Apical Development Research Articles

Shoot apical development during in vitro embryogenesisThis review is one of a selection of papers published on the Special Theme of Shoot Apical Meristems.

Formation of the shoot apical meristem (SAM) has been extensively investigated in zygotic embryos of flowering plants, where it follows a prolonged and dynamic developmental pattern underlined by precise temporal and spatial changes in gene expression. Studies conducted on the plant model system Arabidopsis have revealed that SAM formation is controlled by a genetic network and involves the participation of several regulatory genes expressed at different stages of development. As a general rule apical meristem development in vivo occurs very early; at the globular stage of development in flowering plants and in club-stage embryos of conifers. Once formed, meristems of zygotic embryos are stable structures that become reactivated at the onset of germination. Shoot apical meristem formation during in vitro embryogenesis is demarked by structural events similar to those described for zygotic embryos, although differences can be observed during the late phases of development, where cellular differentiation and formation of intercellular spaces disrupt the architecture of SAMs produced in culture. These events, which denote the “unstable” nature of SAMs of somatic embryos, often result in poor conversion frequency and reduced plant regeneration. By using Picea glauca (Moench) Voss (white spruce) somatic embryos and microspore-derived embryos of Brassica napus L. (canola) as model systems, this review provides methods for improving SAM formation through manipulations of the culture medium which alter the cellular redox status. Meristem marker genes from Arabidopsis, such as WUSCHEL (which is required for the acquisition of stem fate identity), represent a valuable tool for estimating the quality of SAM produced by microspore-derived embryos of canola. In spruce, the identification of two novel meristem marker genes, HBK1 and PgAGO, will allow similar studies in conifers.

Yield Formation in Mediterranean durum wheats under two contrasting water regimes based on path-coefficient analysis

The components of grain yield are altered by adverse growing conditions as the effects of certain environmental factors on crop growth and yield differ depending upon the developmental stages when these conditions occur. Path-coefficient analysis was used to investigate the main processes influencing grain yield and its formation under Mediterranean conditions. Twenty-five durum wheat genotypes, consisting of four Spanish commercial varieties and 21 inbred lines from the ICARDA durum wheat breeding program, were grown during 1997 and 1998 under both rainfed and irrigated conditions in southern Spain. {P}ath-coefficient analysis revealed that under favourable conditions grain yield depended in equal proportion on the three primary yield components (i.e. spikes m−2, grains spike−1, and mean grain weight), whereas in the rainfed experiments, variations in grain yield were due mainly to spikes m−2 and to a lesser extent to grains spike−1. Compensatory effects were almost absent under irrigated treatments; however, under water shortage, spikes m−2 exerted a negative influence on grain spike−1 due mainly to a negative interrelationship between tiller production and apical development. These compensatory effects could partially explain the restricted success in durum wheat breeding observed in water-limited environments of the Mediterranean region. Under rainfed conditions the number of spikes m−2 depended mainly on the ability for tiller production, whereas in the irrigated experiments the final number of spikes was determined mostly by tiller survival.

Relationships among vernalization, shoot apex development and frost tolerance in wheat.

Frost tolerance of wheat depends primarily upon a strong vernalization requirement, delaying the transition to the reproductive phase. The aim of the present study was to learn how saturation of the vernalization requirement and apical development stage are related to frost tolerance in wheat. 'Mironovskaya 808', a winter variety with a long vernalization requirement, and 'Leguan', a spring variety without a vernalization requirement, were acclimated at 2 degrees C at different stages of development. Plant development (morphological stage of the shoot apex), vernalization requirement (days to heading) and frost tolerance (survival of the plants exposed to freezing conditions) were evaluated. 'Mironovskaya 808' increased its frost tolerance more rapidly; it reached a higher level of tolerance and after a longer duration of acclimation at 2 degrees C than was found in 'Leguan'. The frost tolerance of 'Mironovskaya 808' decreased and its ability to re-acclimate a high tolerance was lost after saturation of its vernalization requirement, but before its shoot apex had reached the double-ridge stage. The frost tolerance of 'Leguan' decreased after the plants had reached the floret initiation stage. The results support the hypothesis that genes for vernalization requirement act as a master switch regulating the duration of low temperature induced frost tolerance. In winter wheat, due to a longer vegetative phase, frost tolerance is maintained for a longer time and at a higher level than in spring wheat. After the saturation of vernalization requirement, winter wheat (as in spring wheat) established only a low level of frost tolerance.

