Mitotic activity in the maize root apex after freeze-decapping

Mitosis and nuclear DNA synthesis have been examined in root apices of maize whose caps were removed by a freezing technique. These processes are not impaired by this technique even though cells at the surface of the decapped apex experience a temperature close to 0°±1.5° C for a brief period. We conclude that freeze-decapping is without significant deleterious effects to the apex and therefore the technique is a useful adjunct in studies of the role of the cap on root growth.     

Developmental anatomy of the three-dimensional structure of the vegetative shoot apex

There have been many studies of the vegetative shoot apex, but the majority of such studies have been based on median longitudinal sections. Anatomical studies based on surface views of the apex are relatively few in number. Unfortunately, three-dimensional structures, based on surface and transverse-sectional views as well as longitudinal views, have scarcely been analyzed. However, Hara attempted such analyses (1961, 1962, 1971a, b, c, 1977). He identified radial files of cells in the peripheral meristem and recognized four or five apical sectors in the three-dimensional structure of the shoot apex. The size of the central zone and the sizes and relative positions of the sectors change rhythmically in each plastochron. Further studies on the three-dimensional structure of the shoot apex are needed if we are fully to understand the morphogenesis and histogenesis of vascular plants.     

Developmental control of cell division patterns in the shoot apex

The shoot apical meristem is a group of rapidly dividing cells that generate all aerial parts of the plant. It is a highly organised structure, which can be divided into functionally distinct domains, characterised by specific proliferation rates of the individual cells. Genetic studies have enabled the identification of regulators of meristem function. These factors are involved in the formation and maintenance of the meristem, as well as in the formation of the primordia. Somehow, they must also govern cell proliferation rates within the shoot apex. Possible links between meristem regulators and the cell cycle machinery will be discussed. In order to analyse the role of cell proliferation in development, cell cycle gene expression has been perturbed using transgenic approaches and mutation. The effect of these alterations on growth and development at the shoot apex will be presented. Together, these studies give a first insight into the regulatory networks controlling the cell cycle and into the significance of cell proliferation in plant development.     

A florigenic effect of sucrose in Fuchsia hybrida is blocked by gibberellin-induced assimilate competition

The use of gas chromatography-mass spectrometry-selected ion monitoring along with a (13)C internal standard has allowed sensitive measurements of the sucrose (Suc) content of individual shoot apices of Fuchsia hybrida. With intact plants, as the photosynthetic irradiance increased, so did shoot apex Suc content, reaching saturation at about 500 micromol m(-2) s(-1). These same plants flowered at the higher irradiances, remaining vegetative in 10-h short days at an irradiance of 230 micromol m(-2) s(-1). The strong correlation (r = 0.93) in these studies between flowering and shoot apex Suc content indicates a role for Suc as a stimulus to flowering in this species. However, Suc is not the long-day (LD) "florigen" of F. hybrida because 2 to 4 LD given as a 14-h low-irradiance photoperiod extension (10-15 micromol m(-2) s(-1)) induced flowering but without increase in shoot apex Suc content. Flowering induced by either pathway, the LD- or the Suc-mediated one, was inhibited by applying gibberellin (GA) to the shoot tip. Such inhibition of flowering by GA, at least for the LD pathway, was associated with a reduced apex Suc content, enhanced elongation of subapical stem tissue, and a reduced import into the shoot apex of leaf-sourced assimilate. Thus, our findings show how GA inhibits flowering of F. hybrida and confirm the importance of nutrient diversion in regulating flowering.     

The local cytomechanical properties of growing pollen tubes correspond to the axial distribution of structural cellular elements

Morphological studies of pollen tubes have shown that the configuration of structural cellular elements differs between the growing apex and the distal part of the cell. This polarized cellular organization reflects the highly anisotropic growth behavior of this tip growing cell. Accordingly, it has frequently been postulated that physical properties of pollen tubes such as cell wall plasticity should show anisotropic distribution, but no experimental evidence for this has been published hitherto. Using micro-indentation techniques, we quantify pollen tube resistance to lateral deformation forces and analyze its visco-elasticity as a function of distance from the growing apex. Our studies reveal that cellular stiffness is significantly higher at the distal portion of the cell. This part of the cell is also completely elastic, whereas the apex shows a visco-elastic component upon deformation. To relate these data to the architecture of the particular pollen tube investigated in this study, Papaver rhoeas, we analyzed the distribution of cell wall components such as pectin, callose, and cellulose as well as the actin cytoskeleton in this cell using fluorescence label. Our data revealed that, in particular, the degree of pectin methyl esterification and the configuration of the actin cytoskeleton correlate well with the distribution of the physical properties on the longitudinal axis of the cell. This suggests a role for these cellular components in the determination of the cytomechanics of pollen tubes.     

