Changes of calcium distribution in egg cells,zygotes and two-celled proembryos of rice (<Emphasis Type="Italic">Oryza sativa </Emphasis>L.)

Changes in loosely bound calcium of rice egg cells (pre- and post-pollination) and two-celled proembryos were localized using potassium pyroantimonate precipitation. Egg cells and zygotes (6 h after pollination) have few calcium precipitates (ppts); however within 3 h, (9 -12 h after pollination), ppts become locally abundant in all regions. Statistical analysis indicates that ppts in cytoplasm and nucleoplasm increase by 4.4 and 10.5-fold in zygotes at 9 h and 12 h after pollination, respectively. The ppts labeling the nucleolus 9 h after pollination show a 52-fold increase. In two-celled proembryos, ppts declined to a level similar to that of zygotes 6 h after pollination, except in the nucleolus and cell wall. Following pollination, abundant calcium was detected in the apoplast of the embryo sac.     

莴苣授粉前后中央细胞中钙的分布(简报)

Ca2 作为植物生长发育过程中的必需元素之一,通过特定的时、空分布参与调控植物生长发育的诸多发育过程[1].中央细胞是胚囊中体积最大的细胞,与卵器共同构成雌性生殖单位.在被子植物双受精作用中,卵细胞与一个精细胞融合形成胚,中央细胞与另一个精细胞融合并发育成胚乳,为胚的发育提供营养.     

Calcium changes in ovules and embryo sacs of Plumbago zeylanica L.

Calcium was localized in ovules of Plumbago zeylanica from 1 day before anthesis to 3 days after anthesis using potassium antimonate and transmission electron microscopy in pollinated and emasculated flowers. At 1 day before anthesis, embryo sacs (containing an egg cell, a central cell and zero to three accessory cells) appear mature and contain abundant calcium precipitates (ppts), in contrast to nucellar cells. At anthesis, the vacuoles of nucellar cells have enlarged, and micropylar cells, in particular, are heavily labeled with calcium ppts. As pollen tubes elongate through ovular tissues, ppts diminish in ovular cells and become concentrated in the pollen tube cell wall. After fertilization, the calcium ppts sharply diminish in fertilized ovules; in unfertilized ovules, calcium ppts remain abundant up to 3 days after anthesis (when unfertilized ovules are shed). The distribution of calcium in the ovule changes in apparent response to fertilization, suggesting that calcium content may be related to the attraction and receipt of the pollen tube. In contrast with conventionally-organized embryo sacs with synergids, Plumbago accumulates calcium in the egg cell. Received: 30 December 1999 / Revision accepted: 24 March 2000     

莴苣卵细胞、合子与原胚细胞中钙的分布

用焦锑酸盐沉淀法对莴苣开花前后的卵细胞、合子与原胚细胞中的钙颗粒分布变化进行了观察。结果表明,开花前三天,刚形成的卵细胞内钙颗粒很少,开花前二天的卵细胞内钙颗粒开始增多,开花前一天的卵细胞形成了大液泡,建立了极性,细胞内的钙颗粒又减少。开花后、受精前的卵细胞的钙颗粒主要聚集在细胞核中。受精后合子中的钙颗粒又明显增多,在核质中分布一些较大的钙颗粒,在珠孔端大液泡中聚集了较多的絮状钙。二胞原胚中的钙颗粒又开始减少,多胞原胚细胞中的钙进一步减少,但原胚表面分布一层丰富的钙颗粒。探讨了钙在卵细胞分化成熟、受精以及原胚发育初期中的作用。     