Morphology and mapping analysis of rice (Oryza sativa L.) clustered spikelets (Cl) mutant

The rice clustered spikelets (Cl) mutant exhibits a phenotype that most of branch apical have 2–3 spikelets clustered together. SEM (scanning electron microscope) observation suggested that the Cl gene controlled branch apical development, and influenced the terminal spikelets elongation. The spikelet number was reduced in mutant, indicating that Cl may also have an effect on spikelet number. To map Cl locus, two F2 mapping populations derived from the crosses between the Cl and ZhongHua11, and Cl and ZheFu802 were constructed, respectively. The Cl locus was roughly mapped between two CAPS markers, CK0214 and SS0324. A further fine mapping analysis showed that the Cl locus was mapped between makers R0674E and Cl2560, with genetic distances of 0.2 and 2.1 cM, respectively. Then we found a PAC contig spanning Cl locus, the region was delimited to 196 kb. This result was useful for cloning of the Cl gene. Allelism test demonstrated that Cl was allelic to Cl2, another rice clustered spikelets mutant.

Factors affecting resorption in traumatically intruded permanent incisors in children.

There is a lack of consensus concerning the management of intruded permanent teeth. The objectives of the present study were to determine the prevalence of resorption for intruded permanent teeth and to establish the effect of factors on the timing, prevalence and rate of resorption and to examine the relationship between the timing of onset and the subsequent rate of resorption. Sixty-one intruded permanent incisors treated at the Paediatric Dentistry units in Belfast and Newcastle upon Tyne, during the period 1990-99 with a minimum follow-up period of 1.5 years, were studied. The timing and presence of resorption and its rate of occurrence over time were set as the principal outcomes. There was a significantly earlier onset and higher prevalence of resorption in more severely intruded teeth (P< 0.05). There was also a significant relationship between the degree of apical development and resorption with an increased prevalence in the more fully developed roots (P< 0.001). Resorption was detected significantly earlier in teeth with higher rates of resorption (P< 0.05). However, the treatment method did not significantly affect the prevalence or rate of resorption. In conclusion, the occurrence of root resorption after intrusive trauma appears to be related to the severity of the original injury and the stage of root development rather than the repositioning procedure.

Parâmetros da habilidade competitiva no estabelecimento de populações caracterizam o potencial de produção individual em trigo e aveia

Em monoculturas, normalmente se observa uma variacao de producao entre plantas, que pode estar ocorrendo desde o estabelecimento da populacao, pela habilidade competitiva destas. Este trabalho foi desenvolvido com o objetivo de selecionar parâmetros de habilidade competitiva, durante o estabelecimento, que se relacionam com o potencial de producao, no final do desenvolvimento. Plantas de trigo (EMBRAPA 16) e aveia (UFRGS 15) foram cultivadas em canteiros e a campo, correlacionando-se parâmetros morfologicos, nos estadios de afilhamento e florescimento. No afilhamento, nossos resultados revelaram que o diâmetro do pseudocolmo e o desenvolvimento foliar foram os parâmetros mais relevantes na caracterizacao da habilidade competitiva, pela alta associacao com a massa seca, independente da especie e do ambiente. Nestas populacoes, a ordem com que as plantas emergiram foi inversamente relacionada com o estadio de desenvolvimento apical (para inflorescencia) e com os parâmetros que definiram a habilidade competitiva. Esta habilidade de competicao inicial foi diretamente relacionada com o tamanho e a potencialidade de producao (espiguetas por inflorescencia) das plantas, no estadio de florescimento. Os resultados indicam que o potencial de producao individual pode ser estimado por medidas de parâmetros nao destrutivos, da capacidade de competicao, nos primeiros estadios de desenvolvimento das plantas.

