Involvement of auxin and cytokinins in initiation of growth of isolated pea buds

Axillary buds from the second primary scale excised from 21-day-old pea(Pisum sativum L. cv. Vladan) plants were used as a modelsystem for studying the release of buds from apical dominance. The isolatedbudswere transferred onto basal medium with or without a supplement of growthregulators and cultivated up to 24 h in short-term and up to 4weeks in long-term experiments. In both sets of experiments endogenous IAA,cytokinins and the uptake of labelled zeatin were analysed. The development ofbuds was monitored by image analysis, estimation of their weight, and byanatomical studies. Generative meristems were found in isolated axillary budsalready in 21-day-old plants at the beginning of the experimental period. Theonset of bud growth was recorded as soon as 2 h after the budexcision by image analysis. The bud growth was accompanied by a rapid transientincrease of the endogenous IAA level within the first 2 h,followedby an increase of iPA within 24 h. The uptake of the exogenouscytokinin ([³H]Z) reached its peak between the 6 and 8hafter the release from apical dominance. The cytokinin analyses of bothshort-term and long-term bud cultures revealed the increase of free cytokininsand their glucosides, indicating de novo synthesis ofcytokinins in the buds themselves.     

Differential response of trees and shrubs to browsing and pruning: the effects on Pistacia growth and gall-inducing aphids

The effects of long term browsing on growth and morphology of P. atlantica trees and P. palaestina shrubs, and on the colonization of these hosts by their respective gall-inducing aphid guilds, were studied in natural vegetation pastures in Israel. To simulate apical dominance release by browsing, trees and shrubs were pruned and observed one year later. P. atlantica responded strongly to browsing, producing a crown of dense and sharp dry branches and thorn-like shoots, and pruning, compensating or overcompensating for the lost biomass in the studied morphological variables. Morphological responses of P. palaestina were less evident: we found no compensation in most variables. Three species of aphids produced more galls on browsed than on control shoots of P. atlantica. Two species induced more galls on pruned shoots. The five aphid species on P. palaestina colonized browsed and unbrowsed shoots equally, and produced fewer galls on pruned shoots. The role of apical dominance in architectural responses of trees and shrubs to browsing and pruning, as related to resource availability and timing, is discussed. This revised version was published online in June 2006 with corrections to the Cover Date.     

北美香柏雌球果的发育

用扫描电镜(SEM)观察了北美香柏 Thuja occidentalis 雌球果的发育过程。在北京,北美香柏的雌球果是在八月初由营养芽转变而来,雌球果一般有4~6对苞片,中间2~3对可育,每一苞片腋部着生两枚胚珠,在可育苞片腋部最先观察到一扁平的隆起,并在其上分化出两个胚珠原基,接着分化出珠被和珠心,最后形成扁平而两侧对称的胚珠。在北美香柏雌球果发育过程中,约一半的雌球果在2~3对可育苞片中位于下面的1~2对的腋部产生3个胚珠原基,中间一个较小,并在以后的发育中逐渐退化。由此推测北美香柏的雌球果可能是由祖先类群中每一苞片具多于2个胚珠的雌球果演化而来。在光镜下对雌球果维管系统的观察发现,传粉前幼小雌球果的苞片内仅有一束维管束,传粉后随着苞片基部的居间生长,有4—8束维管束在苞片内形成,但是新发育的维管束木质部和韧皮部相对位置与正常叶性器官一致,这与在以往报道的柏科植物成熟雌球果的苞片中均有反向维管束的发育不同。北美香柏雌球果早期发育和维管束分析结果支持傅德志和杨亲二提出的解释裸子植物生殖器官形态演化的“苞鳞-种鳞复合体”理论。关键词北美香柏;雌球果的发育;胚珠分化;SEM     

Role of diastolic vortices in flow and energy dynamics during systolic ejection

MRI-based computational fluid dynamics simulations were performed in the left ventricles of two adult porcine subjects with varying physiological states (before and after an induced infarction). The hypothesis that diastolic vortices store kinetic energy and assist systolic ejection was tested, by performing systolic simulations in the presence and absence of diastolic vortices. The latter was achieved by reinitializing the entire velocity field to be zero at the beginning of systole. A rudimentary prescribed motion model of a mitral valve was included in the simulations to direct the incoming mitral jet towards the apex. Results showed that the presence or absence of diastolic vortex rings had insignificant impact on the energy expended by walls of the left ventricles for systolic ejection for both the porcine subjects, under all physiological conditions. Although substantial kinetic energy was stored in diastolic vortices by end diastole, it provided no appreciable savings during systolic ejection, and most likely continued to complete dissipation during systole. The role of diastolic vortices in apical washout was investigated by studying the cumulative mass fraction of passive dye that was ejected during systole in the presence and absence of vortices. Results indicated that the diastolic vortices play a crucial role in ensuring efficient washout of apical blood during systolic ejection.     