The role of calcium in exocytosis and endocytosis in plant cells

The role of calcium in the individual cellular events leading to exocytosis is considered. Both vesicle movement processes and vesicle fusion at the cell surface require calcium for completion of specific events in this pathway. Our knowledge of these events is incomplete. In particular the movement of secretory vesicles by the cytoskeleton in response to added calcium is a key event that is beyond our comprehension at present. At the whole cell level, it is shown that external calcium, at the appropriate concentration, is required to elicit secretion at optimal rates. In both plant and animal cells secretion appears to be dependent on, or is triggered by, a rise in the level of internal free calcium ions from about 10^-7 to 10^-6M or even higher. In these eukaryotes internal organelles take up calcium and maintain a low level of calcium in the cell, offsetting the inflow of calcium from the plasma membrane. In some systems the inflow is restricted to a certain part of the plasma membrane, which then acts as a focus for exocytosis and, thereby, establishes a cellular polarity. In plant tissues there appears to be a requirement for some circulation of calcium within the apoplast, to sustain secretion. Recent papers on endocytosis have confirmed its occurrence in plant cells and made significant advances in isolating and characterising the clathrin coats of the coated vesicles involved in the uptake. There is no evidence, at present, for a direct role for calcium in these events. Indirectly, calcium stimulates exocytosis, and hence the delivery of excess membrane to the cell surface, which may be retrieved by an increase in the rate of endocytosis. Quantitative comparisons of the membrane flow occurring in these pathways are not available. Several plant cellular systems have been employed to study secretion and some of these may prove to be superior model systems for the investigation of certain aspects of the control of exocytosis and endocytosis by calcium ions.     

Relationship between calcium decoding elements and plant abiotic-stress resistance

Serving as an important second messenger, calcium ion has unique properties and universal ability to transmit diverse signals that trigger primary physiological actions in cells in response to hormones, pathogens, light, gravity, and stress factors. Being a second messenger of paramount significance, calcium is required at almost all stages of plant growth and development, playing a fundamental role in regulating polar growth of cells and tissues and participating in plant adaptation to various stress factors. Many researches showed that calcium signals decoding elements are involved in ABA-induced stomatal closure and plant adaptation to drought, cold, salt and other abiotic stresses. Calcium channel proteins like AtTPC1 and TaTPC1 can regulate stomatal closure. Recently some new studies show that Ca²⁺ is dissolved in water in the apoplast and transported primarily from root to shoot through the transpiration stream. The oscillating amplitudes of [Ca²⁺]_o and [Ca²⁺]_i are controlled by soil Ca²⁺ concentrations and transpiration rates. Because leaf water use efficiency (WUE) is determined by stomatal closure and transpiration rate, so there may be a close relationship between Ca²⁺ transporters and stomatal closure as well as WUE, which needs to be studied. The selection of varieties with better drought resistance and high WUE plays an increasing role in bio-watersaving in arid and semi-arid areas on the globe. The current paper reviews the relationship between calcium signals decoding elements and plant drought resistance as well as other abiotic stresses for further study.     

陆地棉雌蕊的花粉管生长途径中钙分布的超微细胞化学定位

用焦锑酸盐沉淀法对陆地棉(Gossypium hirsutum L.)授粉前后的柱头、花柱、珠孔与珠心组织中的钙进行了超微细胞化学定位。X射线波谱与能谱分析证明所定位的沉淀确系焦锑酸钙。观察结果表明:在整个花粉管生长途径中的雌蕊组织,钙分布均较其它相邻组织密集;钙主要分布在细胞壁与胞间基质等质外体系统中。在雌蕊中生长的花粉管,其尖端细胞器区也有丰富的钙。     

冬青卫矛叶片质外体适应冬季胁迫的蛋白质组学分析

植物的质外体在感知外界信号和胁迫应答中起重要作用。该研究采用生理生化和蛋白质组学方法,对秋季和冬季冬青卫矛叶片的理化指标、微观结构以及叶片质外体液体中的蛋白变化进行比较分析,探索冬青卫矛叶片质外体响应冻胁迫的分子机制,以期为植物抗冻分子机制研究提供依据。结果表明:(1)冬季冬青卫矛叶片中MDA、可溶性糖含量以及SOD、POD活性均显著升高,气孔张开度减小,叶片厚度变小。(2)冬季冬青卫矛质外体液体中的蛋白质种类和含量显著高于秋季。(3)冬青卫矛叶片质外体液体中共鉴定到838个肽段和194个蛋白质;与秋季相比,冬季冬青卫矛叶片质外体液体中共筛选到43种差异积累蛋白(DAPs),其中26个蛋白质显著上调,17个蛋白质显著下调;蛋白表达模式显示,胚胎发育晚期丰富蛋白质、铁超氧化物歧化酶、过氧化物酶、丝氨酸羧肽酶等在冬季表达量较高,推测它们可能是冬季胁迫响应敏感的蛋白质。(4)KEGG富集分析显示,差异蛋白主要与应激防御、细胞壁修饰、抗病、自由基清除、甘油脂类代谢、淀粉和蔗糖代谢、次生代谢物的生物合成等生物学过程相关。(5)验证实验结果表明,冬季冬青卫矛8个差异积累蛋白与其对应的基因的表达趋势一致。研究认为,冬季冬青卫矛质外体液体中积累的蛋白可通过清除活性氧、促进单糖、寡糖和游离氨基酸等渗透调节物的生成而增强对环境的适应;推测冬青卫矛质外体中积累的单糖和寡糖可能通过增加质外体液体的浓度从而降低冰点,进而提高冬青卫矛对冬季胁迫的耐受性。     

