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PubMed2001

Developmental morphology of the thalloid Hydrobryum japonicum (Podostemaceae).

We describe the unique development and branching of lobed thalli in Hydrobryum japonicum. Lobe formation begins with meristem initiation at random sites near the thallus margin fringed by protective tissues. As the protective tissues are successively peeled off particularly in the growing new lobes, the lobes become naked and then become fringed again by new protective tissues that develop from the marginal part of the new meristems. Subsequently the meristems become less active and are differentiated into parenchymatous ground tissue at maturity. The random pattern of meristem formation during the sporadic development gives rise to a nonorderly branching pattern of the thalli. Some other lobes (∼10%) are regenerated from injured parts of the thalli. The vegetative shoots arise endogenously near the thallus margin and are enclosed by the nonvascular strand nets. The rudimentary shoot apices remain embedded in the thalli. The thalli, though remarkably different from typical roots of other angiosperms, might be extremely transformed roots.

Journal Article↗

PubMed2001

Lateral growth patterns in the Cladoniaceae.

Lateral growth from the apex of vertical structures is widespread in cladoniiform lichens. In the family Cladoniaceae, it is accomplished through a developmental shift in the meristem, in which growth orientation changes from isotropous to anisotropous. In anisotropous development, the more or less constant relationship among the axial, radial, and circumferential planes of growth is altered during ontogeny. The result is pronounced lateral elongation of the apical meristem, a departure from the isotropous body plan of early ontogeny. Development that favors radial and circumferential growth over axial growth is an innovation that provides ontogenetic flexibility but which also entails the loss of control from a single centralized meristem to one or more meristems. A shift from the constraints of symmetry to the risks and potential of asymmetry and a subsequent diversity of heritable thallus forms reflect evolutionary processes in the Cladoniaceae. Similar morphogenetic activities, which are apparently highly conserved, are shared by species that are presumably only distantly related.

Journal Article↗

PubMed2001

Citrus tissue culture employing vegetative explants.

Citrus being a number one fruit of the world due to its high nutritional value, huge production of fruits and fruit products, the citrus industry may be considered a major fruit industry. Though citrus orchard area in India is comparable to USA, the produce is far less, while its export is nil. Biotechnology has played an outstanding role in boosting the citrus industry, e.g., in Spain, which is now the biggest exporter of citrus fruit with the application of micrografting. Amongst the fruit trees, perhaps the maximum tissue culture research has been done in citrus during the past four decades, however, the results of practical value are meagre. The shortfalls in citrus tissue culture research and some advancements made in this direction along with bright prospects are highlighted, restricting the review to vegetative explants only. Whilst utilization of nucellar embryogenesis is limited to rootstocks, the other aspects, like, regeneration and proliferation of shoot meristems measuring 200 microm in length--a global breakthrough--of two commercially important scion species, Citrus aurantifolia and C. sinensis and an important rootstock, C. limonia, improvement of micrografting technique, cloning of the same two scion species as well as some Indian rootstock species, employing nodal stem segments of mature trees, of immense practical value have been elaborated. A rare phenomenon of shift in the morphogenetic pattern of differentiation from shoot bud differentiation to embryoid formation occurred during the long-term culture of stem callus of C. grandis. Stem callus-regenerated plants of C. aurantifolia, C. sinensis and C. grandis showed variation in their ploidy levels and a somaclonal variant of C. sinensis, which produced seedless fruits was isolated. Tailoring of rooting in microshoots to a tap root-like system by changing the inorganic salt composition of the rooting medium, resulting in 100% transplant success, and germplasm preservation through normal growth culture of shoots of C grandis without loss of regeneration capacity during 31 years, observed so far, are some other significant results. Plants of C. aurantifolia and C. sinensis raised from shoot meristem and micrografting were grown in a nethouse and those from nodal stem segments in the field along with the in vitro-raised plants of rootstocks, namely, C. jambhiri, C. karna and C. limonia. All the plants showed normal healthy growth. Significantly enough, the meristem regenerated plants of C. aurantifolia attained the reproductive phase just in 1 year of transplantation to soil similar to those raised from nodal stem segments of mature trees, which also produced normal fruits in the subsequent year while growing under field conditions. Thus, a significant fundamental concept of a maturity factor, carried over through as small a shoot meristem as 200 microm in length to cloned plants has been demonstrated. The concept is of far-reaching significance in citrus industry besides production of pathogen-free orchards.

Botany↗

PubMedDate not supplied

Cold acclimation improves recovery of cryopreserved grass (Zoysia and Lolium sp.).