植物成花素的研究进展

许多植物由营养生长向生殖生长的转换都是由日照长度控制的,而植物叶片可感知日长信号并诱导成花素的合成。成花素从韧皮部运输到茎顶端,使顶端分生组织基因表达发生变化进而成花。其中,FT作为成花素的主要组分,在该转换过程中处于核心地位。本文综合近年的研究,介绍成花素及其作用机理。     

Not worth the risk: apex predators suppress herbivory on coral reefs

Apex predators are known to exert strong ecological effects, either through direct or indirect predator–prey interactions. Indirect interactions have the potential to influence ecological communities more than direct interactions as the effects are propagated throughout the population as opposed to only one individual. Indirect effects of apex predators are well documented in terrestrial environments, however there is a paucity of information for marine environments. Furthermore, manipulative studies, as opposed to correlative observations, isolating apex predator effects are lacking. Coral reefs are one of the most diverse ecosystems, providing a useful model system for investigating the ecological role of apex predators and their influence on lower trophic levels. Using predator models and transplanted macroalgae we examined the indirect effects of predators on herbivore foraging behaviour. We show that the presence of a model reef shark or large coral‐grouper led to a substantial reduction in bite rate and species richness of herbivorous fishes and an almost absolute localized cessation of macroagal removal, due to the perceived risk of predation. A smaller‐sized coral‐grouper also reduced herbivore diversity and activity but to a lesser degree than the larger model predators. These indirect effects of apex predators on the foraging behaviour of herbivores may have flow‐on effects on the biomass and distribution of macroalgae, and the functioning of coral reef ecosystems. This highlights that the ecological interactions and processes that contribute to ecosystem resilience may be more complex than previously assumed.     

Positioning of nuclei in Arabidopsis root hairs: an actin-regulated process of tip growth

In growing Arabidopsis root hairs, the nucleus locates at a fixed distance from the apex, migrates to a random position during growth arrest, and moves from branch to branch in a mutant with branched hairs. Consistently, an artificial increase of the distance between the nucleus and the apex, achieved by entrapment of the nucleus in a laser beam, stops cell growth. Drug studies show that microtubules are not involved in the positioning of the nucleus but that subapical fine F-actin between the nucleus and the hair apex is required to maintain the nuclear position with respect to the growing apex. Injection of an antibody against plant villin, an actin filament-bundling protein, leads to actin filament unbundling and movement of the nucleus closer to the apex. Thus, the bundled actin at the tip side of the nucleus prevents the nucleus from approaching the apex. In addition, we show that the basipetal movement of the nucleus at root hair growth arrest requires protein synthesis and a functional actin cytoskeleton in the root hair tube.     

GAMYB-like Genes, Flowering, and Gibberellin Signaling in Arabidopsis

We have identified three Arabidopsis genes with GAMYB-like activity, AtMYB33, AtMYB65, and AtMYB101, which can substitute for barley (Hordeum vulgare) GAMYB in transactivating the barley alpha-amylase promoter. We have investigated the relationships between gibberellins (GAs), these GAMYB-like genes, and petiole elongation and flowering of Arabidopsis. Within 1 to 2 d of transferring plants from short- to long-day photoperiods, growth rate and erectness of petioles increased, and there were morphological changes at the shoot apex associated with the transition to flowering. These responses were accompanied by accumulation of GAs in the petioles (GA(1) by 11-fold and GA(4) by 3-fold), and an increase in expression of AtMYB33 at the shoot apex. Inhibition of GA biosynthesis using paclobutrazol blocked the petiole elongation induced by long days. Causality was suggested by the finding that, with GA treatment, plants flowered in short days, AtMYB33 expression increased at the shoot apex, and the petioles elongated and grew erect. That AtMYB33 may mediate a GA signaling role in flowering was supported by its ability to bind to a specific 8-bp sequence in the promoter of the floral meristem-identity gene, LEAFY, this same sequence being important in the GA response of the LEAFY promoter. One or more of these AtMYB genes may also play a role in the root tip during germination and, later, in stem tissue. These findings extend our earlier studies of GA signaling in the Gramineae to include a dicot species, Arabidopsis, and indicate that GAMYB-like genes may mediate GA signaling in growth and flowering responses.     