Dynamic characteristics of floret primordium development in wheat

This study was undertaken to determine the time-course of floret primordium initiation and degeneration in relation to thermal time after sowing in various cultivars and tiller positions of wheat ( Triticum aestivum L.). The field experiment was carried out on the experiment station of Nanjing Agricultural University of China, and included three sowing dates: early planting of 30 September, middle planting of 30 October and late planting of 2 March and two wheat cultivars: winter type ‘Jing 411’ and spring type ‘Yangmai 158’. Quadratic and linear relationships could describe floret initiation, degeneration and abortion in response to growing-degree-days after sowing for main stem and first and second tillers of middle planting plants of both cultivars. The floret development patterns on the third tillers for all treatments deviated from the basic models and were associated with poor floret fertility. The growing-degree-days from sowing to floret initiation were significantly different between the three sowing dates and between the two cultivars. These results help quantify the dynamic processes of apical development and explain the differences in floret development of wheat with different environments, cultivars and tiller positions.

Apical development in oats predicted by leaf stage

Phenological development of the shoot apex in three oat cultivars was monitored and related to the appearance and growth of leaves. Because oat leaves appear and expand differently from wheat leaves, a new continuous decimal leaf scale was developed for this purpose. There were usually two visible laminae expanding at the same time, until the appearance of the flag leaf. Thus, leaf tip number n + 2 became visible after leaf number n had reached its maximum length. The leaf stages corresponding to the formation of the flag leaf primordium (FP), double ridge (DR) and the formation of the terminal spikelet (TS) were strongly and linearly related to the final number of leaves, even though the leaf stage at any of these events varied substantially with sowing date. However, the number of leaves still to emerge after the specified apical events was nearly constant, regardless of sowing dates. The study showed that apical events can be predicted from observations of the leaf stage, provided that the final leaf number can be predicted.

Production of stable transgenic sunflowers (Helianthus annuus L.) from wounded immature embryos by particle bombardment and co-cultivation with Agrobacterium tumefaciens

Split embryonic axes of 21-day old immature sunflower (Helianthus annuus L.) embryos were bombarded by microparticles and then co-cultured with disarmed Agrobacterium tumefaciens strain EHA105 bearing a binary vector carrying nptII and uidA genes. Apical shoot bud development and organogenesis induced on the explants led T0 transgenic plants. Southern blot analysis revealed complex integration patterns in T0 plants. The uidA gene segregated as a dominant trait and single-insertion events were observed in T1 plants. Patterns similar to those of T1 plants were observed in T2 progeny.

Non-vital immature permanent incisors: factors that may influence treatment outcome.

This study examines the treatment of non-vital immature permanent incisors using the calcium hydroxide apexification technique. The objectives of the present study were to determine the speed and location of barrier formation and those factors discernible at presentation and during treatment which may be related to it. Forty-four non-vital immature incisors undergoing calcium hydroxide apexification were reviewed in detail. All cases were reviewed every 8-12 weeks for up to 18 months, or until apexification occurred. Details of the time and nature of the injuries and treatment were recorded. The degree of apical development prior to treatment was assessed, and barrier formation, location and time were noted. Mean time to barrier formation was 34.2 weeks (range 13-67 weeks). The strongest predictor of rapid barrier formation was the rate of change of calcium hydroxide and a barrier also formed more rapidly in cases with narrower initial apical width. There was evidence of displacement and a higher mean time for barrier formation in half of the cases. The presence of an abscess was the weakest predictor of rapid barrier formation and the effect was not significant (P = 0.280). The barrier was located at the apex in 28 cases (63.6%) and the distance from the barrier to the apex for the remaining 16 (36.4%) varied from 1 mm to 5 mm. The number of placements of calcium hydroxide varied from 1 to 4 with a mean of 1.9, and there was a higher mean number of calcium hydroxide placements in the cases where the barrier was located at the apex.

