The Casparian strip in pea epicotyls: Effects of light on its development

The Casparian strip, which is specific to roots, was studied in the epicotyls of dark-grown seedlings of pea (Pisum sativum L.) where it was found to have the same morphology and properties as the strip in roots. In dark-grown seedlings, the distance between the upper-most position of the Casparian strip and the bending point of the hook (about 37 mm) did not change during growth of the seedlings. In the uppermost 0.5-mm region of the region in which the Casparian strip could be detected by fluorescence microscopy, the plasma membrane was not firmly attached to the cell wall. The development of the Casparian strip continued for about 42 h after dark-grown seedlings were transferred to the light, indicating that (i) the cells that have been determined to form the Casparian strip in darkness form the strip in the light, and that (ii) it takes about 42 h for the cells to complete formation of the strip. Cells in the hook of dark-grown seedlings did not form a Casparian strip when such seedlings were transferred to the light. The Casparian strip was formed in rapidly elongating internodes of light-grown seedlings when the seedlings were transferred to darkness. Light did not control the formation of the Casparian strip in roots.Abbreviation PBS phosphate-buffered saline     

Effects of brefeldin A on the development of the Casparian strip in pea epicotyls

Summary The Casparian strip, a structure that is present in roots, is also present in epicotyls of dark-grown pea seedlings. In a dark-grown epicotyl, the cells in each stage of the development of the Casparian strip have been suggested to be lined up basipetally in the region 3 to 37 mm below the bending point of the hook, in order of the developmental stage. Brefeldin A (BFA), a specific inhibitor of secretory transport, was administrated at 200 M. to dark-grown pea epicotyls for 2 h via a thread passed through the epicotyl 40 mm below the bending point. The basipetal sequence of development of the modification of the cell wall at the Casparian strip, as judged by fluorescence microscopy, stopped 5 h after the start of 2 h treatment with BFA and resumed after 30 h. This basipetal sequence of development did not stop in control seedlings. Electron micrographs of endodermal cells in epicotyls treated with BFA showed striking morphological changes in the Golgi stacks and the ER. Histological examination made 20 h after the start of the experiment revealed that the basipetal sequence of development of the cell wall modification stopped at a point which was present at 25.2 ± 1.6 mm (mean with SD, n=5) from the bending point of the hook at the start while the basipetal sequence of development of the tight adhesion of the plasma membrane to the cell wall at the Casparian strip stopped 0.9 ± 0.5 mm (mean with SD, n=5) below this point. These results indicate the involvement of secretory transport not only in the introduction of the modification of the cell wall but also in the completion of the tight adhesion of the plasma membrane.Abbreviations BFA brefeldin A - PBS phosphate-buffered saline - ER endoplasmic reticulum     

植物凯氏带形成分子机制及功能特点的研究进展

凯氏带位于被子植物初生根内皮层细胞,环绕细胞1周,是与质膜紧密结合的非极性带状增厚结构。凯氏带作为植物根中离子径向运输障碍,调节离子的质外体吸收途径,迫使土壤中的离子通过内皮层细胞膜,选择性地进入中柱。凯氏带发现于1865年,但直至拟南芥凯氏带蛋白的发现和凯氏带阻滞作用物质基础被揭示,凯氏带的形成机理和功能才逐渐为人们所认知。凯氏带的物质基础为木质素,其形成需要由凯氏带蛋白和受体激酶组成的合成平台。细胞内部的木质素单体经ABCG载体运输到凯氏带的形成区,经ESB1dirigent蛋白、RBOHF氧化酶和PER64过氧化物酶等催化,合成木质素。该文对近年来国内外有关凯氏带形成的分子机制和功能特点方面的研究进展进行综述,为进一步理解和解析凯氏带的形成机理和功能提供参考。     

Radial widening of the Casparian strip follows induced radial expansion of endodermal cells

The Casparian strip, the barrier to apoplastic transport that is located at the endodermis in roots and stems, is formed by individual endodermal cells and is constructed as a highly organized mesh within the primary wall. Since little is known about the mechanism of formation of the strip, we tried to obtain morphological evidence for the existence, prior to suberization and lignification, of some regulatory system at the expected site of the strip. Endodermal cells in etiolated pea stems were induced to expand in the radial direction by piercing the stems through the cortex before formation of the strip. The radial width of the strip increased significantly with the expansion of the radial walls of these endodermal cells. The expansion of the cells occurred before the formation of the strip. However, strips that had already been formed when the stems were pierced did not increase in width despite an induced expansion of the radial walls. These observations suggest that some positional information exists in the radial wall of endodermal cells that defines the future site of formation of the strip and its width.     

