Mathematical Analysis of Plant Growth zea mays Primary Roots

A new mathematical treatment is given to the concept of relative elemental growth rate, the limiting value at a point of the relative rate of tissue growth. The new point of view requires consideration of a coordinate system in which each point on the root surface is defined by its initial position on the root axis. The resultant technique of plotting growth parameters provides the ability to distinguish between waves of growth propagating toward the root tip away from cells and waves which tend to propagate along with the cells. Thus it is possible to model the plant growth-regulatory process in terms of auxin and other regulators, considering both cell-associated maxima of regulator concentration and maxima which propagate through the tissue.     

The Role of the Plasma Membrane in the Response of Plant Roots to Aluminum Toxicity

Al³⁺, the predominant form of solubilized aluminum at pH values below 5.0, has been shown to exert a profound inhibitory effect on root elongation. Al is known to accumulate at the root apex. The plasma membrane represents the first potential target for Al toxicity, due to its pronounced binding to phospholipids. Al appears to alter both the structure and functions of the plasma membrane, and a great deal of research has been conducted concerning the interactions between Al and the plasma membrane. In this review, recent findings regarding the interactions between Al and the plasma membrane are described, specifically findings involving Al-induced alterations in the structure and function of the plasma membrane.Key Words: acid soil, aluminum, plasma membrane, tolerance, toxicity     

植物根系有机酸的分泌和解铝毒作用

对植物有机酸的解铝毒作用及释放机制,尤其是对铝刺激的阴离子通道的研究进展作了介绍.     

铝胁迫下植物根系的有机酸分泌及其解毒机理

酸性土壤中的铝毒害问题,已成为限制植物生长发育的主要因素之一.耐铝植物通过根系分泌有机酸来解除或减轻铝的毒害是外部解铝毒的重要机制.文章对铝胁迫下植物根系分泌有机酸的种类,有机酸解铝毒机理、解铝毒能力,有机酸分泌方式及调控其分泌的主要因素等相关研究进行综述.     

Photomorphogenesis of Plant Roots in Vitro

Photomorphogenetic phenomena of excised roots of various plants cultured aseptically in White medium were studied. Continuous white fluorescent light (3000 lx) could generally inhibit extension growth of the primary roots and lateral roots, depress emergence of lateral roots and raise of fresh weight of roots. If greening of the roots could be induced by light, however, the light would increase the fresh weight and formation of lateral roots. Addition of 75 mg/l inositol significantly improved root development. Continuous irradiation stimulated greening of excised roots from some plants (e. g. cucumber, bird rape, India mustard and maize) depending on their hereditary potency. Chlorophyll a, chlorophyll b and carotenoids accumulated slowly. After 1 day or 5 days under the light, chlorophyll or carotenoids could be detected separately. Total amount of plastid pigments per unit of fresh weight in roots was much lower than in leaves. More sucrose applied could pro- mote synthesis of chlorophyll and growth of roots. Chloroplasts appeared in small parenchyma cells located in the stele. They did not come frorn proplastid, but were transformed from a kind of leueoplast-amyloplast. Electron microscopic photographes indicated that starch grains diminished gradually while lamella structure appeared first within enveloped and then filled inner-space of plastid. Many thylakoids stacked to form irregular-shaped gram. These chloroplasts seemed to be different from those in mesophyll. Special inhibitor of carotenoid biosynthesis (norflurazon) prevented chlorophyll synthesis with high efficiency while another photosynthetic inhibitor (motoxuron) increased chlorophyll accumulation in isolated roots.     

Phytase Activity of Plant Roots

Experiments were conducted to study the phytase activity ofpea, gram, wheat, and barley plant roots grown under sterileand unsterile conditions. The results indicate substantial phytaseactivity of the plant roots studied. The substrate concentrationappeared to affect the activity. Plants grown under sterileconditions exhibited less phytase activity as compared withplants grown in rich garden soil.     

Uptake of Proteins by Plant Roots

The patterns of uptake of fluorescein-labelled lysozyme (Fl-lysozyme) by barley, maize, onion, tomato and vetch are similar as revealed by fluorescence microscopy. Penetration of the root cap and through the epidermis into the cortex increases with time of exposure and decreases with higher salt concentrations. In fact, one molar ethylammonium chloride can remove most of the absorbed protein from treated roots and the space observed to be stained by Fl-lysozyme in this manner can be visualized as “free space”. Results with sterile and non-sterile barley roots were indistinguishable. At low ionic strength, Fl-lysozyme can penetrate cells and complex with nucleoli. Such cell protoplasts appear “coagulated”. Uptake results with fluorescein per se were unlike those with protein. The uptake of a much larger molecule, ferritin, is confined to the epidermis and root cell walls. Localized, absorbed protein and root growth inhibition by basic proteins have yet to be related.     