Movement of Lucifer Yellow CH in potato tuber storage tissues: A comparison of symplastic and apoplastic transport

The fluorescent dye Lucifer Yellow CH (LYCH) was introduced directly into the symplast of potato (Solanum tuberosum L.) tuber storage parenchyma by microinjection and also into the apoplast through cuts made in the stolon cortex. Microinjected LYCH moved away rapidly from a single storage cell and spread radially via the symplast. When the microinjected tissue was subsequently fixed in glutaraldehyde and sectioned the dye was seen clearly to be localised in the cytoplasm but not in the vacuole. In comparison, when LYCH was introduced into cuts made in the stolon cortex the dye entered the tuber by the xylem and subsequently spread apoplastically. No movement of dye was observed in the phloem. In glutaraldehyde-fixed tissues, in which LYCH was introduced to the apoplast, the dye was found within xylem vessels, in the cell walls and in intercellular spaces. Wall regions, possibly associated with plasmodesmata, became stained by the dye as it moved through the apoplast. Three hours after introduction of the dye to the stolon, intense deposits of LYCH were found in the vacuoles of all cells in the tuber, many aligned along the tonoplast. Differentiating vascular parenchyma elements contained large amounts of dye within enlarging vacuoles. However, with the exception of plasmolysed and-or damaged cells, LYCH was absent from the cytoplasm following its introduction to the plasmalemma it is suggested that the most likely pathway from the cell wall to the vacuole was by endocytosis, the dye being transported across the cytoplasm in membrane-bound vesicles. Clathrin-coated vesicles were abundant in the storage cells, providing a possible endocytotic pathway for dye movement. The significance of these observations is discussed in relation to the movement of LYCH in plant tissues and to the movement of solutes within and between storage cells of the tuber.Abbreviation LYCH Lucifer Yellow CH     

Ion distribution measured by electron probe X-ray microanalysis in apoplastic and symplastic pathways in root cells in sunflower plants grown in saline medium

Little is known about how salinity affects ions distribution in root apoplast and symplast. Using x-ray microanalysis, ions distribution and the relative contribution of apoplastic and symplastic pathways for delivery of ions to root xylem were studied in sunflower plants exposed to moderate salinity (EC=6). Cortical cells provided a considerably extended Na+ and Cl- storage facility. Their contents are greater in cytoplasm (root symplast) as compared to those in intercellular spaces (root apoplast). Hence, in this level of salinity, salt damage in sunflower is not dehydration due to extracellular accumulation of sodium and chloride ions, as suggested in the Oertli hypothesis. On the other hand, reduction in calcium content due to salinity in intercellular space is less than reduction in the cytoplasm of cortical cells. It seems that sodium inhibits the radial movement of calcium in symplastic pathway more than in the apoplastic pathway. The cell wall seems to have an important role in providing calcium for the apoplastic pathway. Redistribution of calcium from the cell wall to intercellular space is because of its tendency towards xylem through the apoplastic pathway. This might be a strategy to enhance loading of calcium to xylem elements and to reduce calcium deficiency in young leaves under salinity. This phenomenon may be able to increase salt tolerance in sunflower plants. Supplemental calcium has been found to be effective in reducing radial transport of Na+ across the root cells and their loading into the xylem, but not sodium absorption. Supplemental calcium enhanced Ca2+ uptake and influx into roots and transport to stele.     

Changes of pH in <Emphasis Type="Italic">Petunia hybrida</Emphasis> (Hort.) styles induced by pollination and influence of proton pump and ion channels on its regulation

Ionselective microelectrode method was used to study changes of pH in transmitting tissue of style in Petunia hybrida (Hort.). Effect of pollination and pollen tube growth were examined. Subsequently solutions of ions and various stimulators or blockers of ion channels were applied on pollinated styles to examine the possible role of ion channels in pH stabilisation. It was confirmed in the present study that: (1) there is a pH gradient in the transmitting tissue of a petunia unpollinated style with the stigma region being more acidic; (2) pollination causes further acidification of transmitting tissue: (3) the gradient of pH first vanishes at 24 h after pollination then is reversed up to 72 h after pollination; (4) active transport of ions plays an important role in pH regulation in transmitting tissue. The presented results confirm the role of pH changes and Ca²⁺ as a mediator in controlling proton influx into the apoplast of the transmitting tissue during pollen tube growth.     