The structure of barley roots in relation to the transport of ions into the stele

Summary Three successive states are recognized in the development of endodermal cells in seminal and nodal axes and primary lateral roots of barley: 1. Casparian bands in the anticlinal walls; 2. suberin lamella around the whole inner face of the wall; 3. unevenly deposited cellulosic wall thickening. These states develop asynchronously, the cells adjacent to the protoxylem pole cells always being last to mature. All cells have progressed to at least the secondary state by 32 cm from the tip in seminal axes, 48 cm from the tip in nodal axes, but only 6 cm from the tip in primary laterals. The asynchronous development gives the appearance of passage cells adjacent to the protoxylem pole cells, although all cells eventually attain the same state and degree of wall thickening. Long distance transport of calcium shows a close correlation with the incidence of suberin lamellae in the three types of root examined; it is suggested that formation of a suberin lamella effectively blocks calcium movement into the stele and, therefore, long distance transport. Plasmodesmata are present in comparable frequencies through both tangential and radial endodermal walls; they appear to maintain intercellular continuity until a late stage in development.     

Effects of salt on growth,ion accumulation,photosynthesis and leaf anatomy of the mangrove,<Emphasis Type="Italic"> Bruguiera parviflora</Emphasis>

The effects of a range of salinity (0, 100, 200 and 400 mM NaCl) on growth, ion accumulation, photosynthesis and anatomical changes of leaves were studied in the mangrove, Bruguiera parviflora of the family Rhizophoraceae under hydroponically cultured conditions. The growth rates measured in terms of plant height, fresh and dry weight and leaf area were maximal in culture treated with 100 mM NaCl and decreased at higher concentrations. A significant increase of Na⁺ content of leaves from 46.01 mmol m^-2 in the absence of NaCl to 140.55 mmol m^-2 in plants treated with 400 mM NaCl was recorded. The corresponding Cl^- contents were 26.92 mmol m^-2 and 97.89 mmol m^-2. There was no significant alteration of the endogenous level of K⁺ and Fe²⁺ in leaves. A drop of Ca²⁺ and Mg²⁺ content of leaves upon salt accumulation suggests increasing membrane stability and decreased chlorophyll content respectively. Total chlorophyll content decreased from 83.44 g cm^-2 in untreated plants to 46.56 g cm^-2 in plants treated with 400 mM NaCl, suggesting that NaCl has a limiting effect on photochemistry that ultimately affects photosynthesis by inhibiting chlorophyll synthesis (ca. 50% loss in chlorophyll). Light-saturated rates of photosynthesis decreased by 22% in plants treated with 400 mM NaCl compared with untreated plants. Both mesophyll and stomatal conductance by CO₂ diffusion decreased linearly in leaves with increasing salt concentration. Stomatal and mesophyll conductance decreased by 49% and 52% respectively after 45 days in 400 mM NaCl compared with conductance in the absence of NaCl. Scanning electron microscope study revealed a decreased stomatal pore area (63%) in plants treated with 400 mM NaCl compared with untreated plants, which might be responsible for decreased stomatal conductance. Epidermal and mesophyll thickness and intercellular spaces decreased significantly in leaves after treatment with 400 mM NaCl compared with untreated leaves. These changes in mesophyll anatomy might have accounted for the decreased mesophyll conductance. We conclude that high salinity reduces photosynthesis in leaves of B. parviflora, primarily by reducing diffusion of CO₂ to the chloroplast, both by stomatal closure and by changes in mesophyll structure, which decreased the conductance to CO₂ within the leaf, as well as by affecting the photochemistry of the leaves.     