Free‐Air CO2 Enrichment Effects on Apex Dimensional Growth of Spring Wheat

Although primordium initiation in wheat (Triticum aestivum L.) has been extensively researched, a complete description of the growth dynamics of the apex at elevated CO2 concentrations is lacking. This study determined the rates of main stem and tiller apical elongation and widening in plants grown under two levels of CO2 concentration. Spring wheat was grown at the University of Arizona's Maricopa Agricultural Center at elevated (550 μmol mol−1) or ambient (370 μmol mol−1) CO2 concentrations. Individual plant samples were collected at different developmental stages and dissected. After dissection, the lengths and widths of the apices of the main stem (MS), coleoptile tiller (TO), primary tillers (T1, T2, and T3), and secondary tillers (T00, T01, T02, T10, Tll, and T12) were measured with a stage micrometer. Apex dimensions were fitted to an exponential model. Elevated CO2 increased the apex lengths of T2 at the double ridge stage, and of T3 and T10 at the double ridge and the terminal spikelet stages, and the apex widths of T2 at double ridge stage, and of T2, T3, T10, and Tll at the flag leaf appearance stage. Combining these results with a parallel study, the longer apices did not have more spikelet primordia, but wider apices had more floret primordia. Elevated CO2 changed apex elongation or widening patterns within a plant by enhancing elongation or widening rates of the MS, and laterformed tillers. Earlier‐formed tillers were less responsive to elevated CO2 levels. This information will be used in modeling wheat apical development and grain production in the elevated atmospheric CO2 environments of the future.

Effect of salinity on salt accumulation and reproductive development in the apical meristem of wheat and barley

Two cultivars of wheat and two of barley differing in salt tolerance were grown at 0, 100 and 175 mМ NaCl, and the development of the apex was followed as it turned from vegetative to reproductive. Apices were taken for ion analysis when most of the spikelet primordia had been produced and the process of differentiation into floral organs was starting. K + concentrations in apices were unaffected by the salinity treatment, Na + concentrations were generally low in all treatments, below 50 mМ, while Cl − levels were very low, below 30 mМ. These levels of Na + and Cl − are too low to affect metabolism. In spite of these negligible effects on ion levels in the apex, salinity affected reproductive development: fewer spikelet primordia formed, and final spikelet numbers at ear emergence were reduced. Salinity greatly advanced the time of floral initiation in the wheat cultivars. This indicates that modification of apical development by salinity was not determined by salt in the apex itself, but mediated by signals or substrates derived from elsewhere in the plant.

Directional cell-to-cell communication in the Arabidopsis root apical meristem I. An ultrastructural and functional analysis

As a foundation for studies on directional intercellular communication and its regulation in apical development, the network of plasmodesmata inArabidopsis root apical meristems was characterized by quantitative electron microscopy and dye-coupling analysis, using symplasmic probes, and real-time imaging in confocal laser scanning microscopy. A tissue-specific plasmodesmatal network, which interconnected the cells in the root apical meristem, was characterized by the following features, (a) Plasmodesmatal distribution and density were found to be tissue-specific, (b) Primary and secondary plasmodesmata were differentially grouped and regulated. Primary plasmodesmata were formed in large numbers in the transverse walls of each tissue, and were subject to deletion during cell differentiation. Secondary plasmodesmata were mostly distributed in longitudinal walls between cell files and common walls between neighboring tissues; they also provided a symplasmic path between different initial tiers in the meristem. Small fluorescent tracers moved through the plasmodesmatal network of the root apical meristem in two distinct phases. At low concentrations molecules trafficked in a non-tissue-specific manner, whereas at higher concentrations, their distribution reflected the presence of tissue-specific movement consistent with plasmodesmatal distribution. These findings are discussed in terms of the role of tissue-specific plasmodesmatal domains in the control of root development.

