that it is positional control that is most important in the determination of cell fate. Positional signals can be perpetuated from more mature to initial cells to guide the pattern of meristem cell differentiation. This offers an alternative to the general opinion that meristems are the source of patterning information.
Two monoclonal antibodies (ZUM 15 and ZUM 18) directed against carrot (Daucus carota L.) seed arabinogalactan proteins (AGPs) were used to isolate specific AGP fractions. For both carrot and tomato (Lycopersicon esculentum Mill.) seed AGPs analyzed by crossedelectrophoresis, the ZUM 15 and ZUM 18 AGP fractions showed one identical peak. However, the Rf values for the two species were different: 0.82 for carrot seed AGPs and 0.52 for tomato seed AGPs. When the fractionated AGPs (carrot or tomato) were added to carrot cell lines they had a dramatic effect on the culture. One AGP fraction (ZUM 15 AGPs) was able to induce vacuolation of embryogenic cells. Those cells failed to produce embryos. The other AGP fraction (ZUM 18 AGPs) increased the percentage of embryognic cells from about 40% up to 80% within one week and this subsequently resulted in the formation of more embryos on hormone-free medium. This activity was higher than that of unfractionated carrot seed AGPs, while the optimum concentration was 50-fold lower. Since both ZUM 18 AGPs (carrot or tomato) yielded identical responses it can be concluded that neither the Rf value nor the source are essential for biological activity. The dose-response curve of ZUM 18 AGPs showed a sharp optimum. When the AGPs that also bound to the antibody ZUM 15 were removed, the dose-response curve of the remaining AGPs (containing only the ZUM 18 epitope, not the ZUM 15 epitope) resembled a saturation curve. Regardless of its concentration, the fraction in which AGP molecules contained both epitopes showed no appreciable embryogenesis-promoting activity. The biological activity of AGPs was therefore determined by the presence of embryogenesis-enhancing and-inhibiting epitopes. The inhibiting and enhancing epitopes can be located on separate molecules or one single AGP molecule.
To isolate and study genes controlling floral development, we have carried out a large-scale transposon-mutagenesis experiment in Antirrhinum majus. Ten independent floral homeotic mutations were obtained that could be divided into three classes, depending on whether they affect (1) the identity of organs within the same whorl; (2) the identity and sometimes also the number of whorls; and (3) the fate of the axillary meristem that normally gives rise to the flower. The classes of floral phenotypes suggest a model for the genetic control of primordium fate in which class 2 genes are proposed to act in overlapping pairs of adjacent whorls so that their combinations at different positions along the radius of the flower can specify the fate and number of whorls. These could interact with class 1 genes, which vary in their action along the vertical axis of the flower to generate bilateral symmetry. Both of these classes may be ultimately regulated by class 3 genes required for flower initiation. The similarity between some of the homeotic phenotypes with those of other species suggests that the mechanisms controlling whorl identity and number have been highly conserved in plant evolution. Many of the mutations obtained show somatic and germinal instability characteristic of transposon insertions, allowing the cell-autonomy of floral homeotic genes to be tested for the first time. In addition, we show that the deficiens (def) gene (class 2) acts throughout organ development, but its action may be different at various developmental stages, accounting for the intermediate phenotypes conferred by certain def alleles. Expression of def early in development is not necessary for its later expression, indicating that other genes act throughout the development of specific organs to maintain def expression. Direct evidence that the mutations obtained were caused by transposons came from molecular analysis of leaf or flower pigmentation mutants, indicating that isolation of the homeotic genes should now be possible.
Genetic and anatomical analyses of the developing Arabidopsis embryogenic root reveal that stereotypic patterns of cell division are not required for pattern formation.
Homeotic mutants, that is, mutants with a normal organ in a place where an organ of another type is typically found, were first recognized in plants. The earliest descriptions of mutants in which petals replace stamens, giving double flowers, go back to ancient Greece and Rome. Similar accounts can be found in the botanical literature of China more than a thousand years ago, and in the books of the herbalists of Renaissance Europe (Meyerowitz et al., 1989). The use of such mutants (and similar but noninherited developmental abnormalities) to understand developmental processes in plants is more recent, dating from Linnaeus in the mid-eighteenth century (see Cullen and Stevens, 1990), and from Goethe (1790), who derived the ideas of organ homology and homological comparison from plants showing what was then called abnormal metamorphosis. Our term homeosis dates from Bateson’s work (1894) on organismal variation, in which he expanded Masters’treatment (1889) of abnormal metamorphosis in plants to animals and introduced the term homoeosis as a replacement for the older term. Goethe (1790) used homeotic variation in plants as the basis for a specific model explaining the developmental origin of different organ types in flowers. In his view, the four types of floral organs (sepals, petals, stamens, and carpels) are all modified leaves. As sap rises through developing flowers it is progressively refined, thereby inducing different organ types in different positions.
Homeotic genes controlling the identity of flower organs have been characterized in several plant species. To determine whether cells expressing these genes are specified to follow particular developmental fates, we have studied the pattern of cell lineages in developing flowers of Antirrhinum. Each flower has four whorls of organs, and progenitor cells of these can be marked at particular stages of development using a temperature-sensitive transposon. This allows the cell lineages in the flower to be followed, as well as giving information about rates of cell division.
We show here that, prior to the emergence of organ primordia, cells in the floral meristem have not been allocated organ identities. After this time, lineage restrictions arise between whorls, correlating with the onset of expression of genes that control organ identity. A further lineage restriction appears slightly later on, between the dorsal and ventral surfaces of the petal. Our results further suggest that the rates of cell division fluctuate during key stages of meristern development, perhaps as a consequence of meristem-identity gene expression.
The patterns of lineage restriction and organ-identity gene expression in early floral meristems are consistent with some cells being allocated specific identities at about this stage of development. Plant cells cannot move relative to each other, so lineage restrictions in plants may reflect particular orientations and/or rates of growth at boundary regions.
During postgenital tissue fusions, some plant epidermal cells redifferentiate into parenchyma, a different cell type. Diffusible
factors cause this response in the fusing gynoecium of the Madagascar periwinkle (Catharanthus roseus). Surgical manipulations of the gynoecium showed that epidermal cells from normally nonfusing surfaces could trasmit and
respond to the diffusible factors. Furthermore, the diffusible fators could be trapped in agar-impregnated barriers, as shown
by the redifferentiation of carpel epidermal cells from nonfusing regions when the factor-loaded barriers were appressed to
them.
In multicellular plants, development starts with an asymmetric division of the zygote into two differentiated cells. The nature
and distribution of fate-determining factors operating during embryogenesis remain largely obscure. Laser microsurgery was
used here to dissect two-celled embryos of the alga Fucus spiralis. Removal of protoplasts from the cell wall induced dedifferentiation. However, isolated cells within the walls followed their
restricted fate. Moreover, contact of one cell type with the isolated cell wall of the other cell type caused its fate to
be switched. The cell wall thus appears to maintain the differentiated state and to direct cell fate in plant development.
Radicle development(s)
- Doerner