A light-enhanced metabolism of sulfite in cells of Cucumis sativus L. cotyledons

The effects of light and several photosynthetic inhibitors on the rate of sulfite metabolism in cells obtained from Cucumis sativus L. cotyledons was studied. The cells were treated with 200 M Na₂SO₃ and the disappearance of sulfite was monitored using either dithiobisnitrobenzoic acid or fuchsin. The rate of sulfite disappearance in light was double the dark rate. Disalicylidene propanediamine at 1 mM increased this light-enhanced metabolism approx. 50%; neither 1 M 3,4-dichlorophenyl-N,N-dimethylurea nor 0.1 mM cyanazine, which completely inhibited CO₂-dependent oxygen evolution, affected the rate of sulfite metabolism. Addition of 200 M Na₂SO₃ to the cells partially inhibited ¹⁴CO₂ fixation. The rate of sulfite consumption by the cells did not affect this inhibition. We conclude that light-dependent sulfite metabolism is cucumber cells may utilize reduced ferredoxin generated as a result of photosynthetic electron transport. An injurious interaction between CO₂ fixation and sulfite appears to occur independently of the sulfite-metabolism process.Abbreviations DCMU 3,4-dichlorophenyl-N,N-dimethylurea - DSPD disalicylidene propanediamine - DTNB 5,5-dithiobis-(2-nitrobenzoic acid)     

Germination responses of Diplotaxis harra to temperature and salinity

Diplotaxis harra (Forssk.) Boiss, an annual herb in the family of Brassicaceae, is widely distributed in many sandy and gypseous areas in southern Tunisia. Laboratory experiments were carried out to assess the effects of temperature and salinity on seed germination and recovery responses after seed transfer to distilled water. The germination responses of the seeds in complete darkness were determined over a wide range of temperatures (5, 10, 15, 20, 25 and 30 °C) and salinities (0, 50, 100, 150 and 200 mM NaCl). Germination was inhibited by either an increase or decrease in temperature from the optimal temperature (15 °C). Highest germination percentages were obtained under non-saline conditions and an increase in NaCl concentrations progressively inhibited seed germination. Rate of germination decreased with an increase in salinity at all temperatures but comparatively higher rates were obtained at 15 °C. Salt stress decreased both the percentage and the rate of germination. An interaction between salinity and temperature yielded no germination at 200 mM NaCl. Seeds were transferred from salt solution to distilled water after 20 days, and those from low salinities recovered at all temperatures. At NaCl concentration of 200 mM, the recovery of germination was completely inhibited.     

Sodium,potassium, chloride and proline concentrations of chloroplasts isolated from a halophyte,Mesembryanthemum crystallinum L.

Concentrations of four major solutes (Na⁺, K⁺, Cl^-, proline) were determined in isolated, intact chloroplasts from the halophyte Mesembryanthemum crystallinum L. following long-term exposure of plants to three levels of NaCl salinity in the rooting medium. Chloroplasts were obtained by gentle rupture of leaf protoplasts. There was either no or only small leakage of inorganic ions from the chloroplasts to the medium during three rapidly performed washing steps involving precipitation and re-suspension of chloroplast pellets. Increasing NaCl salinity of the rooting medium resulted in a rise of Na⁺ und Cl^- in the total leaf sap, up to approximately 500 and 400 mM, respectively, for plants grown at 400 mM NaCl. However, chloroplast levels of Na⁺ und Cl^- did not exceed 160–230 and 40–60 mM, respectively, based upon a chloroplast osmotic volume of 20–30 l per mg chlorophyll. At 20 mM NaCl in the rooting medium, the Na⁺/K⁺ ratio of the chloroplasts was about 1; at 400 mM NaCl the ratio was about 5. Growth at 400 mM NaCl led to markedly increased concentrations of proline in the leaf sap (8 mM) compared with the leaf sap of plants grown in culture solution without added NaCl (proline 0.25 mM). Although proline was fivefold more concentrated in the chloroplasts than in the total leaf sap of plants treated with 400 mM NaCl, the overall contribution of proline to the osmotic adjustment of chloroplasts was small. The capacity to limit chloroplast Cl^- concentrations under conditions of high external salinity was in contrast to an apparent affinity of chloroplasts for Cl^- under conditions of low Cl^- availability.Abbreviation Chl chlorophyll     

Growth and Differentiation of Root Endodermis in Primula acaulis Jacq.