Mechanical role of the leaf sheath in rattans

Leaf sheaths of rattans are long, tubular and persistent and unlike many self-supporting palms, extend far from the apex of the plant. The mechanical role of the leaf sheath was investigated in eight rattan species of the subfamily Calamoideae. The main objective was to analyse its influence on the mechanical architecture and contribution to the climbing habit. Bending mechanical properties were measured along climbing axes before and after removal of leaf sheaths. Results were related to stem and leaf sheath geometry and mechanical properties. Contribution of the leaf sheath to axial flexural rigidity was high (c. 90%) in the early stages of growth and towards the apex of older climbing axes for all climbing palms tested. Senescence and loss of the leaf sheath strongly influenced axial stiffness. A nonclimbing species, Calamus erectus, showed a different mechanical architecture. Although lacking secondary growth, palms have been able to develop successful climbers with a mechanical architecture broadly analogous to, although developmentally different from, dicotyledonous lianas. The role of the leaf sheath in modulating mechanical properties during ontogeny ought not to be neglected in studies on monocotyledons, as it possibly contributed significantly to the ways in which different growth forms have evolved in the group.     

Evidence that actin reinforces the extensible hyphal apex of the oomyceteSaprolegnia ferax

Summary Filamentous actin in the apices of growing hyphae of the oomyceteSaprolegnia ferax is distributed such that it could compensate for weakness in the expanding apical cell wall and thus play a role in morphogenesis of the tip. The tapered extensible portion of the hyphal tip where the cell wall is plastic contains a cap of actin which differs in organization from the actin in subapical, inextensible regions of the hypha. Rapidly growing hyphae which are expected to have a longer plastic cell wall region contain longer actin caps. Furthermore, the weakest point in the hyphal apex, demonstrated by osmotic shock-induced bursting, was within the taper where the wall is plastic but never in the extreme apex where actin was most densely packed and presumably the strongest. Treatment of hyphae with cytochalasin E/dimethyl sulphoxide induced rapid changes in actin caps. Cap disruption was accompanied by transient growth rate increases, subsequent rounding and swelling of apices and a shift of osmotically induced burst points closer to the apex. These correlated changes are consistent with a role for the actin cap in tip morphogenesis. The association between regions of plasticity in the apical cell wall, the extent of the actin cap, the location of the weakest point in the apex and the effects of damage to the actin cap suggest that the cap functions to support the apex in regions where the cell wall is weak.Abbrevations CE cytochalasin E - DMSO dimethyl sulphoxide - RP rhodamine phalloidin Dedicated to the memory of Professor Oswald Kiermayer     

Meristems: start your signaling

All of the cells of the shoot apex are derived from a small number of stem cells in the center of the shoot meristem. Hence, cell lineage plays no role in establishing pattern formation or cell fate at the shoot apex. Growing evidence has implicated a number of cell signaling pathways in regulating shoot meristem development and organ formation. These signaling pathways include receptor-mediated signaling, protein movement via plasmodesmata, and potential feedback loops.     

Shoot Apex Demand Determines Assimilate and Nutrients Partitioning and Nutrient-uptake Rate in Tobacco Plants

Our previous experiment revealed that apex-removed plants have larger root systems but a lower K＋-uptake rates than intact tobacco plants. Since the apex is not only a center of growth and metabolism, but also an important place of auxin synthesis and export, the aims of this study were to distinguish whether the apex demand or auxin synthesized in the apex regulates assimilate and nutrients partitioning within plant, and to explain the reason for the lower K＋-uptake rate of the apex-removed plant. In comparison with the control plant, covering the shoot apex with a black transparent plastic bag reduced net increases in dry matter and nutrients; however, the distribution of the dry matter and nutrients between shoot and roots and nutrient-uptake rates were not changed. Removal of the shoot apex shifted the dry mass and nutrients distributions to roots, and reduced the rate of nutrient uptake. Application of 1-naphthylacetic acid （NAA） could partly replace the role of the removed apex, stimulated assimilate and nutrient deposition into the treated tissue, and enhanced the reduced plasma membrane ATPase activity of roots to the control level. However, treatment of the apex-removed plants with NAA could not rescue the reduced nutrient uptake rate and the shifted assimilates and nutrients partitioning caused by excision of the apex. Higher nutrient uptake rate of the intact plants could not be explained by root growth parameters, such as total root surface area and number of root tips. The results from the present study indicate that strong apex demand determined assimilates and nutrients partitioning and nutrient-uptake rate in tobacco （Nicotiana tabacum） plants.     

Strigolactones,a novel class of plant hormone controlling shoot branching

For several decades, auxin and cytokinin were the only two hormones known to be involved in the control of shoot branching through apical dominance, a process where the shoot apex producing auxin inhibits the outgrowth of axillary buds located below. Grafting studies with high branching mutants and cloning of the mutated genes demonstrated the existence of a novel long distance carotenoid derived signal which acted as a branching inhibitor. Recently, this branching inhibitor has been shown to belong to the strigolactones, a group of small molecules already known to be produced by roots, exuded in the rhizosphere and as having a role in both parasitic and symbiotic interactions.     