Influence of calcium on blue-light-induced chloroplast movement in Lemna trisulca L.

The presence of calcium is essential for chloroplast movement induced by blue light in Lemna trisulca L. The regulatory role of calcium was confirmed by the inhibition of chloroplast movement by cytochalasin B and trifluoperazine. The calcium concentration in tissues was modified by ethylene glycol-bis(2-aminoethylether)-N,N,N, N-tetraacetic acid (EGTA), the calcium ionophore A23187 and La³⁺. Only a long period of incubation (12h) in EGTA or La³⁺ caused distrubances in chloroplast movement. This indicates that calcium influx is not essential for chloroplast movement. Those conditions that dramatically changed the internal calcium concentration, either applications of calcium ionophore A23187 and EGTA, or ionophore and La³⁺, markedly decreased the amplitude of response to blue-light pulses. This demonstrates that disturbances of chloroplast movement are observable only when internal stores of calcium are affected by Ca²⁺-antagonists. We suggest that the calcium involved in blue-light-induced chloroplast movement is derived from intracellular stores. The addition of Mg²⁺ to EGTA buffer counteracted its effect, indicating that Mg²⁺, as well as Ca²⁺, might possibly be involved in chloroplast movement.Abbreviations EGTA ethylene glycol-bis(2-aminoethylether)-N,N,N,N-tetraacetic acid - Hepes 4(2-hydroxyethyl-1-piperazine) ethanesulfonic acid - A23187 calcium ionophore We express our gratitude to Professor W. Korohoda for valuable critical comments on this paper and stimulating discussion. We also thank Mr. P. Malec for help in preparing the experiment with trifluoperazine and Mr. A. Waloszek for taking the photographs. We are indebted to Mr. Tim Kline (International House, Krakow, Poland) for improving the English style. This research was supported by grant No. 1042/P2/92/03 from the State Committe for Scientific Research.     

The suberin lamella,a possible barrier to water movement from the veins to the mesophyll ofThemeda triandra forsk

Summary Precipitation of ferrous ions by ferricyanide in transpiring leaves ofThemeda triandra Forsk. produced crystalline deposits, which were visible with the light and electron microscope. Prussian blue crystals were formed within the lumina of the tracheary elements and the apoplast, or cell wall continuum of the vascular tissues and bundle-sheath cells. Little if any deposition was noted within the lignified secondary thickenings of the tracheary elements. The localization pattern suggests that the ferrous ions moved from the lumina of the tracheary elements via the exposed primary walls. Prussian blue crystals were abundant in the outer tangential and radial walls of the bundle-sheath cells. By contrast, crystals were lacking in the walls of neighbouring mesophyll cells, suggesting that the suberin lamella in the bundle-sheath walls effectively inhibited the apoplastic movement of ferrous ions and possibly may impede, or restrict the movement of water across the bundle-sheath/mesophyll interface.     

Temperature and growth-induced water potential

When the steins of dark-grown soybean [Glycine max (L.) Merr.] seedlings grew rapidly at favorable temperatures in saturating humidities, a water potential of about 0·2 MPa was induced by growth ($pS_o-$pS_w, where $pS_o is the water potential of the basal nonelongating tissue and $pS_w is the water potential of the elongating tissue). If this water potential was caused by high concentrations of solute in the apoplast, as has been proposed, lowering the temperature should have little effect on the potential. On the other hand, if the water potential was caused by apoplast tensions generated by growth, then the tensions should disappear as growth is inhibited by low temperatures. We observed that the growth-induced water potential became too small to detect when growth was inhibited by temperatures as low as 13—5 °C. The disappearance was observed as a rise in apoplast water potential using a thermocouple psychrometer for intact plants, a rise in cell turgor using a miniature pressure probe and a decrease in apoplast tensions using a pressure chamber. The disappearance was not caused by a loss of solute from the apoplast because the tensions fully accounted for the growth-induced water potential at all temperatures. The results are consistent with the lack of solute measured directly in the apoplast solutions at high temperatures (Nonami & Boyer 1987). Therefore, it was concluded that little solute was present in the apoplast at any temperature, and the growth-induced water potential was associated mostly with a tension that moved water from the xylem and into the surrounding cells to meet the demand of cell enlargement.     