Adventitious roots of Primula acaulis Jacq. are characterized by broad cortex and narrow stele during the primary development. Secondary thickening of roots occurs through limited cambial growth together with secondary dilatation growth of the persisting cortex. Close to the root tip, at a distance of ca. 4 mm from the apex, Casparian bands (state I of endodermal development) within endodermal cells develop synchronously. During late, asynchronous deposition of suberin lamellae (state II of endodermal development), a positional effect is clearly expressed - suberization starts in the cells opposite to the phloem sectors of the vascular cylinder at a distance of 30 – 40 mm from the root tip. The formation of secondary walls in endodermis (state III of endodermal development) correlates with the beginning of secondary growth of the root at a distance of ca. 60 mm. Endodermis is the only cortical layer of primrose, where not only cell enlargement but also renewed cell division participate in the secondary dilatation growth. The original endodermal cells additionally divide anticlinally only once. Newly-formed radial walls acquire a typical endodermal character by forming Casparian bands and deposition of suberin lamellae. A network of endodermal Casparian bands of equal density develops during the root thickening by the tangential expansion of cells and by the formation of new radial walls with characteristic wall modifications. These data are important since little attention has been paid up till now to the density of endodermal network as a generally significant structural and functional trait of the root. This revised version was published online in July 2006 with corrections to the Cover Date.     

Endodermal cell-cell contact is required for the spatial control of Casparian band development in Arabidopsis thaliana

Background and Aims

Apoplasmic barriers in plants fulfil important roles such as the control of apoplasmic movement of substances and the protection against invasion of pathogens. The aim of this study was to describe the development of apoplasmic barriers (Casparian bands and suberin lamellae) in endodermal cells of Arabidopsis thaliana primary root and during lateral root initiation.

Methods

Modifications of the endodermal cell walls in roots of wild-type Landsberg erecta (Ler) and mutants with defective endodermal development – scarecrow-3 (scr-3) and shortroot (shr) – of A. thaliana plants were characterized by light, fluorescent, confocal laser scanning, transmission and cryo-scanning electron microscopy.

Key Results

In wild-type plant roots Casparian bands initiate at approx. 1600 µm from the root cap junction and suberin lamellae first appear on the inner primary cell walls at approx. 7000–8000 µm from the root apex in the region of developing lateral root primordia. When a single cell replaces a pair of endodermal and cortical cells in the scr-3 mutant, Casparian band-like material is deposited ectopically at the junction between this ‘cortical’ cell and adjacent pericycle cells. Shr mutant roots with an undeveloped endodermis deposit Casparian band-like material in patches in the middle lamellae of cells of the vascular cylinder. Endodermal cells in the vicinity of developing lateral root primordia develop suberin lamellae earlier, and these are thicker, compared wih the neighbouring endodermal cells. Protruding primordia are protected by an endodermal pocket covered by suberin lamellae.

Conclusions

The data suggest that endodermal cell–cell contact is required for the spatial control of Casparian band development. Additionally, the endodermal cells form a collet (collar) of short cells covered by a thick suberin layer at the base of lateral root, which may serve as a barrier constituting a ‘safety zone’ protecting the vascular cylinder against uncontrolled movement of water, solutes or various pathogens.     

The fine structure and development of the trabeculae and the trabecular ring in Selaginella kraussiana

In the shoot apex of Selaginella kraussiana A. Br., air spaces develop between the endodermal cells, isolating the two steles from the cortex. The endodermal cells elongate to form trabeculae connecting the two tissues. As each trabecular cell develops, cuticularization occurs on the external surface at a point mid-way along the cell and forms the trabecular ring. The plasmalemma of the trabecular cell becomes attached to the wall beneath the main concentration of the ring, resembling the connection between the plasmalemma and the Casparian strip of a primary endodermis. The trabeculae retain a functional cytoplasm throughout growth.     

Glycinebetaine biosynthesis and its control in detached secondary leaves of spinach

