Search

Background The fern Dicranopteris linearis is a hyperaccumulator of rare earth elements (REEs), aluminium (Al) and silicon (Si). However, the physiological mechanisms of tissue-level tolerance of high concentrations of REE and Al, and possible interactions with Si, are currently incompletely known. Methods A particle-induced X-ray emission (μPIXE) microprobe with the Maia detector, scanning electron microscopy with energy-dispersive spectroscopy and chemical speciation modelling were used to decipher the localization and biochemistry of REEs, Al and Si in D. linearis during uptake, translocation and sequestration processes. Results In the roots >80 % of REEs and Al were in apoplastic fractions, among which the REEs were most significantly co-localized with Si and phosphorus (P) in the epidermis. In the xylem sap, REEs were nearly 100 % present as REEH3SiO42+, without significant differences between the REEs, while 24–45 % of Al was present as Al-citrate and only 1.7–16 % Al was present as AlH3SiO42+. In the pinnules, REEs were mainly concentrated in necrotic lesions and in the epidermis, and REEs and Al were possibly co-deposited within phytoliths (SiO2). Different REEs had similar spatial localizations in the epidermis and exodermis of roots, the necrosis, veins and epidermis of pinnae of D. linearis. Conclusions We posit that Si plays a critical role in REE and Al tolerance within the root apoplast, transport within the vascular bundle and sequestration within the blade of D. linearis.

Background and Aims In the present study, we show that development of endodermis and exodermis is sensitively regulated by water accessibility. As cadmium (Cd) is known to induce xeromorphic effects in plants, maize roots were exposed also to Cd to understand the developmental process of suberin lamella deposition in response to a local Cd source. Methods In a first experiment, maize roots were cultivated in vitro and unilaterally exposed to water-containing medium from one side and to air from the other. In a second experiment, the roots were placed between two agar medium layers with a strip of Cd-containing medium attached locally and unilaterally to the root surface. Key Results The development of suberin lamella (the second stage of exodermal and endodermal development) started asymmetrically, preferentially closer to the root tip on the side exposed to the air. In the root contact with Cd in a spatially limited area exposed to one side of the root, suberin lamella was preferentially developed in the contact region and additionally along the whole length of the root basipetally from the contact area. However, the development was unilateral and asymmetrical, facing the treated side. The same pattern occurred irrespective of the distance of Cd application from the root apex. Conclusions These developmental characteristics indicate a sensitive response of root endodermis and exodermis in the protection of vascular tissues against abiotic stresses.

Background and aims Root absorptive characteristics rely on the presence of apoplastic barriers. However, little is known about the establishment of these barriers within a complex root system, particularly in a major portion of them - the lateral roots. In Zea mays L., the exodermis differentiates under the influence of growth conditions. Therefore, the species presents a suitable model to elucidate the cross-talk among environmental conditions, branching pattern and the maturation of barriers within a complex root system involved in the definition of the plant-soil interface. The study describes the extent to which lateral roots differentiate apoplastic barriers in response to changeable environmental conditions. Methods The branching, permeability of the outer cell layers and differentiation of the endo- and exodermis were studied in primary roots and various laterals under different types of stress of agronomic importance (salinity, heavy metal toxicity, hypoxia, etc.). Histochemical methods, image analysis and apoplastic tracer assays were utilized. Key results The results show that the impact of growth conditions on the differentiation of both the endodermis and exodermis is modulated according to the type/diameter of the root. Fine laterals clearly represent that portion of a complex root system with a less advanced state of barrier differentiation, but with substantial ability to modify exodermis differentiation in response to environmental conditions. In addition, some degree of autonomy in exodermal establishment of Casparian bands (CBs) vs. suberin lamellae (SLs) was observed, as the absence of lignified exodermal CBs did not always fit with the lack of SLs. Conclusions This study highlights the importance of lateral roots, and provides a first look into the developmental variations of apoplastic barriers within a complex root system. It emphasizes that branching and differentiation of barriers in fine laterals may substantially modulate the root system-rhizosphere interaction.