Effect of moisture stress at late vegetable growth stage on apical development of rice and varietal differences

Effect of moisture stress at late vegetable growth stage on apical development of rice and varietal differences

Free‐Air CO2 Enrichment Effects on Rate and Duration of Apical Development of Spring Wheat

Rates and durations of individual phases of wheat (Triticum aestivum L.) apical development are among the most important factors that determine yield components. Because atmospheric CO2 has been increasing steadily, it is important to evaluate the effects of elevated CO2 on wheat development. This study was conducted to determine rates and durations of leaf, spikelet, and floret primordium initiation in a Free‐Air Carbon Dioxide Enrichment (FACE) system. Spring wheat (cv. Yecora Roja) was planted at the University of Arizona Maricopa Agricultural Center. The two CO2 concentrations were 550 (elevated) and 370 (ambient) μmol mol−1 CO2. Individual plant samples were collected every 3 to 4 d. We dissected the main stem (MS), coleoptile tiller (T0), primary tillers (T1, T2, and T3) and secondary tillers (T00, T01, T02, T10, Tll, and T12) and counted primordia. Apex primordium data were fitted to a four‐piece linear‐spline segmented regression model with the SAS proc NLIN. No influence of elevated CO2 (550 μmol−1 on leaf primordium initiation of MS was detected. Nevertheless, CO2 enrichment significantly increased rates of spikelet primordium initiation of MS, T1, T2, T10, and Tll, and diminished the durations of spikelet development phase of MS, TI, T2, T3, T10, and T11. Within the floret phase, CO2 enrichment significantly increased rates of floret primordium initiation of MS, TO, T1, T2, and T3, and diminished the time to the completion of floret primordium initiation of MS, T0, T1, T3, and T11. The information from this study will be utilized to predict wheat apical development and grain production in the elevated atmospheric CO2 environments of the future.

Effects of Photoperiod, Temperature and Asynchrony between Thermoperiod and Photoperiod on Development to Panicle Initiation in Sorghum

Abstract The duration of the vegetative phase (i.e. days from sowing to panicle initiation) in sorghum [Sorghum bicolor(L.) Moench] is affected by photoperiod and temperature. Plants of several contrasting genotypes of sorghum were grown in controlled-environment growth cabinets with either synchronous or asynchronous photoperiods and thermoperiods. Apical development was recorded. Diurnal asynchrony between photoperiod and thermoperiod reduced durations to panicle initiation when the temperature warmed after lights went on and cooled after lights went off, but increased these durations when the temperature warmed before lights went on and cooled before lights went off. These effects were shown in the maturity lines 60M and SM100 and also in the USA cv. RS610 and the Sudanese landrace IS22365, but their magnitude varied with genotype, photothermal regime, and the degree of asynchrony. The greatest effect was detected in IS22365 grown at 30/21 °C (12 h/12 h) with a 12 h d−1photoperiod when the temperature warmed 2.5 h before lights went on and cooled 2.5 h before lights went off, when the duration from sowing to panicle initiation was 69 d compared with 37 d in the control (synchronous photoperiod and thermoperiod in each diurnal cycle). Reciprocal transfers of plants of IS22365 between short and long days revealed that asynchrony principally affected the duration of the photoperiod-insensitive pre-inductive phase of development; i.e. asynchrony affected the time (age) at which the plants were first able to respond to photoperiod. In that investigation in controlled-environment growth chambers, the subsequent photoperiod-sensitive inductive phase continued until panicle initiation. Subsequent reciprocal transfer experiments in controlled-environment glasshouses in four different alternating temperature regimes employed synchronous photoperiods and thermoperiods in short (11 h) days with temperature warming 1.5 h after the beginning of the day in long (12.5 h) days. In those investigations, photoperiod sensitivity ended some time before (2.5–8.1 d, mean 5.7 d) panicle initiation in IS22365, Naga White and Seredo. Moreover, whereas the duration of the photoperiod-insensitive pre-inductive phase was affected by temperature, the durations of the photoperiod-sensitive inductive and the photoperiod-insensitive post-inductive phases were not.

Darkness promotes flowering in the absolute long-day requiring plant,Lolium temulentumL. Ceres

Vegetative plants of the long-day grass Lolium temulentum L. Ceres were exposed to threshold long days or light breaks. Protracted darkness given just afterwards clearly promoted flowering and was weakly inductive on its own. The promotive effect of darkness was restricted to floral induction since further apical development was weak.