In secondary leaves from spinach plants pretreated in vermiculite for 24 h with 300 mM NaCl, glycinebetaine accumulated at a rate of circa 0.16 mol 100 g^-1 Chl d^-1 (2 mol g^-1 FW d^-1), about three times the rate of control plants. The soluble carbohydrate and free amino acid contents did not increase significantly following salinisation until after 4 d when the relative growth rate also decreased. Leaf proline levels remained very low throughout the experimental period. K⁺ on a tissue water basis remained constant at 200 mM while Cl^- and Na⁺ levels increased linearly to reach 175 and 100 mM respectively after 5 d of saline treatment. The osmotic pressure of leaf tissue also increased from 300 to 500 mosmol kg^-1. These experimental conditions were considered suitable to study glycinebetaine biosynthesis and its induction by salinity in the absence of marked growth inhibition or metabolic disturbance. Radioactive labelled [¹⁴C]serine, ethanolamine and choline (all 1 mol, 13.3 MBq in 10 l) were fed to detached secondary leaves via the petiole 24 h after the exposure of plants to salt. The rate of isotope incorporation into water soluble products, lipids and residue was measured over a further 24 h. The major metabolic fate of exogenous [¹⁴C]choline and [¹⁴C]ethanolamine was incorporation into glycinebetaine while less ¹⁴C-label was found in phosphatidyl choline and phosphatidyl ethanolamine. Incorporation rates were identical in control and salinised leaves and were adequate to account for observed values of glycinebetaine accumulation previously reported in spinach. In contrast the labelling of glycinebetaine from [¹⁴C]serine was twice as great in salinated plants as in the controls. These results, together with short term labelling experiment with [¹⁴C]ethanolamine using leaf slices, were consistent with the formation of glycinebetaine via serine, ethanolamine and its methylated derivatives to choline with some control being exerted at the serine level. However a flux through the phosphorylated intermediates is not excluded.From a consideration of these results and the published data on barley subjected to water stress (Hanson and Scott, 1980 Plant Physiol. 66, 342–348) there appear to be significant differences in the biosynthetic pathways in spinach and barley.Abbreviations BHT butylated hydroxytoluerte (2,6-di-tert-butyl-4-methylphenol) - C₁ one-carbon fragment - 1,2DG diglyceride moiety - DW day weight - MCW methanol-chloroform-water (12:5:1, by vol.) - PA phosphatidic acid - PC phosphatidyl choline - PMME phosphatidyl monomethylethanolamine - PDME phosphatidyl dimethylethanolamine - PE phosphatidyl ethanolamine - PPO 2,5-diphenyloxazole - POPOP 1,4-bis(5-phenyloxazoyl) benzene     

Root apoplastic barriers block Na+ transport to shoots in rice (Oryza sativa L.)

Rice is an important crop that is very sensitive to salinity. However, some varieties differ greatly in this feature, making investigations of salinity tolerance mechanisms possible. The cultivar Pokkali is salinity tolerant and is known to have more extensive hydrophobic barriers in its roots than does IR20, a more sensitive cultivar. These barriers located in the root endodermis and exodermis prevent the direct entry of external fluid into the stele. However, it is known that in the case of rice, these barriers are bypassed by most of the Na(+) that enters the shoot. Exposing plants to a moderate stress of 100 mM NaCl resulted in deposition of additional hydrophobic aliphatic suberin in both cultivars. The present study demonstrated that Pokkali roots have a lower permeability to water (measured using a pressure chamber) than those of IR20. Conditioning plants with 100 mM NaCl effectively reduced Na(+) accumulation in the shoot and improved survival of the plants when they were subsequently subjected to a lethal stress of 200 mM NaCl. The Na(+) accumulated during the conditioning period was rapidly released when the plants were returned to the control medium. It has been suggested that the location of the bypass flow is around young lateral roots, the early development of which disrupts the continuity of the endodermal and exodermal Casparian bands. However, in the present study, the observed increase in lateral root densities during stress in both cultivars did not correlate with bypass flow. Overall the data suggest that in rice roots Na(+) bypass flow is reduced by the deposition of apoplastic barriers, leading to improved plant survival under salt stress.     

Effects of salinity and nitrogen on growth,ion relations and proline accumulation in Triglochin bulbosa

The effects of salinity and nitrogen on growth, ion relations and prolineaccumulation in the monocotyledonous halophyte, Triglochin bulbosa,was investigated in hydroponic culture over 5 months. The experimentaldesign was a 3 × 3 factorial with three salinity treatments (0, 150 and 300 mol m^-3 NaCl) and three levels of N (5, 10 and 20 gml^-1 N as NaNO₃). Total and root dry biomass accumulationwere significantly affected by salinity, but not by N or N × salinityinteraction. Increase in NaCl from 0 to 150 mol m^-3 had no effecton total or root dry biomass, while further increase in salinity to 300mol m^-3 significantly reduced biomass by 21% and 25%respectively. Shoot dry biomass, which was significantly affected by N andnot by salinity, increased with increase in N from 5 to 10 gml^-1. Ion concentrations in roots and shoots were significantlyaffected by salinity, but not by N or N × salinity interaction. Theconcentration of Na⁺ and Cl^- in roots and shoots increasedprogressively with an increase in salinity, while that of K⁺ decreased. Under non-saline conditions, Na⁺/K⁺ ratios were low (0.41to 0.44) and increased significantly with an increase in salinity in both rootsand shoots. Shoot sap osmotic potentials decreased progressively with anincrease in salinity. Increase in N in the hydroponic solution from 5 to20 g ml^-1 significantly increased root and shoot N by 66%and 41% respectively. Tissue concentrations of proline were significantlyaffected by salinity and substrate N but not by N × salinity interaction. Theconcentration of proline in roots and shoots increased significantly by334% and 48%, respectively, with an increase in salinity from 0 to 300mol m^-3 NaCl. Increase in substrate N from 5 to 20 g ml^-1 significantly increased proline in roots and shoots by 66% and41% respectively. The significance of substrate N on the accumulationof proline is discussed in relation to salt tolerance.     