† Background and Aims Most studies of exodermal structure and function have involved species with a uniseriate exodermis. To extend this work, the development and apoplastic permeability of Iris germanica roots with a multiseriate exodermis (MEX) were investigated. The effects of different growth conditions on MEX maturation were also tested. In addition, the exodermises of eight Iris species were observed to determine if their mature anatomy correlated with habitat. † Methods Plants were grown in soil, hydroponics (with and without a humid air gap) or aeroponics. Roots were sectioned and stained with various dyes to detect MEX development from the root apical meristem, Casparian bands, suberin lamellae and tertiary wall thickenings. Apoplastic permeability was tested using dye (berberine) and ionic (ferric) tracers. † Key Results The root apical meristem was open and MEX development non-uniform. In soil-grown roots, the exodermis started maturing (i.e. Casparian bands and suberin lamellae were deposited) 10 mm from the tip, and two layers had matured by 70 mm. In both hydro- and aeroponically grown roots, exodermal maturation was delayed. However, in areas of roots exposed to an air gap in the hydroponic system, MEX maturation was accelerated. In contrast, maturation of the endodermis was not influenced by the growth conditions. The mature MEX had an atypical Casparian band that was continuous around the root circumference. The MEX prevented the influx and efflux of berberine, but had variable resistance to ferric ions due to their toxic effects. Iris species living in well-drained soils developed a MEX, but species in water-saturated substrates had a uniseriate exodermis and aerenchyma. † Conclusions MEX maturation was influenced by the roots’ growth medium. The MEX matures very close to the root tip in soil, but much further from the tip in hydro- and aeroponic culture. The air gap accelerated maturation of the second exodermal layer. In Iris, the type of exodermis was correlated with natural habitat suggesting that a MEX may be advantageous for drought tolerance.

Using epifluorescent and histochemical techniques, we examined anatomical differences in the shoot organs of Typha latifolia, T. angustifolia and T. glauca. The leaf lamina of T. latifolia and T. glauca had enlarged epidermal cells and a thickened cuticle above the subepidermal vascular bundles; that of T. angustifolia lacked these characteristics. Leaf sheaths were similar among the species and all lacked the epidermal thickenings found in the lamina. The fertile stems had typical scattered vascular bundles with a band of fibres that was most prominent in T. glauca. The sterile stems were only 1 cm in length and contained a multiseriate hypodermis and a uniseriate endodermis over part of their length. The rhizomes were similar except for a pronounced band of fibres surrounding the central core in T. angustifolia. The rhizome was also characterized by an outer cortical region with a large multiseriate hypodermis/exodermis and a uniseriate endodermis with Casparian bands, suberin lamellae and secondarily thickened walls.

Background and aims Roots have complex anatomical structures, and certain localized cell layers develop suberized apoplastic barriers. The size and tightness of these barriers depend on the growth conditions and on the age of the root. Such complex anatomical structures result in a composite water and solute transport in roots. Methods Development of apoplastic barriers along barley seminal roots was detected using various staining methods, and the suberin amounts in the apical and basal zones were analysed using gas chromatography-mass spectometry (GC-MS). The hydraulic conductivity of roots ( Lp r ) and of cortical cells ( Lp c ) was measured using root and cell pressure probes. Key results When grown in hydroponics, barley roots did not form an exodermis, even at their basal zones. However, they developed an endodermis. Endodermal Casparian bands first appeared as 'dots' as early as at 20 mm from the apex, whereas a patchy suberin lamellae appeared at 60 mm. The endodermal suberin accounted for the total suberin of the roots. The absolute amount in the basal zone was significantly higher than in the apical zone, which was inversely proportional to the Lp r . Comparison of Lp r and Lp c suggested that cell to cell pathways dominate for water transport in roots. However, the calculation of Lp r from Lp c showed that at least 26 % of water transport occurs through the apoplast. Roots had different solute permeabilities ( P sr ) and reflection coefficients ( σ sr ) for the solutes used. The σ sr was below unity for the solutes, which have virtually zero permeability for semi-permeable membranes. Conclusions Suberized endodermis significantly reduces Lp r of seminal roots. The water and solute transport across barley roots is composite in nature and they do not behave like ideal osmometers. The composite transport model should be extended by adding components arranged in series (cortex, endodermis) in addition to the currently included components arranged in parallel (apoplastic, cell to cell pathways).