Leaf carbohydrate status in Lolium temulentum during the induction of flowering.

Unifoliated plants of Lolium temulentum L. ev. Ceres, a qualitative long-day grass, were induced to flower by one 24-h long day (LD) or by one 8-h short day (SD) advanced by 1 2 h in the normal regime, so-called 'displaced short day' (DSD). Standard light for SD and DSD was a mixture of fluorescence and incandescence at 400 μmol m-2 s-1 whereas the extension period of the 24-h LD was solely incandescence at 10-15 μmol m-2 s-1 . The DSD system was first characterized by the timings of floral induction, stimulus translocation and apical development. Carbohydrates in the blade tissues and in leaf exudate were analysed comparatively in vegetative and induced plants. Fructans were not detected in the leaf tissues whereas sucrose and starch were found to be present in similar amounts. In SD, their contents exhibited a diurnal fluctuation and were not in large excess. The common change observed during the two inductive treatments was that starch remained at a high level during the LD extension, even though the lighting was unsuitable for photosynthesis, and increased transiently in DSD. Sucrose was the major sugar contained in the leaf exudate. Its content increased when flowering was induced, but not at the same time in the two systems. In LD, sucrose exudation rose when plants were returned to standard light after the inductive cycle, i.e. after the LD stimulus had left the leaf blade. By contrast, during the DSD, sucrose was transported at the same time as the floral stimulus. Results are discussed together with the methods used to time stimulus translocation and their implications.

Effects of Basic FGF and TGF??1 on F-Actin and ZO-1 Organization During Cat Endothelial Wound Healing

Previous studies suggest the existence of two separate and distinct mechanisms of endothelial wound healing (i.e., cell migration and cell spreading), which may be controlled by unique, injury-dependent, wound-related factors. The purpose of our study was to evaluate potential biologic mediators regulating healing of the growth arrested cat endothelium by using an ex vivo, organ-culture model. Three buttons were punched from each cornea of 11 cats with a 6-mm trephine. A 1- to 2-mm diameter endothelial scrape injury (SI) was made, and buttons were cultured in (a) serum-free media (SFM), (b) serum plus media (20% fetal calf serum), (c) SFM plus basic fibroblast growth factor (bFGF), (d) SFM plus bFGF and heparin, (e) SFM plus transforming growth factor-beta 1 (TGF beta 1), or (f) SFM plus TGF beta 1 and anti-TGF beta 1. At various times from 8-48 h after injury, buttons were stained with phalloidin and anti-ZO-1, and imaged by using laser scanning confocal microscopy. Evaluation of SI in cat corneal buttons under serum-free conditions showed maintenance of normal endothelial differentiation, indicating that the organ-culture SI model mimics in vivo SI. Addition of TGF beta 1 produced a dramatic reorganization of apical F-actin and development of stress fibers, as well as the loss of normal cell border-associated ZO-1 distribution. The effects of TGF beta 1 were blocked by the neutralizing antibodies to TGF beta 1. Addition of serum or bFGF produced much less pronounced changes in F-actin and ZO-1 distribution. These results suggest that TGF beta 1 may play a critical role in modulating the wound-healing response of the corneal endothelium.

Changes in lipid composition during floral development of Brassica campestris

Apical tissues of Brassica campestris, grown under controlled environmental conditions, were analysed for their lipid content. The principal lipids were sterols, phospholipids and sphingolipids. The major sterols were identified as sitosterol, stigmasterol, campesterol and cholesterol, the phospholipids as phosphatidylethanolamine (PE) and phosphatidylcholine (PC), and the sphingolipids as cerebrosides. In the early stages of apical development, unusually high proportions of cholesterol and cerebrosides were found. However, their relative proportions gradually decreased as the apex developed; a concomitant increase in sitosterol was observed. These results suggested a specific association between these lipids and the development of the shoot apex. PE increased steadily during apical development, whereas PC increased more rapidly, but then declined at the later stage. The relative proportion of campesterol increased in the apex during the late stages of development and appeared to be involved in petal formation, which coincided with the decrease in PC.