Salinity induced nuclear and DNA degradation in meristematic cells of soybean (Glycine max (L.)) roots

Changes in the nuclei of meristematic root cells of soybean (Glycine max (L.) Merr. cv. Acme) in response to severe salinity were studied. Root growth was inhibited by 200 mM NaCl, when 1 mM CaCl_2 was present in the culture media. Increasing CaCl_2 up to 5 mM partially prevented this inhibition. However, inhibition also occurred with 100~mM NaCl without CaCl_2. We examined the meristematic cells under a series of NaCl treatments. Nuclear deformation of the cells occurred with 24 h of 150 mM or higher NaCl, and was followed by degradation of nuclei in the apical region of the root. TEM observation and agarose gel electrophoretic analysis confirmed that root tip nuclear DNA deformed or degraded with 150 mM or higher NaCl concentrations.     

Re-organization of microtubules during preprophase band development inAdiantum protonemata

Summary Microtubule organization during preprophase band development was investigated using immunofluorescence microscopy in filamentous protonemal cells (approx. 600 m in length, 20 m in width) ofAdiantum capillus-veneris L. Protonemata pre-cultured under red light were transferred to continuous blue light or total darkness to induce synchronous cell division. Preprophase bands were found under both light conditions. In an early stage of development, the preprophase band which is transverse to the cell axis overlapped with an interphase cortical array of microtubules which is random or parallel to the cell axis. The interphase cortical array disappeared thereafter. While the width of the preprophase band became narrow during development under dark conditions, under blue light conditions it did not.Spatial and temporal aspects of the disappearance of the interphase cortical array of microtubules were also investigated. The interphase cortical array began to disappear at nearly the same time as the beginning of preprophase band formation. Under blue light, the disruption of cortical microtubules started at approx. 150 m from the tip (approx. 120 m from the nucleus), and spread toward the tip as far as the nuclear region and toward the base to an area approx. 300–400 m from the tip. Cortical microtubules remained in the basal part of the protonema. The pattern of disappearance between the tip and nucleus could not be determined. Under dark conditions, the pattern of the disappearance of cortical microtubules was somewhat different in many cells from that encountered with exposure to blue light. Microtubules first re-oriented from longitudinal to transverse, and then gradually disappeared. In some cells, the pattern of disappearance was similar to that observed under blue light.Abbreviations DAPI 4, 6-diamidino-2-phenylindole - ICM interphase cortical microtubules - PBS phosphate buffered saline - PPB preprophase band - MT microtubule     

Regulation of a plant aquaporin by a Casparian strip membrane domain protein‐like

The absorption of soil water by roots allows plants to maintain their water status. At the endodermis, water transport can be affected by initial formation of a Casparian strip and further deposition of suberin lamellas and regulated by the function of aquaporins. Four Casparian strip membrane domain protein‐like (CASPL; CASPL1B1, CASPL1B2, CASPL1D1, and CASPL1D2) were previously shown to interact with PIP2;1. The present work shows that CASPL1B1, CASPL1B2, and CASPL1D2 are exclusively expressed in suberized endodermal cells, suggesting a cell‐specific role in suberization and/or water transport regulation. When compared with wild‐type plants, and by contrast to caspl1b1*caspl1b2 double loss of function, caspl1d1*caspl1d2 double mutants showed, in some control or NaCl stress experiments and not upon abscisic acid (ABA) treatment, a weak enlargement of the continuous suberization zone. None of the mutants showed root hydraulic conductivity (Lp_r) phenotype, whether in control, NaCl, or ABA treatment conditions. The data suggest a slight negative role for CASPL1D1 and CASPL1D2 in suberization under control or salt stress conditions, with no major impact on whole root transport functions. At the molecular level, CASPL1B1 was able to physically interact with PIP2;1 and potentially could influence the regulation of aquaporins by acting on their phosphorylated form.     