Cold acclimation of Lolium L. and Zoysia Willd. Grass cultivars significantly increased regrowth of cryopreserved meristems. One wk of cold acclimation improved recovery following cryopreservation but extended acclimation (4-8 wk) resulted in the best regrowth. Cold acclimation also significantly increased the dehydration tolerance of both Zoysia and Lolium meristems. Lolium apices cold acclimated for 4 wk produced 60-100% regrowth following cryopreservation by slow freezing or encapsulation-dehydration. Cold-acclimated Zoysia had greater than 60% regrowth following encapsulation-dehydration when beads were dehydrated to less than 22% water content. Non-acclimated meristems of both genera had little or no regrowth. Thawed meristems grew quickly without callus formation and the plantlets produced were transplanted to pots in the greenhouse after 4 to 6 wk. Samples of each cultivar were stored in liquid nitrogen as part of the U.S. National Plant Germplasm System.

Journal Article↗

PubMed1986

The flowering process as an example of plastic development.

The field of flowering has been characterized by simplistic ideas, as exemplified by the attempts to classify plants on the basis of the kinds of environmental factors required for their transition to flower initiation and the claims that the process of flower formation in photoperiodic and cold-requiring plants is independent of correlative influences from various organs, e.g. the roots. Other examples of this simplified picture are the concept of a specific leaf-generated floral hormone and floral inhibitor and the search at the level of the meristem of a specific evocational event which would set in motion the whole sequence of other events and commit the meristem to flower initiation. Finally, there is the belief that because flower morphology is basis to species classification the morphogenesis of flowers is a rather invariable process. All these ideas are essentially erroneous and it is shown that all aspects of the flowering process are much more flexible than is usually believed. Floral induction may be completed by many, if not all, plants in several alternative sets of environmental factors. At least in some plants, the alternative inductive factors are perceived by different organs, indicating that these factors affect most probably entirely different processes. Thus, at induction, plasticity is extremely large and the fate of any shoot meristem appears to be controlled by a complex and flexible array of promoters and inhibitors arising from all plant parts. At meristem evocation, there are a number of events which are fundamentally the same in many plants, but so far no single initial critical event has been found. The various evocational changes appear to form sets of interconnected systems and this complex network seems to embody some plasticity since it has been possible to suppress experimentally some of the most universal evocational events or alter their temporal order without impairing evocation itself. At later stages, it has been observed that all the morphological characters of inflorescences and flowers may be experimentally altered. However, if the occasional and extreme malformations (monstrosities) caused by some growth substances are excluded, morphogenetic processes do not appear flexible to the point that the reproductive structures of one species are transformed into those of a taxonomically unrelated species. Thus, despite the fact that these processes are never absolutely fixed, plasticity at morphogenesis appears less than that at induction.(ABSTRACT TRUNCATED AT 400 WORDS)

Light↗

PubMed2004

Plant hormones and homeoboxes: bridging the gap?

Plant hormones are signalling molecules that control growth and development. Growth of the aerial parts of higher plants requires the continuous activity of the shoot apical meristem, a small mound of cells at the apex of a plant. KNOTTED1-like HOMEOBOX (KNOX) genes are involved in regulating meristem activity, however, little is known about how this regulation is mediated. Recent evidence suggests that KNOX transcription factors may control meristem development by regulating the balance of activities of multiple hormones.

Cell Differentiation↗

PubMed1998

Additional vegetative growth in maize reflects expansion of fates in preexisting tissue, not additional divisions by apical initials.

The maize shoot is usually determinate: the apical meristem produces a fixed number of vegetative nodes before it switches to tassel development. Culturing maize meristems, however, delays their determinacy. Cultured meristems may form up to twice the usual number of vegetative nodes. Clonal analysis of the "extra" vegetative nodes reveals that these nodes are the product of conversion, roughly equivalent to a homoeotic transformation, of tissue that otherwise would form the base of the tassel. Altered activity of the apical initials does not generate the extra vegetative growth. The conserved, stereotypical activity of the apical initials even in the face of radically prolonged vegetative growth suggests that apical initials in this annual grass may acquire a highly restricted fate (presporogenous tissue) early in embryogenesis.

Cell Count↗

PubMed1998

Infection of tobacco or Arabidopsis plants by CMV counteracts systemic post-transcriptional silencing of nonviral (trans)genes.

Cucumber mosaic cucumovirus (CMV) infection but not tomato black ring nepovirus infection counteracted post-transcriptional gene silencing (PTGS) of nitrate reductase (Nia) or beta-glucuronidase (uidA) transgenes in newly developing leaves of tobacco and Arabidopsis plants. PTGS did not affect meristems of noninfected silenced plants, indicating that the interfering effect of CMV is not likely to occur in the meristem. Models are proposed to explain how CMV (which has no sequence similarity to the Nia or uidA transgenes) can inhibit cellular factors involved in the RNA degradation step of PTGS and/or inhibit the systemic spread of the silencing signal to tissues emerging from the meristem.

