Anomalous somatic embryos in Acca sellowiana (O. Berg) Burret (Myrtaceae)

Somatic embryogenesis represents a valuable tool for the studies on the basic aspects of plant embryo development. Today this process is used as a potencial technique for large-scale plant micropropagation although, so far, it has been applied to only a small number of species. However, when somatic embryos are malformed they are considered economically useless. In Acca sellowiana (O. Berg) Burret, an important fruit-producing crop, large amounts of anomalous somatic embryos (76.3%) were found just after 40 days of culture of explants in a 2,4-D containing medium. Among the anomalous forms found in the cotiledonary stage, 12.2% consisted of fused embryos, 40.4% displayed fused cotyledons, 13.0% presented supernumerary cotyledons, and 10.7% showed absence or poorly developed cotyledons, including those without the shoot apical meristem. Histological analyses indicated that the altered embryos were formed either directly from cotyledons, hypocotyl and radicle of the zygotic embryos used as explants, or indirectly from calli formed from these tissue parts. It is suggested that the formation of anomalous somatic embryos, as well as a low frequency of conversion into emblings reflect physiological and/or genetic disturbances triggered by the presence of 2,4-D in the medium. In vitro experimental alternative approaches are discussed in order to lessen the occurrence of malformed somatic embryos.

Anatomy, ultrastructure and secretion of Hibiscus pernambucensis Arruda (Malvaceae) extrafloral nectary

This paper reports on the extrafloral nectary (EFN) of Hibiscus pernambucensis, a native shrub species occurring in mangrove and restinga along Brazil's coastline. EFNs occur as furrows with a protuberant border on the abaxial surface veins of the leaf blade. Each nectary consists of numerous secretory multicellular trichomes, epidermal cells in palisade-like arrangements and non-vascularized parenchyma tissue. Nectar secretion is prolonged, since secretion starts in very young leaves and remains up to completely expanded leaves. Reduced sugars, lipids, and proteins were histochemically detected in all the nectary cells; phenolic substances were detected in the vacuoles of the epidermal palisade cells and in some secretory trichome cells. The secretory cells that constitute the body of trichomes have large nuclei, dense cytoplasm with numerous mitochondria, dictyosomes, scattered lipid droplets and plastids with different inclusions: protein, lipid droplets or starch grains; vacuoles with different sizes have membranous material, phenolic and lipophilic substances. The palisade cells show thick periclinal walls, reduced cytoplasm with voluminous lipid drops and developed vacuoles. The nectary parenchyma cells contain abundant plasmodesmata and cytoplasm with scattered lipid droplets, mitochondria, plastids with starch grains and endoplasmic reticulum. Mucilage idioblasts are common in the inner nectary parenchyma. Protoderm and ground meristem participate in the formation of EFN. Our data indicate that all nectary regions are involved in nectar production and secretion, constituting a functional unit. Longevity of the extrafloral nectaries is likely associated with the presence of mucilage idioblasts, which increases the capacity of the nectary parenchyma to store water.

Origin of successive cambia on stem in three species of Menispermaceae

The lianas observed in this study, Abuta convexa (Vell.) Diels, Abuta imene (Mart.) Eichler, and Chondrodendron platiphyllum (A. St.-Hil.) Miers, all have successive cambia in their stems. The terminology applied to stem histology in species with successive cambia is as diverse as the interpretations of the origins of this cambial variant. Therefore, this study specifically investigates the origin of successive cambia through a developmental analysis of the above-mentioned species, including an analysis of the terminology used to describe this cambial variation. For the first time, we have identified several developmental stages giving rise to the origins of successive cambia in this family. First, the pericycle originates in 1-3 layers of conjunctive tissue. After the differentiation of the first ring, the conjunctive tissue undergoes new divisions, developing approximately 10 rows of parenchyma cells. In the middle portion, a layer of sclereids is formed, again subdividing the conjunctive tissue into two parts: internal and external. New cambia originate in the internal part, from which new secondary vascular strands will originate, giving rise to the second successive vascular ring of the stem. The external part remains parenchymatous during the installation of the second ring and will undergo new periclinal division, repeating the entire process. New cambia will originate from the neoformed strands, which will form only rays. In the literature, successive cambia are formed by a meristem called "diffuse lateral meristem."However, based on the species of Menispermaceae studied in this report, it is demonstrated that the diffuse lateral meristem is the pericycle itself.