Scanning Electron Microscopy of Plant Roots

A glycol methacrylate infiltration and polymerization techniquewas used to prepare clover roots inoculated with Rhizobium forscanning reflection electron microscopy. Root hairs and epidermalcells were coated with many bacteria; some bacteria seemed tobe embedded in the wall surface. Root hair tips were often smoothbut some older root hair surfaces showed a fibrillar meshworkpattern. Small granules c. 0.18 µm diameter were presenton the root hair and epidermal cell walls. The root cap, someroot hairs, and some epidermal cells were covered by an amorphousfilm thought to be the mucigel.     

Allosteric Regulation of Potassium Uptake in Plant Roots

In uptake experiments from nutrient solutions containing 2.0 mM K⁺ labelled with ⁸⁶Rb⁺, the relationship between potassium uptake efficiency and internal potassium concentration of the roots, [K⁺]_i was found to be partly sigmoidal for intact plants of spring wheat (Triticum aestivum L.), glasshouse cucumber (Cucumis sativus L.), birch (Betula verrucosa Ehrh.), lingonberry (Vaccinium vitis-idaea L.), Scots pine (Pinus silvestris L.) and Norway spruce (Picea abies (L.) Karst.), The results were interpreted in terms of sigmoidal enzyme kinetics for allosteric regulation. Hill plots of the data gave straight lines at specific [K⁺]_i intervals for the species. The slopes of the lines are the Hill coefficient, which could be regarded as a measure of the minimal number of allosteric sites. The Hill coefficient varied between - 14.4 and - 15.9. When divided by four, these values are fairly consistent with those in the literature. It is suggested that four active uptake sites interact with four groups of allosteric sites, each group containing four such sites, or that one active uptake site interacts with all the allosteric sites. Thus the results are evidence that the mechanism regulating K⁺ uptake is basically similar for the investigated plants. It is the interval of [K⁺]_i mediating highly negatively cooperative allosteric regulation that differs among species. For some of the species, n decreased from about 15 and approached unity at high [K⁺]_i values. This may indicate that only few sites are still available, making cooperativity unimportant. Alternatively high vacuolar [K⁺]_i concentrations may give rise to an incorrect evaluation of data from Hill plots, since the cytoplasmic K⁺ content likely regulates the allosteric mechanism. Moreover, it is suggested that gene-controlled carrier synthesis is responsible for the varying maximum K⁺ uptake efficiency among species.     

不同浓度Al3+胁迫对水稻根生长的影响

铝毒害是酸性土壤限制农作物产量的主要因素之一。本文以水稻为材料,探讨了不同浓度的AlCl3(0、50、100、150 mmol.L-1)胁迫处理对水稻幼苗根生长的影响。结果表明,随着Al3+浓度和胁迫时间的增加,水稻幼苗根的生长受到抑制,长度和鲜重都减小,根尖细胞死亡的程度增大,根中H2O2含量上升;与对照(未经DMTU预处理)相比,经dimethylthiourea(内源过氧化氢抑制剂)预处理的水稻幼苗在Al3+胁迫下根细胞死亡程度减小,H2O2含量降低,根的生长抑制得到缓解,表明内源H2O2的积累是造成Al3+对水稻幼苗根生长抑制的原因之一。     

Identification of the Toxic Principle in a Sample of Poaefusarin

A sample of poaefusarin (a mycotoxin suspected of being one of the toxins involved in alimentary toxic aleukia in the U.S.S.R.) was received from a Soviet scientist for evaluation and comparison with other mycotoxins. Although poaefusarin is presumed to be a steroid, analyses by thin-layer chromatography, gas-liquid chromatography, and infrared, ultraviolet, and mass spectrometry could not confirm the presence of a steroid structure. However, 2.5% of the sample was made up of the trichothecene T-2 toxin, an amount sufficient to explain the toxicity found in the rat and rabbit skin toxicity tests. In addition, neosolaniol (0.14%), T-2 tetraol (0.6%), and zearalenone (F-2) (0.43%) were present in the sample. Since the toxicity was found to be associated only with T-2 toxin, no attempt was made to determine the nature of the other nontoxic components of the sample.     