Arabidopsis↗

PubMed2006

In vitro culture methods in sorghum with shoot tip as the explant material.

Twenty-four diverse genotypes of sorghum were evaluated for response to callus induction and plant regeneration with two media viz., MS and NBKNB using shoot tips as the start material to identify a model genotype. None of the genotypes tested showed promising results. Therefore, alternative methods of in vitro pathways using shoot meristem isolated from shoot tips were explored. Shoot apical meristems were isolated and were induced to multiple shoots or multiple shoot buds pathway by manipulation of thidiazuron (TDZ), 6-benzyl adenine (BAP) and 2, 4-dichlorophenoxy acetic acid (2, 4-D). Choice of the pathway whether large-scale multiplication of shoots or production of target tissues for transformation can be exercised based on the needs and applications. A simple procedure, for large scale handling of shoot tips is described in detail. Electron microscopic studies revealed that meristems isolated from 7-day-old seedlings are superior because of possessing greater number of transformation competent cells.

Analysis of Variance↗

PubMed2006

High frequency early in vitro flowering of Dendrobium Madame Thong-In (Orchidaceae).

We have successfully developed a method to induce early in vitro flowering of the self-pollinated seedlings of a tropical orchid hybrid, Dendrobium Madame Thong-In. Transition of vegetative shoot apical meristem to inflorescence meristem was observed when young protocorms were cultured in modified KC liquid medium. In contrast, protocorms cultured on Gelrite-solidified medium only produced axillary shoots and roots. CW was required to trigger the transitional shoot apical meristem and BA enhanced inflorescence stalk initiation and flower bud formation. However, normal flower development was deformed in liquid medium but developed fully upon transferring to two-layered (liquid over Gelrite-solidified) medium. Under optimal condition, in vitro flowering was observed about 5 months after seed sowing. Segregation of flower colours was observed in these seedlings and seedpods formed upon artificial pollination of the in vitro flowers.

Benzyl Compounds↗

PubMed2001

Immunohistochemical localisation of ubiquitin and the proteasome in sunflower (Helianthus annuus cv. Giganteus).

This study provides an immunohistochemical demonstration of the involvement of the ubiquitin- and proteasome-dependent pathway during differentiation and organogenesis in plants. The localisation of ubiquitin and the proteasome was studied in meristems, leaves, stems and roots of sunflower (Helianthus annuus L. cv. Giganteus). By using a new technique that enhances very low antigen signals, we obtained information on the structural distribution of the ubiquitin- and proteasome-dependent pathway, and of the importance of this pathway during organogenesis and plant development. Ubiquitin and the proteasome showed overall similarities in their cellular localisation. The highest antigenic signal was observed in the root and shoot apical meristems, in leaf primordia and vascular tissue. The cambium showed less expression than the apical meristems. During adventitious root formation in cuttings, no sign of increased expression was observed within dedifferentiating tissue, but as organogenesis progressed, the antigenic signal of ubiquitin and the proteasome gradually increased in the developing roots. Comparison of immunochemical results and Western blots demonstrated that important changes in the cellular antigen signal could only be detected by immunochemistry.

Cell Differentiation↗

PubMed1998

Cell fate and cell differentiation status in the Arabidopsis root.

Post-embryonic development in plants is mainly achieved by its meristems. Within the Arabidopsis root meristem, both the fate and origin of its cells can be predicted with high accuracy. Mutants defective in the determination of root cell fates show that the corresponding genes are first required during embryogenesis. The sub-specification of cell fates, such as during epidermal root hair formation, involves transcription factors and phytohormones. In the Arabidopsis root, initial cell fate specification events must be followed by position-dependent reinforcement of cellular identity. A major question remains as to whether the signals that are involved in initiating the cellular pattern are the same or different from the signals used to reinforce it. The integrity of the root meristem is kept by balancing cell proliferation and cell differentiation, and differentiation-inhibiting signals originating from the quiescent centre are involved.

Arabidopsis↗

PubMed2004

Histological analysis in shoot organogenesis from hypocotyl explants of Kandelia candel (Rhizophoraceae).