Cytotoxic and genotoxic potential of powdered juices

Abstract Powdered juices are widely consumed by the population especially because of their convenient preparation, availability in various fruit flavors and low cost when compared to other industrialized beverages. They have complex formulation, consisting of several classes of food additives. However, there are no scientific studies on the toxicity of these foods. Thus, this study evaluated the toxicity at the cellular level of industrialized powdered juices of orange and guava flavors of three different food companies. This analysis was made using root meristem cells of Allium cepa L., at the exposure times of 24 and 48 hours, and two concentrations, 30 g/1000 mL, considered ideal for consumption according to the label of the products, and 30 g/500 mL. Both flavors of juices, of the three companies, in both concentrations and the two exposure times promoted significant antiproliferative effect to root meristem cells and caused a statistically significant number of mitotic spindle changes and micronuclei in cells of the test system used. Therefore, under the studied conditions, all the samples of juice powder exhibited cytotoxic and genotoxic potential.

Phases of dormancy in yam tubers (Dioscorea rotundata)

center dot Background and Aims The control of dormancy in yam (Disocorea spp.) tubers is poorly understood and attempts to shorten the long dormant period (i.e. cause tubers to sprout or germinate much earlier) have been unsuccessful. The aim of this study was to identify and define the phases of dormancy in Dioscorea rotundata tubers, and to produce a framework within which dormancy can be more effectively studied. center dot Methods Plants of 'TDr 131' derived from tissue culture were grown in a glasshouse simulating temperature and photoperiod at Ibadan (7 degrees N), Nigeria to produce tubers. Tubers were sampled on four occasions: 30 d before shoot senescence (149 days after planting, DAP), at shoot senescence (179 DAP), and twice during storage at a constant 25 degrees C (269 and 326 DAP). The development of the apical shoot bud was described from tissue sections. In addition, the responsiveness of shoot apical bud development to plant growth regulators (gibberellic acid, 2-chloroethanol and thiourea) applied to excised tuber sections was also examined 6 and 12 d after treatment. center dot Key Results and Conclusions Three phases of tuber dormancy are proposed: Phase I, from tuber initiation to the appearance of the tuber germinating meristem; Phase II, from the tuber germinating meristem to initiation of foliar primordium; and Phase III, from foliar primordium to appearance of the shoot bud on the surface of the tuber. Phase I is the longest phase (approx. 220 d in 'TDr 131'), is not affected by PGRs and is proposed to be an endo-dormant phase. Phases II and III are shorter (< 70 d in total), are influenced by PGRs and environmental conditions, and are therefore endo-/eco-dormant phases. To manipulate dormancy to allow off-season planting and more than one generation per year requires that the duration of Phase I is shortened.

Water deficits promote flowering in Rhododendron via regulation of pre and post initiation development

Flowering is generally considered to be advanced by water deficits in many woody perennial species. A long-standing paradigm being that as a plant senses severe environmental conditions resources are diverted away from vegetative growth and towards reproduction before death. It is demonstrated that in Rhododendron flowering is promoted under water deficit treatments. However, the promotion of flowering is not achieved via all increase in floral initiation, but through separate developmental responses. If regulated deficit irrigation (RDI) is imposed prior to the time of initiation, fewer vegetative nodes are formed before the apical meristems switch to floral initiation, and chronologically, floral initiation occurs earlier. Both RDI and partial rootzone drying (PRD) treatments stimulate the development of more flowers Oil each inflorescence if the treatments are continued after the plant has undergone floral initiation. However, floral initiation is inhibited by soil water deficits. If the soil water deficit continues beyond the stages of floral development then anthesis call occur prematurely oil the fully formed floral buds without a need for a winter chilling treatment. It is hypothesised that inhibition of floral initiation in plants experiencing severe soil water deficits results from the inhibitory action Of ABA transportation to the apical meristem from stressed roots. It is demonstrated that ABA applications to well-watered Rhododendron inhibit floral initiation. (c) 2008 Elsevier B.V. All rights reserved.