太白红杉顶芽动态及其对分枝格局的影响

应用计盒维数方法比较研究了与太白红杉顶芽动态相关的不同分枝格局对空间占据能力的差异.结果表明,Ⅰ型分枝有利于对所拥有的空间进行有效填充和利用,Ⅱ型分枝有利于扩展枝条所占的面积,Ⅲ型分枝既扩展了面积,又实现了对所拥有空间的有效填充和利用.总的来讲,顶芽的死亡使分枝对空间的占据能力和对光能利用的效率有所提高,同时对植株冠型的调整也有重要作用.太白红杉枝条顶芽的死亡是外界严酷环境条件的被迫"产物".     

稳定同位素技术在鲨鱼摄食和洄游行为研究中的应用

随着稳定同位素分析技术的不断成熟,其在生态学领域中的应用也增长迅速,并成为动物摄食生态学的重要研究工具.鲨鱼因其在生物系统进化过程中的独特地位和海洋生态系统中的重要作用已成为海洋食物网研究的重点,然而国内针对鲨鱼摄食习性和洄游行为等方面的研究仍处于起步阶段.本文在总结了国内外鲨鱼稳定同位素分析组织样品选取和样品预处理方法的基础上,系统归纳了稳定同位素技术在鲨鱼摄食生态学,尤其在其摄食和洄游行为研究领域中的应用,着重分析稳定同位素技术在鲨鱼稳定同位素判别值和更新速率、食性分析、营养级评估、洄游路径分析和生态位分布等核心问题上的应用现状和发展前景,以期为国内学者开展鲨鱼类基础生物、生态学研究提供有益参考.     

HISTOCHEMICAL AND AUTORADIOGRAPHIC STUDIES OF FLORAL INDUCTION IN CHENOPODIUM ALBUM

Gifford , Ernest M., Jr. , and Herbert B. Tepper . (U. California, Davis.) Histochemical and autoradiographic studies of floral induction in Chenopodium album. Amer. Jour. Bot. 49(7): 706–714. Illus. 1962.—Chenopodium album was induced to flower using short-day photoperiods. Changes in the chondriome, starch, total protein, ribonucleic acid (RNA), deoxyribonucleic acid (DNA), and histone distribution in cells of vegetative and inflorescence shoot apices were studied. The distal cells of the vegetative apex (especially the axial tunica cells) possess larger nucleoli and vacuoles, less granular mitochondria, and more differentiated plastids than do other cells of the apex; the distal cells stain lightly with dyes that indicate the presence of DNA and histone. RNA is distributed relatively uniformly in the shoot apex; the cells at sites of leaf initiation and young leaf primordia contain slightly higher concentrations of RNA than the axial cells of the shoot apex. Protein is uniformly distributed throughout the vegetative as well as the inflorescence apex. Upon induction, the chemical and morphological differences between cells in the shoot apex gradually disappear. RNA concentration of cells in the apex increases, reaching a maximum after 4 inductive cycles. Protein concentration of cells also increases, but this increase lags behind that of RNA.     

Intracellular calcium handling heterogeneities in intact guinea pig hearts

Regional heterogeneities of ventricular repolarizing currents and their role in arrhythmogenesis have received much attention; however, relatively little is known regarding heterogeneities of intracellular calcium handling. Because repolarization properties and contractile function are heterogeneous from base to apex of the intact heart, we hypothesize that calcium handling is also heterogeneous from base to apex. To test this hypothesis, we developed a novel ratiometric optical mapping system capable of measuring calcium fluorescence of indo-1 at two separate wavelengths from 256 sites simultaneously. With the use of intact Langendorff-perfused guinea pig hearts, ratiometric calcium transients were recorded under normal conditions and during administration of known inotropic agents. Ratiometric calcium transients were insensitive to changes in excitation light intensity and fluorescence over time. Under control conditions, calcium transient amplitude near the apex was significantly larger (60%, P < 0.01) compared with the base. In contrast, calcium transient duration was significantly longer (7.5%, P < 0.03) near the base compared with the apex. During isoproterenol (0.05 microM) and verapamil (2.5 microM) administration, ratiometric calcium transients accurately reflected changes in contractile function, and, the direction of base-to-apex heterogeneities remained unchanged compared with control. Ratiometric optical mapping techniques can be used to accurately quantify heterogeneities of calcium handling in the intact heart. Significant heterogeneities of calcium release and sequestration exist from base to apex of the intact heart. These heterogeneities are consistent with base-to-apex heterogeneities of contraction observed in the intact heart and may play a role in arrhythmogenesis under abnormal conditions.