Changing proton concentrations at the surfaces of gravistimulated Phleum roots

The pH patterns at the surfaces of both vertically growing roots of Phleum pratense L. and roots tilted by 45° were recorded using H ⁺-sensitive microelectrodes. During vertical growth the root cap exhibited lower pH values than the meristematic zone. The highest pH values were found at the border between meristematic and elongation zones. In the apical part of the elongation zone the values strongly decreased basipetally. They reached a minimum value of pH 5.4–5.5 (medium pH of about 6.0) at a distance of 700 m from the root tip. This region of strongest acidification usually coincided with that of the highest relative rates of elongation. The region of the first visible curvature following gravistimulation was positioned at 100–200 m more apically. The pH values increased in the basal elongation zone towards the mature zone. The H⁺-flux pattern around a vertically growing Phleum root was characterized by high influxes in the meristematic zone and smaller effluxes in the elongation zone. Tilting the root by 45° induced changes in the pH values of the upper and lower sides of a Phleum root. At a distance of 300–500 m from the root tip, the upper side was strongly acidified while the pH of the lower side slightly increased compared with the values during vertical orientation. pH differences of up to 0.9 pH units between the two sides of a root were detected. These differences decreased basipetally and could not be measured more distant than 700–800 m from the tip. Compared with a vertically growing root, the H⁺-flux pattern of a Phleum root tilted by 45° exhibited effluxes on the entire upper organ flank while the pattern was scarcely altered on the lower side. The curvature-initiating zone in Phleum roots is positioned within that section of the root in which pH changes occur after tilting. The region of highest pH differences, however, is nearer to the apex of the root than the curvature-initiating zone. The pH changes began 8.2 min after a root had been tilted. The bending process started after 17.2 min, i.e. after double the time needed for differential acidification. After reorienting a root, which had just begun to bend, to its previous vertical position the inversion of the pH gradient could be measured within the same mean time of about 8 min. This is again significantly earlier than the beginning of the rebending process. The results indicate that, during the graviresponse, ionic movements occur much earlier than the changes in hormonal activities reported in the literature.Abbreviation CIZ curvature-initiating zone A preliminary report was presented at the 29th Plenary Meeting of the Committee on Space Research (COSPAR) in Washington D.C., USA, 28 Aug – 5 Sept 1992 (Zieschang and Sievers 1993)This work was supported by the Deutsche Forschungsgemeinschaft. We thank Professor H. Felle (Botanisches Institut, Universität Gießen, Gießen, FRG) for practical instructions concerning the method of H⁺-sensitive microelectrodes and Professor W. Simonis (Botanisches Institut, Universität Würzburg, Würzburg, FRG) for allowing to use the microelectrode amplifier.     

Anatomical Changes in Prosopis tamarugo Phil. Seedlings Growing at Different Levels of NaCl Salinity

Seedlings of Prosopis tamarugo were grown in artificial substrateswith additions, of 200, 400 and 600 mM NaCl, and without salttreatment. Salinity induced anatomical changes in the roots,stems and leaflets. The diameters of the roots of seedlingsgrown in the increasing salt concentrations (up to 400 mM) wereprogressively smaller and differentiation of the stelar tissueswas delayed. At an NaCl concentration of 600 mM, the root structurewas strongly altered. On the contrary, stem diameter increasedas salinity rose. In the stems of seedlings grown in a concentrationof 200 mM NaCl, secondary xylem differentiation appeared earlierthan in the controls. At a concentration of 400 mM NaCl, disorganizationof the vascular cylinder of the stem was evident. Leaflets ofseedlings grown in 200 mM NaCl showed a delay in structuraldifferentiation: no water-storage cells or ‘special cells’could be seen. The leaflets from plants grown in 400 mM NaCl,had larger numbers of intercellular air spaces; probably anindication of the beginning of tissue disorganization. A progressivedecrease in cell size of leaflets as salinity rose was alsodemonstrated. Prosopis tamarugo Phil., seedling, salinity, anatomical changes