Temperature Sensing by Primary Roots of Maize

Zea mays L. seedlings, grown on agar plates at 26°C, reoriented the original vertical direction of their primary root when exposed to a thermal gradient applied perpendicular to the gravity vector. The magnitude and direction of curvature can not be explained simply by either a temperature or a humidity effect on root elongation. It is concluded that primary roots of maize sense temperature gradients in addition to sensing the gravitational force.     

In Situ Speciation and Distribution of Toxic Selenium in Hydrated Roots of Cowpea

The speciation and spatial distribution of selenium (Se) in hydrated plant tissues is not well understood. Using synchrotron-based x-ray absorption spectroscopy and x-ray fluorescence microscopy (two-dimensional scanning [and associated mathematical model] and computed tomography), the speciation and distribution of toxic Se were examined within hydrated roots of cowpea (Vigna unguiculata) exposed to either 20 µm selenite or selenate. Based upon bulk solution concentrations, selenate was 9-fold more toxic to the roots than selenite, most likely due to increased accumulation of organoselenium (e.g. selenomethionine) in selenate-treated roots. Specifically, uptake of selenate (probably by sulfate transporters) occurred at a much higher rate than for selenite (apparently by both passive diffusion and phosphate transporters), with bulk root tissue Se concentrations approximately 18-fold higher in the selenate treatment. Although the proportion of Se converted to organic forms was higher for selenite (100%) than for selenate (26%), the absolute concentration of organoselenium was actually approximately 5-fold higher for selenate-treated roots. In addition, the longitudinal and radial distribution of Se in roots differed markedly: the highest tissue concentrations were in the endodermis and cortex approximately 4 mm or more behind the apex when exposed to selenate but in the meristem (approximately 1 mm from the apex) when exposed to selenite. The examination of the distribution and speciation of Se in hydrated roots provides valuable data in understanding Se uptake, transport, and toxicity.Selenium (Se) is an essential micronutrient for humans and other animals (Rayman, 2008). At elevated concentrations, however, it is toxic, and the concentration range from deficiency to lethality is unusually narrow (Terry et al., 2000). Plants represent a direct entrance to the wider food chain as the main sources of dietary Se (Rayman, 2008). The uptake and accumulation of Se by plants is an important process in controlling the health risks resulting from Se deficiency or toxicity. Se toxicity to plants has been observed in arid and semiarid soils derived from seleniferous rocks and shales, although anthropogenic contamination is also of concern (Terry et al., 2000). Therefore, it is important that the mechanisms of Se uptake, transformation, and toxicity in plants are understood in order to reduce health risks.Selenite (Se[IV]) and selenate (Se[VI]) are the two dominant inorganic species in soils depending upon the redox potential and pH (Elrashidi et al., 1987). The mechanism of Se[VI] uptake is well known: it is taken up by plant roots via the high-affinity sulfate transporters (Terry et al., 2000) due to the similarity between Se[VI] and sulfate. By contrast, little is known about the uptake mechanism involved in Se[IV] in plant roots. Some studies suggested that Se[IV] is taken up via passive diffusion (Shrift and Ulrich, 1969; Arvy, 1993). Recently, Zhao et al. (2010) reported that the uptake of Se[IV] is mediated by the silicon (Si) influx transporter Lsi1 (OsNIP2;1) in rice (Oryza sativa). Furthermore, Se[IV] uptake was found to occur via both passive diffusion and phosphate transporters in the marine coccolithophore Emiliania huxleyi (Araie et al., 2011). Apart from the difference in their mechanisms of uptake, they also differ in their mobility within plants (Li et al., 2008). Se[VI] is relatively easily translocated from roots to shoots, whereas Se[IV] tends to accumulate within the roots (Arvy, 1993). Despite this important progress, much less is known about the sites of uptake of Se[IV]/Se[VI] and their possible chemical transformations in hydrated plant roots. This information regarding the in situ distribution and chemical forms of Se would be helpful in elucidating the mechanism(s) responsible for Se uptake, transformation, and toxicity in plants.Recent advances in synchrotron-based techniques allow in situ measurement of the distribution of metal(loid)s in hydrated fresh plant tissues (Kopittke et al., 2011, 2012; Lombi et al., 2011a). In particular, the prototype Maia detector system, jointly developed by the Australian Synchrotron, the Commonwealth Scientific and Industrial Research Organization, and the Brookhaven National Laboratory, represents a new-generation x-ray fluorescence detector and real-time processing approach that provides unprecedented capabilities in in situ element imaging and measurement (Lombi et al., 2011b). The Maia uses an annular array of 384 silicon-diode detectors positioned in a backscatter geometry to subtend a large solid angle (approximately 1.3 steradian) and to achieve high count-rate capacity (Kirkham et al., 2010). Data acquisition times are approximately 10 to 100 times faster in the Maia than for other detectors, thereby allowing analysis of highly hydrated biological specimens (e.g. roots) without observable damage (Lombi et al., 2011a). This has allowed us to overcome the analytical challenges of examining the two-dimensional and virtual three-dimensional distribution of low-concentration metal(loid)s in hydrated and fresh plant tissues (Kopittke et al., 2011, 2012; Lombi et al., 2011a, 2011c).In this study, we investigated the speciation and quantified the longitudinal and radial distribution of Se in hydrated roots of cowpea (Vigna unguiculata) exposed to either Se[IV] or Se[VI]. Cowpea is a model species of rhizotoxicity and is also one of the most important food legume crops in the semiarid tropics, where Se toxicity is often a concern. The chemical forms of Se in cowpea roots were first examined using x-ray absorption spectroscopy (XAS). Second, with x-ray fluorescence microscopy (µ-XRF), we used two-dimensional imaging (coupled with an associated mathematical model to calculate concentrations of Se within various tissues of the root cylinder) to determine the spatially resolved distribution of Se within root tissues. Additionally, sequential tomography was used to provide virtual three-dimensional reconstructions of Se distribution in roots, enabling comparison of computed tomography with the mathematical model.     