A qualitative host-pathogen interaction in the Theobroma cacao-Moniliophthora perniciosa pathosystem

The aim of this study was to test whether resistance of clones of Theobroma cacao ( cocoa) varied between isolates of Moniliophthora (formerly Crinipellis) perniciosa, the cause of witches' broom disease. Developing buds of vegetatively propagated T. cacao grown in greenhouses in the UK were inoculated with 16 000 spores of M. perniciosa per meristem in water, under conditions where water condensed on the inoculated shoot for at least 12 h after inoculation. The proportion of successful inoculations varied between clones and was inversely correlated with time to symptom production or broom formation. A specific interaction was demonstrated among three single-spore isolates of M. perniciosa and the clone Scavina 6 (SCA 6) and a variety of susceptible clones. Isolates Castenhal-I and APC3 were equally likely to infect SCA 6 and the other clones, but isolate Gran Couva A9 never infected SCA 6, although it was as virulent on the other clones. The interaction was maintained when the wetness period was extended to 70 h. Offspring of SCA 6 x Amelonado matings were all susceptible to both Castenhal-I and GC-A5, with no evidence of greater variability in susceptibility to GC-A5 than Castanhal-I. This suggests recessive inheritance of a single homozygous factor conferring resistance to GC-A5, from SCA 6. The progenies were slightly more susceptible to Castanhal-I than GC-A5. The implications for managing the disease are discussed.

Phases of dormancy in yam tubers (Dioscorea rotundata)

Background and Aims The control of dormancy in yam (Disocorea spp.) tubers is poorly understood and attempts to shorten the long dormant period (i.e. cause tubers to sprout or germinate much earlier) have been unsuccessful. The aim of this study was to identify and define the phases of dormancy in Dioscorea rotundata tubers, and to produce a framework within which dormancy can be more effectively studied. center dot Methods Plants of 'TDr 131' derived from tissue culture were grown in a glasshouse simulating temperature and photoperiod at Ibadan (7 degrees N), Nigeria to produce tubers. Tubers were sampled on four occasions: 30 d before shoot senescence (149 days after planting, DAP), at shoot senescence (179 DAP), and twice during storage at a constant 25 degrees C (269 and 326 DAP). The development of the apical shoot bud was described from tissue sections. In addition, the responsiveness of shoot apical bud development to plant growth regulators (gibberellic acid, 2-chloroethanol and thiourea) applied to excised tuber sections was also examined 6 and 12 d after treatment. center dot Key Results and Conclusions Three phases of tuber dormancy are proposed: Phase I, from tuber initiation to the appearance of the tuber germinating meristem; Phase II, from the tuber germinating meristem to initiation of foliar primordium; and Phase III, from foliar primordium to appearance of the shoot bud on the surface of the tuber. Phase I is the longest phase (approx. 220 d in 'TDr 131'), is not affected by PGRs and is proposed to be an endo-dormant phase. Phases II and III are shorter (< 70 d in total), are influenced by PGRs and environmental conditions, and are therefore endo-/eco-dormant phases. To manipulate dormancy to allow off-season planting and more than one generation per year requires that the duration of Phase I is shortened.

Leafy, terminal flower1 and agamous are functionally conserved but do not regulate terminal flowering and floral determinacy in Impatiens balsamina

In Impatiens balsamina a lack of commitment of the meristem during floral development leads to the continuous requirement for a leaf-derived floral signal. In the absence of this signal the meristem reverts to leaf production. Current models for Arabidopsis state that LEAFY (LFY) is central to the integration of floral signals and regulates flowering partly via interactions with TERMINAL FLOWER1 (TFL1) and AGAMOUS (AG). Here we describe Impatiens homologues of LFY, TFL1 and AG (IbLFY, IbTFL1 and IbAG) that are highly conserved at a sequence level and demonstrate homologous functions when expressed ectopically in transgenic Arabidopsis. We relate the expression patterns of IbTFL1 and IbAG to the control of terminal flowering and floral determinacy in Impatiens. IbTFL1 is involved in controlling the phase of the axillary meristems and is expressed in axillary shoots and axillary meristems which produce inflorescences, but not in axillary flowers. It is not involved in maintaining the terminal meristem in either an inflorescence or indeterminate state. Terminal flowering in Impatiens appears therefore to be controlled by a pathway that uses a different integration system than that regulating the development of axillary flowers and branches. The pattern of ovule production in Impatiens requires the meristem to be maintained after the production of carpels. Consistent with this morphological feature IbAG appears to specify stamen and carpel identity, but is not sufficient to specify meristem determinacy in Impatiens.