In vitro culture of hypocotyl explants from Kandelia candel, a common mangrove species, on hormone-free Murashige and Skoog (MS) medium resulted in shoot formation. Since the hypocotyls showed good potential for in vitro shoot multiplication, the process of bud primordium formation was analyzed from a histological viewpoint. A wound periderm first appeared at the top, exposed cut surface of the explants. The wound-induced meristem continued to divide giving rise to suberized cells oriented towards the cut surface. After formation of the suberized cell layers, the meristem and its inner derivatives differentiated into multilayered, uniformly packed parenchyma cells. Bud primordia differentiated from the dense cytoplasmic cells of the wound-induced meristem just beneath the suberized layer near the severed vascular bundles. Each explant produced several visible shoot buds. Furthermore, histological sections revealed that additional bud primordia were present within the explant just underneath the suberized cells and that these bud primordia appeared to be arrested in their development. The fact that additional bud primordia were present within the explant suggests that further manipulation of the explant is helpful to maximize the potential of this system.

Hypocotyl↗

PubMed2006

Developmental morphology of branching flowers in Nymphaea prolifera.

Nymphaea and Nuphar (Nymphaeaceae) share an extra-axillary mode of floral inception in the shoot apical meristem (SAM). Some leaf sites along the ontogenetic spiral are occupied by floral primordia lacking a subtending bract. This pattern of flower initiation in leaf sites is repeated inside branching flowers of Nymphaea prolifera (Central and South America). Instead of fertile flowers this species usually produces sterile tuberiferous flowers that act as vegetative propagules. N. prolifera changes the meristem identity from reproductive to vegetative or vice versa repeatedly. Each branching flower first produces some perianth-like leaves, then it switches back to the vegetative meristem identity of the SAM with the formation of foliage leaves and another set of branching flowers. This process is repeated up to three times giving rise to more than 100 vegetative propagules. The developmental morphology of the branching flowers of N. prolifera is described using both microtome sections and scanning electron microscopy.

Flowers↗

PubMed2005

Pattern formation in the monocot embryo as revealed by NAM and CUC3 orthologues from Zea mays L.

All aerial parts of a higher plant originate from the shoot apical meristem (SAM), which is initiated during embryogenesis as a part of the basic body plan. In contrast to dicot species, the SAM in Zea mays is not established at an apico-central, but at a lateral position of the transition stage embryo. Genetic and molecular studies in dicots have revealed that members of the NAC gene family of plant-specific transcription factors such as NO APICAL MERISTEM (NAM) from Petunia or the CUP-SHAPED COTYLEDON (CUC) genes from Arabidopsis contribute essential functions to the establishment of the SAM and cotyledon separation. As an approach to the understanding of meristem formation in a monocot species, members of the maize NAC family highly related to the NAM/CUC genes were isolated and characterized. Our phylogenetic analysis indicates that two distinct NAM and CUC3 precursors already existed prior to the separation of mono- and dicot species. The allocation of the two maize paralogues, ZmNAM1 and ZmNAM2 together with PhNAM, AtCUC2 and AmCUP in one sub-branch and the corresponding expression patterns support their contribution to SAM establishment. In contrast, the ZmCUC3 orthologue is associated with boundary specification at the SAM periphery, where it visualizes which fraction of cells in the SAM is committed to a new leaf primordium. Other maize NAC gene family members are clearly positioned outside of this NAM/CUC3 branch and also exhibit highly cell type-specific expression patterns.

Amino Acid Sequence↗

PubMed2006

Transcription factors and hormones: new insights into plant cell differentiation.

Plant development is a continuous process, mainly due to the presence of stem cell niches within the root and shoot. The interplay between a host of transcription factors determines whether the cells within the meristem maintain their stem cell state, differentiate into leaves or form secondary meristems, which develop into shoots and flowers. Several recent studies provide new insight into how transcription factors and phytohormones interact within meristems to control cell proliferation and differentiation.

Cell Differentiation↗

PubMed2005

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.

Cell Differentiation↗

PubMed2005

Shoot and inflorescence branching.

A major aspect of postembryonic plant development is the formation of secondary axes of growth: vegetative branches, inflorescence branches, or flowers. The first step in side-shoot development is the establishment of lateral meristems in the axils of leaves. GRAS-, MYB-, and bHLH-type transcription factors act as key regulators of early steps in this process. The REVOLUTA subfamily of HD-ZIP transcription factors controls the organization of lateral meristems. Whereas the development of lateral meristems into lateral buds is only poorly understood, recent studies have provided new insights into the regulation of lateral bud outgrowth. The MORE AXILLARY GROWTH (MAX) genes of Arabidopsis and the RAMOSUS (RMS) genes of pea are involved in the production, perception, and transduction of a signal that inhibits lateral bud outgrowth. Synthesis of this not-yet-identified hormone is positively regulated by the main shoot tip through auxin signalling.

Gene Expression Regulation, Developmental↗