植物根系吸水机理的研究进展

近年来,植物根系吸水机理在细胞、组织和整体水平上的研究进展非常迅速,对阐明植物抗旱机制及其高效利用有限水资源途径的探讨具有重要意义。本文主要对植物根的复合结构和根系在土壤中的分布、根系中水流性质等方面的最新研究状况进行了概述,特别详细地论述了水通道蛋白的表达及功能与根系中水分运动的关系、以及根系输水的调节和根系吸水过程中的信号传导方面的研究动态,并且评价了根的复合运输模型和根系吸水的数学模型等,最后就其可能生理意义及其应用前景作了评述。     

荞麦根吸收铝和分泌草酸的部位

荞麦根在Al诱导下分泌草酸。从根尖开始依次划分为 0～ 0 .5、0 .5～ 1.0、1.0～ 1.5、1.5～ 2 .0cm区段 ,在 0 .5～ 1.0cm区Al的吸收最多 ,草酸的分泌部位主要在 0～ 0 .5cm区 ,显示Al诱导下 ,Al的吸收和草酸的分泌不在同一部位     

Distribution and Toxic Effects of Cadmium and Lead on Maize Roots

Two-day-old seedlings of maize (Zea mays L.) were incubated on Cd and Pb nitrate solutions at the concentrations that inhibited root growth approximately by 50% after two-day-long incubation (LC₅₀; 10^–4 and 10^–3 M, respectively) or completely terminated growth of the primary root after one-day-long incubation (LC; 5 × 10^–4 and 10^–2 M, respectively). Cd and Pb contents were measured using an anodic inversion voltammetric technique in a flow injection system and a histochemical method. At LC₅₀, Cd and Pb were discerned, by histochemical techniques, in all root apical tissues, whereas in the root hair zone, the heavy metals were primarily accumulated in the apoplast of the rhizodermis and cortex and to a lesser extent, in the vascular tissues and parenchyma cells surrounding the metaxylem vessels. Insignificant accumulation of Cd and Pb in the pericycle probably explains why root branching was tolerant to these agents. At LC, Cd and Pb were found in the apoplast of all root tissues, in accordance with the practically complete inhibition of root growth and branching. Irrespectively of Cd and Pb concentrations in the external solution, the metal contents in the root apex exceeded those in the basal region. Procion dyes were used to assess cell death inflicted by Cd and Pb. At LC, the root cap and meristematic cells perished, together with the rhizodermal cells and the outer cortical cells of the root apex, whereas only the rhizodermal cells in the root apical region died at LC₅₀. The evidence that Cd and Pb cross the endodermal barrier at LC presumes that, at lower metal concentrations, the Casparian strip and plasmalemma of the endodermis regulate the transport of these metals into the central cylinder. The authors conclude that the identical barriers control Cd and Pb transport in root tissues.     

植物根内通气组织形成机理的研究进展

植物能否在湿地或淹涝环境中生长, 很大程度上取决于植物是否具有健全发达的通气组织。在结合形态学和分子生物学等方面研究的基础上, 概述了植物根内通气组织的形成过程, 主要涉及生理功能、诱导因子和相关酶等, 推测细胞程序性死亡是溶生性通气组织形成的机理, 乙烯在整体信号转导网络中起关键性中介作用。