Mechanisms and function of flower and inflorescence reversion

Flower and inflorescence reversion involve a switch from floral development back to vegetative development, thus rendering flowering a phase in an ongoing growth pattern rather than a terminal act of the meristem. Although it can be considered an unusual event, reversion raises questions about the nature and function of flowering. It is linked to environmental conditions and is most often a response to conditions opposite to those that induce flowering. Research on molecular genetic mechanisms underlying plant development over the last 15 years has pinpointed some of the key genes involved in the transition to flowering and flower development. Such investigations have also uncovered mutations which reduce floral maintenance or alter the balance between vegetative and floral features of the plant. How this information contributes to an understanding of floral reversion is assessed here. One issue that arises is whether floral commitment (defined as the ability to continue flowering when inductive conditions no longer exist) is a developmental switch affecting the whole plant or is a mechanism which assigns autonomy to individual meristems. A related question is whether floral or vegetative development is the underlying default pathway of the plant. This review begins by considering how studies of flowering in Arabidopsis thaliana have aided understanding of mechanisms of floral maintenance. Arabidopsis has not been found to revert to leaf production in any of the conditions or genetic backgrounds analysed to date. A clear-cut reversion to leaf production has, however, been described in Impatiens balsamina. It is proposed that a single gene controls whether Impatiens reverts or can maintain flowering when inductive conditions are removed, and it is inferred that this gene functions to control the synthesis or transport of a leaf-generated signal. But it is also argued that the susceptibility of Impatiens to reversion is a consequence of the meristem-based mechanisms controlling development of the flower in this species. Thus, in Impatiens, a leaf-derived signal is critical for completion of flowering and can be considered to be the basis of a plant-wide floral commitment that is achieved without accompanying meristem autonomy. The evidence, derived from in vitro and other studies, that similar mechanisms operate in other species is assessed. It is concluded that most species (including Arabidopsis) are less prone to reversion because signals from the leaf are less ephemeral, and the pathways driving flower development have a high level of redundancy that generates meristem autonomy even when leaf-derived signals are weak. This gives stability to the flowering process, even where its initiation is dependent on environmental cues. On this interpretation, Impatiens reversion appears as an anomaly resulting from an unusual combination of leaf signalling and meristem regulation. Nevertheless, it is shown that the ability to revert can serve a function in the life history strategy (perenniality) or reproductive habit (pseudovivipary) of many plants. In these instances reversion has been assimilated into regular plant development and plays a crucial role there.

Root growth models: towards a new generation of continuous approaches

Models of root system growth emerged in the early 1970s, and were based on mathematical representations of root length distribution in soil. The last decade has seen the development of more complex architectural models and the use of computer-intensive approaches to study developmental and environmental processes in greater detail. There is a pressing need for predictive technologies that can integrate root system knowledge, scaling from molecular to ensembles of plants. This paper makes the case for more widespread use of simpler models of root systems based on continuous descriptions of their structure. A new theoretical framework is presented that describes the dynamics of root density distributions as a function of individual root developmental parameters such as rates of lateral root initiation, elongation, mortality, and gravitropsm. The simulations resulting from such equations can be performed most efficiently in discretized domains that deform as a result of growth, and that can be used to model the growth of many interacting root systems. The modelling principles described help to bridge the gap between continuum and architectural approaches, and enhance our understanding of the spatial development of root systems. Our simulations suggest that root systems develop in travelling wave patterns of meristems, revealing order in otherwise spatially complex and heterogeneous systems. Such knowledge should assist physiologists and geneticists to appreciate how meristem dynamics contribute to the pattern of growth and functioning of root systems in the field.

Decimal growth stages for precision wheat production in changing environments?

The utility of the decimal growth stage (DGS) scoring system for cereals is reviewed. The DGS is the most widely used scale in academic and commercial applications because of its comprehensive coverage of cereal developmental stages, the ease of use and definition provided and adoption by official agencies. The DGS has demonstrable and established value in helping to optimise the timing of agronomic inputs, particularly with regard to plant growth regulators, herbicides, fungicides and soluble nitrogen fertilisers. In addition, the DGS is used to help parameterise crop models, and also in understanding the response and adaptation of crops to the environment. The value of the DGS for increasing precision relies on it indicating, to some degree, the various stages in the development of the stem apex and spike. Coincidence of specific growth stage scores with the transition of the apical meristem from a vegetative to a reproductive state, and also with the period of meiosis, is unreliable. Nonetheless, in pot experiments it is shown that the broad period of booting (DGS 41–49) appears adequate for covering the duration when the vulnerability of meiosis to drought and heat stress is exposed. Similarly, the duration of anthesis (61–69) is particularly susceptible to abiotic stresses: initially from a fertility perspective, but increasingly from a mean grain weight perspective as flowering progresses to DGS 69 and then milk development. These associations with DGS can have value at the crop level of organisation: for interpreting environmental effects, and in crop modelling. However, genetic, biochemical and physiological analysis to develop greater understanding of stress acclimation during the vegetative state, and tolerance at meiosis, does require more precision than DGS can provide. Similarly, individual floret analysis is needed to further understand the genetic basis of stress tolerance during anthesis.