Modeling the formation of root apices

The formation of new root apices from small groups of cells with different cellular patterns has been simulated using an existing model based on growth tensors. To generate an apex, a steady growth field was used. The pattern of cells evolved to approach the steady state. Two extreme types of progressions have been obtained : one leading to an apex with a single or a few apical cells, and the other to an apex with a quiescent centre. The change of structure while applying a steady growth tensor indicates that development may involve a succession of discrete growth tensors.     

Photopigments underlying color vision in ringtail lemurs (Lemur catta) and brown lemurs (Eulemur fulvus)

A recent examination of color vision in the ringtail lemur produced evidence that these prosimians could make color discriminations consistent with a diagnosis of trichromatic color vision. However, it was unclear if this behavior reflected the presence of three classes of cone or whether lemurs might be able to utilize signals from rods in conjunction with those from only two classes of cone. To resolve that issue, spectral sensitivity functions were obtained from ringtail lemurs (Lemur catta) and brown lemurs (Eulemur fulvus) using a noninvasive electrophysiological procedure, electroretinographic flicker photometry. Results from experiments involving chromatic adaptation indicate that these lemurs routinely have only a single class of cone photopigment in the middle to long wavelengths (peak sensitivity of about 545 nm); they also have a short-wavelengthsensitive cone pigment with peak of about 437 nm. The earlier behavioral results are suggested to have resulted from the ability of lemurs to jointly utilize signals from rods and cones. The cone pigment complements of these lemurs differ distinctly from those seen among the anthropoids. © 1993 Wiley-Liss, Inc.     

Observation on the incorporation cone in the rat

At the time of in vivo sperm–egg fusion in the rat, a small region of the oolemma under the head of the fertilizing sperm is observed to be free of microvilli. The microvilli-free region increases in area, and by one hour after sperm–egg contact extends over an area 20–30 μ in circumference and bulges out to form an “incorporation cone” visible by light microscopy. The microvilli-free incorporation cone reaches its maximum size at about two hours after sperm–egg interaction. It soon becomes smaller and has disappeared three to four hours after sperm–oocyte fusion. The cone cytoplasm is characterized by a 0.1 μ zone of thin filaments below the plasma membrane. Cytochalasin-B, 2.5 μg/ml, prevents formation of the cone or destroys the intact cone. It is suggested that micro filaments may be involved in the formation of the incorporation cone.     

Intercellular communication inAzolla roots: II. Electrical coupling

Summary There is a predictable and well defined variation in numbers of plasmodesmata in roots ofAzolla. As the apical cell of the root ages, it lays down walls with progressively fewer plasmodesmata, thereby gradually cutting itself off from the rest of the root (Gunning 1978). Electrical coupling was examined between the apical cell and an adjacent merophyte in roots of various lengths. The apical cell becomes increasingly electrically isolated from the rest of the root as it ages. Electrical coupling is strongly correlated with the number of the plasmodesmata between the coupled cells. The resistance of a plasmodesma, as estimated from equivalent electrical circuits, was 150–600 times more resistive than a value based on theoretical considerations. No evidence was found for a change in the physiology of plasmodesmata as the root ages. Coupling experiments, both on root hairs and at the apex, gave some suggestion that plasmodesmata may be less resistive towards the apical cell than away from it.     

New World Monkeys and Color

The visual worlds of most primates are rich with potential color signals, and many representatives of the order have evolved the biological mechanisms that allow them to exploit these sources of information. Unlike the catarrhines, platyrrhines typically have sex-linked polymorphic color vision that provides individuals with any of several distinct types of color vision, including both trichromatic and dichromatic variants. In recent years, this polymorphism has been the target of an expanding range of research efforts. As a result, researchers now reasonably understand the proximate biology underlying the polymorphisms, and a number of ideas have emerged as to their evolution. Progress has also been made in illuminating how color vision capacities may be related to the particular visual tasks that New World monkeys face.     

AFM-based Mapping of the Elastic Properties of Cell Walls: at Tissue,Cellular, and Subcellular Resolutions

We describe a recently developed method to measure mechanical properties of the surfaces of plant tissues using atomic force microscopy (AFM) micro/nano-indentations, for a JPK AFM. Specifically, in this protocol we measure the apparent Young’s modulus of cell walls at subcellular resolutions across regions of up to 100 µm x 100 µm in floral meristems, hypocotyls, and roots. This requires careful preparation of the sample, the correct selection of micro-indenters and indentation depths. To account for cell wall properties only, measurements are performed in highly concentrated solutions of mannitol in order to plasmolyze the cells and thus remove the contribution of cell turgor pressure.In contrast to other extant techniques, by using different indenters and indentation depths, this method allows simultaneous multiscale measurements, i.e. at subcellular resolutions and across hundreds of cells comprising a tissue. This means that it is now possible to spatially-temporally characterize the changes that take place in the mechanical properties of cell walls during development, enabling these changes to be correlated with growth and differentiation. This represents a key step to understand how coordinated microscopic cellular changes bring about macroscopic morphogenetic events.However, several limitations remain: the method can only be used on fairly small samples (around 100 µm in diameter) and only on external tissues; the method is sensitive to tissue topography; it measures only certain aspects of the tissue’s complex mechanical properties. The technique is being developed rapidly and it is likely that most of these limitations will be resolved in the near future.