Calcium transport between tissues and its distribution in the plant

Abstract. The low cytosol concentration of free Ca²⁺ makes the symplast of roots an ineffective pathway for the supply of the calcium needed for healthy growth in the aerial parts of plants. Ca²⁺ moves rapidly across the cortical apoplast by diffusion and mass flow but is probably diverted across the plasmamembranes of endodermal cells by Casparian bands. A proposal is made to account for the movement of calcium across the endodermis and it is estimated that Ca-fluxes are likely to be appreciably greater than in the regulation of cell Ca level by cortical cells.
Ca transport in the xylem occurs by mass flow of free Ca²⁺, and some organically complexed Ca, and by chromatographic movement along Ca-exchange sites in the xylem walls. Delivery of Ca to transpiring leaves and to weakly transpiring meristematic zones is discussed in relation to the two modes of Ca movement in the xylem. Competition between sinks is intensified when [Ca²⁺] in xylem is low and transpiration is great.
Tropic growth responses involve pumping of vacuolar calcium into the apoplast followed by its migration along gradients of electrical potential which develop in the apoplast after geo-stimulation. An attempt is made to estimate plasmalemma efflux during this process.
Redistribution from mature tissues to meristems in the pholem is likely to be small, if it occurs at all, since sieve tubes cannot have more than micro-molar concentrations of free-Ca²⁺ in them.     

The kinetics of calcium and magnesium entry into mycorrhizal spruce roots

 The entry of calcium and magnesium from external sources into mycorrhizal roots of 3-year-old Norway spruce trees (Piceaabies [L.] Karst.) was monitored. Roots of intact plants were exposed for various periods of time, ranging from 2 min to 48 h, to nutrient solutions which contained the stable-isotope tracers ²⁵Mg and ⁴⁴Ca. After labelling, samples of roots were excised from the plants, shock-frozen, cryosubstituted and embedded. The resulting isotope composition in this material was analysed by a laser-microprobe-mass-analyser (LAMMA) at relevant positions within cross-sections of the roots. For both elements, we determined (i) the fractions of the isotopes originating from the plant prior to labelling, and (ii) the fraction of isotopes originating from the corresponding tracer that penetrated into the root. Both divalent cations rapidly penetrated across the cortical apoplast and reached the endodermis. After 2 min of exposure to the labelling solution, an initial transient gradient of the tracers could be observed within the root cortex. Subsequently, calcium as well as magnesium equilibrated between the apoplast of the entire cortex and the external tracer with a half-time, t_1/2, of about 3 min. In contrast, the kinetics of radial movement into the vascular stele showed a delay with a t_1/2 of 100–120 min. We take this as strong evidence that there exists a free apoplastic path for divalent cations in the cortex and that the endodermis is a major barrier to the further passage of Mg and Ca into the xylem. While ²⁵Mg in the labelling solution exchanged rapidly with Mg in the cortical apoplast, the exchange across the plasma membrane with Mg present in the protoplasm of the same cortical cells was almost 2 orders of magnitude slower. The kinetics of Ca and Mg entry at +6 °C were similar to those obtained at a root temperature of +22 °C. Received: 23 December 1998 / Accepted: 17 September 1999     

The assessment of enriched apoplastic extracts using proteomic approaches

In plant tissues the extracellular environment or apoplast, incorporating the cell wall, is a highly dynamic compartment with a role in many important plant processes including defence, development, signalling and assimilate partitioning. Soluble apoplast proteins from Arabidopsis thaliana, Triticum aestivum and Oryza sativa were separated by two‐dimensional electrophoresis. The molecular weights and isoelectric points for the dominant proteins were established prior to excision, sequencing and identification by matrix‐assisted laser‐desorption ionisation time of flight mass spectrometry (MALDI ‐ TOF MS). From the selected spots, 23 proteins from O. sativa and 25 proteins from A. thaliana were sequenced, of which nine identifications were made in O. sativa (39%) and 14 in A. thaliana (56%). This analysis revealed that: (i) patterns of proteins revealed by two‐dimensional electrophoresis were different for each species indicating that speciation could occur at the level of the apoplast, (ii) of the proteins characterised many belonged to diverse families reflecting the multiple functions of the apoplast and (iii), a large number of the apoplast proteins could not be identified indicating that the majority of extracellular proteins are yet to be assigned. The principal proteins identified in the aqueous matrix of the apoplast were involved in defence, i.e. germin‐like proteins or glucanases, and cell expansion, i.e. β‐D‐glucan glucohydrolases. This study has demonstrated that proteomic analysis can be used to resolve the apoplastic protein complement and to identify adaptive changes induced by environmental effectors.     