Morphological architecture of Actinocephalus (Koern.) sano (Eriocaulaceae-Poales)

Actinocephalus exhibits perhaps more diversity in habit than any other genus of Eriocaulaceae. This variation is largely a result of differences in the arrangement of the paraclades. Based on the analysis of stem architecture of all 25 species of Actinocephalus, the following patterns were established: (1) leaf rosette, with no elongated axis, instead the axillary paraclades originating directly from the short aerial stem, (2) rosette axis continuing into an elongated axis with spirally arranged paraclades, (3) an elongated axis originating from a rhizome, with ramified paraclades, and (4) an elongated axis originating from a short aerial stem, with paraclades arranged in a subwhorl. The elongated axis exhibits indeterminate growth only in pattern 4. Patterns 3 and 4 are found exclusively in Actinocephalus; pattern I occurs in many other genera of Eriocaulaceae, while pattern 2 is also found in Syngonanthus and Paepalanthus. Anatomically, each stem structure (i.e., paraclade, elongated axis, short aerial stem, rhizome) is thickened in a distinctive way and this can be used to distinguish them. Specifically, elongated axes and paraclades lack thickening, thickening of short aerial stems results from the primary thickening meristem and/or the secondary thickening meristem. Thickening of rhizomes results from the activity of the primary thickening meristem. (c) 2008 Elsevier GmbH. All rights reserved.

Axillary bud development in pineapple nodal segments correlates with changes on cell cycle gene expression, hormone level, and sucrose and glutamate contents

During the process of lateral organ development after plant decapitation, cell division and differentiation occur in a balanced manner initiated by specific signaling, which triggers the reentrance into the cell cycle. Here, we investigated short-term variations in the content of some endogenous signals, such as auxin, cytokinins (Cks), and other mitogenic stimuli (sucrose and glutamate), which are likely correlated with the cell cycle reactivation in the axillary bud primordium of pineapple nodal segments. Transcript levels of cell cycle-associated genes, CycD2;1, and histone H2A were analyzed. Nodal segments containing the quiescent axillary meristem cells were cultivated in vitro during 24 h after the apex removal and de-rooting. From the moment of stem apex and root removal, decapitated nodal segment (DNS) explants showed a lower indol-3-acetic acid (IAA) concentration than control explants, and soon after, an increase of endogenous sucrose and iP-type Cks were detected. The decrease of IAA may be the primary signal for cell cycle control early in G1 phase, leading to the upregulation of CycD2;1 gene in the first h. Later, the iP-type Cks and sucrose could have triggered the progression to S-phase since there was an increase in H2A expression at the eighth h. DNS explants revealed substantial increase in Z-type Cks and glutamate from the 12th h, suggesting that these mitogens could also operate in promoting pineapple cell cycle progression. We emphasize that the use of non-synchronized tissue rather than synchronous cell suspension culture makes it more difficult to interpret the results of a dynamic cell division process. However, pineapple nodal segments cultivated in vitro may serve as an interesting model to shed light on apical dominance release and the reentrance of quiescent axillary meristem cells into the cell cycle.

Dark-induced hormone changes coincide with the resumption of light-inhibited shoot growth in Catasetum fimbriatum (Orchidaceae)

Catasetum fimbriatum plants cultivated in the absence of light exhibit continuous shoot growth leading to the formation of nodes and internodes. On the other hand, when these plants are incubated in the presence of light, shoot longitudinal growth is inhibited and pseudobulbs develop just below the shoot apical meristem. These facts provide evidence of a possible influence of light on mitotic cell division in the shoot apex as well as on pseudobulb initiation. The effects of light and dark on the interruption and/or maintenance of shoot apex mitotic activity and the subsequent formation of pseudobulbs in the sub-meristematic regions were investigated by means of histological and hormonal studies. The interruption of shoot apex development occurred around the 150th d of light incubation and seems to have resulted from the establishment of a strong storage sink in the region of the future pseudobulb, in detriment to the continuous activity of the shoot apical meristem. The reduced total cytokinins/IAA ratio in the apex, mainly due to high levels of IAA, could be a key factor in the interruption of cell divisions. Transfer to the dark brings about the resumption of shoot apex development of plants through the re-entrance of cells in the cell cycle which coincides with a significant increase in the total cytokinins/IAA ratio. (C) 2009 Elsevier GmbH. All rights reserved.