The fungal sheath of ectomycorrhizal pine roots: an apoplastic barrier for the entry of calcium,magnesium, and potassium into the root cortex?

The apoplastic permeability of the fungal sheath of two different ectomycorrhizal associations of Pinus sylvestris L. was analysed by laser microprobe mass analysis (LAMMA) and energy-dispersive X-ray spectroscopy (EDXS) after stable isotope labelling with 25Mg, 41K and 44Ca. Entry of 25Mg and 44Ca into the outer cortical apoplast of non-mycorrhizal roots was detected after 4 min of labelling. After a longer exposure time the endodermis with its Casparian band acted as an efficient apoplastic diffusion barrier for the radial movement of 25Mg and 44Ca into the stele. A fraction of approximately one-third of the apoplastic cations of the root cortex could not be exchanged against the external label even after longer exposure times. The ectomycorrhizal sheath of the two fungal species used, Pisolithus tinctorius (Pers.) Coker & Couch and Suillus bovinus (L. ex Fr.) Kuntze, does not completely inhibit the apoplastic movement of ions into the mycorrhizal root cortex, but retarded the penetration of isotopes into the cortical apoplast. In roots inoculated with S. bovinus, a clear labelling of the cortical apoplast could first be detected after 24 h of exposure to the stable isotope solution. At this time the labelling of the cortical apoplast in these mycorrhizal roots was higher than those of non-mycorrhizal roots and, with EDXS, changes in the element composition of the apoplast were detected. The results indicated that possibly hydrophobins localized in the fungal cell wall might be involved in the increased hydrophobicity of mycorrhizal roots and the lower permeability of the ectomycorrhizal sheath.     

Inhibition of polar calcium movement and gravitropism in roots treated with auxin-transport inhibitors

Primary roots of maize (Zea mays L.) and pea (Pisum sativum L.) exhibit strong positive gravitropism. In both species, gravistimulation induces polar movement of calcium across the root tip from the upper side to the lower side. Roots of onion (Allium cepa L.) are not responsive to gravity and gravistimulation induces little or no polar movement of calcium across the root tip. Treatment of maize or pea roots with inhibitors of auxin transport (morphactin, naphthylphthalamic acid, 2,3,5-triiodobenzoic acid) prevents both gravitropism and gravity-induced polar movement of calcium across the root tip. The results indicate that calcium movement and auxin movement are closely linked in roots and that gravity-induced redistribution of calcium across the root cap may play an important role in the development of gravitropic curvature.Abbreviations 9-HFCA 9-hydroxyfluorenecarboxylic acid - NPA naphthylphthalamic acid - TIBA 2,3,5-triiodobenzoic acid - IAA indole-3-acetic acid     

The use of lanthanum to study the functional development of the Casparian strip in corn roots

Summary Lanthanum in combination with electron microscopy was used to study the functional development of the Casparian strip in the primary root of corn (Zea mays L. WF9 X M14). The Casparian strip was first visible between 13 and 16 mm from the root tip, and was first an effective barrier to lanthanum movement through the apoplast from cortex to stele between 14 and 18 mm from the apex. Until the Casparian strip was formed, lanthanum ion freely penetrated the stelar apoplast. By 18 mm from the apex lanthanum was completely restricted to the extracellular spaces of the cortex. Treatment with 0.02 or 0.03% (v/v) Triton X-100 increased the permeability of the plasma membrane, and allowed lanthanum ion to penetrate the cortical symplast. After the detergent treatment, the endodermis no longer functioned as a barrier to La³⁺ movement from cortex to stele. This confirms that an intact plasma membrane is necessary for the primary endodermis to control the passage of solutes into the vascular tissues of the stele.