Structural features of the mature root cortex and its apoplastic permeability to dyes have been determined for two dicotyledonous wetland plants of differing habitats: Nymphaea odorata, growing rooted in water and mud, and Caltha palustris, growing in temporal wetlands among cattails. In mature roots, movement of the apoplastic dyes, berberine and safranin, into the roots was blocked at the hypodermis, indicating the presence of an exodermis. A hypodermis with an exodermis, i.e. Casparian bands in the outermost uniseriate layer plus suberin lamellae, is present in both species. In N. odorata, hypodermal walls are further modified with cellulosic secondary walls. Roots of N. odorata and C. palustris have an endodermis with Casparian bands only. A honeycomb aerenchyma is produced by differential expansion in N. odorata and includes astrosclereids and diaphragms, while roots of C. palustris have no aerenchyma, but some irregular lacunae are found in old roots. These aspects of cortex structure are related to an open meristem organization, with unusual patterns of cell divisions in certain ground meristem cells (called semi-regular hexagon cells) of N. odorata. The correlation between aerenchyma pattern and hypodermal structure appears to be related to habitat differences.

Water loss from roots back into drying soil is a problem of practical importance in plants growing under conditions of very low substrate water potential, such as dry or saline areas. Root exodermis is relatively impermeable and has been suggested to play a protective role against water loss. The relative water retention ability was compared in root segments from exodermal (maize, onion, sunflower, Rhodes grass and sorghum) and non-exodermal species (Pisum satiaum, Vicia faba and wheat). Apical and basal segments from exodermal roots, with dierent degrees of exodermis development, were also compared, as were segments from sorghum roots in which the exodermis thickness had been modified by subjecting the plants to a 30 d water stress treatment. Water retention was significantly higher in segments from exodermal roots. In each root, water loss was higher in apical than in basal segments, regardless of the presence of exodermis. In sorghum, prolonged drought treatment increased exodermis thickening in nodal roots, however, no dierences in rates of water loss were observed in segments obtained from control and droughted plants. Soil sheaths formed around roots of Rhodes grass growing in very dry soil with the epidermis adhering tightly to the sheath. In plants growing in the field, soil sheaths may be more eective than the exodermis in preventing root water loss. # 1999 Annals of Botany Company

The fine lateral roots ofLysinema ciliatumR. Br., an epacrid from habitats subject to periodic drought in Western Australia, are hair roots resembling those of Ericaceae. The finest (ultimate) hair roots have a cortex consisting only of an endodermis and an exodermis. Both layers have Casparian strips on the radial walls. The exodermis develops to state III very close to the root tip, showing wall thickening and a suberized lamella encircling each cell. In many roots collected after tip-growth had ceased and the tip had fully differentiated this suberized exodermis completely encircled the apex. In older hair roots the epidermis collapses or is sloughed off leaving the suberized exodermis as the outermost layer. The very fine hair roots have a very small stele containing only one xylem tracheid, and phloem consisting of a single sieve element with companion cell. The very small diameter of the single tracheid indicates a high resistance to water flow along the hair roots. This may tend to conserve soil moisture in the region of the hair roots, leading to improved survival and prolonged function of mycorrhizas in the field.

Mycorrhizal root systems ofLeucopogon parviflorus(Andr.) Lindl. were collected from wild populations at three sites on the coast of New South Wales, Australia and examined by light and electron microscopy. The structure of the hair roots is typical of the family, there being an epidermal layer in which ericoid mycorrhizas are formed, two cortical layers (an exodermis and endodermis) and a very small stele. The colour, size and coil structure of the fungal symbionts indicate that there were at least two different fungi that consistently formed ericoid mycorrhizal structures at these sites. Transmission electron microscopy of the endophytes showed only ascomycete fungi. Plants from two of the populations were used for fungal isolations. Fungi were isolated by incubating surface sterilized hair-root pieces in a solution of bovine serum albumin with penicillin and streptomycin. Twenty-one different culture types were obtained, four of which were common to both sites. Two of the common culture types were dark, sterile, slow-growing cultures similar to the ericoid endophyteHymenoscyphus ericae(Read) Korf & Kernan.

Hair roots of Lysinema ciliatum R. Br. and some other Epacridaceae have thick-walled cells in the epidermis. These are preferentially colonized with mycorrhizal fungi. Individual epidermal cells containing hyphal coils separate at the middle lamella and are released into the soil. Other colonized cells remain attached to the roots, usually in groups, surrounded by bare exodermis, where epidermal cells have either collapsed or been sloughed off. It is suggested that these colonized thick walled cells can serve to prolong the mycorrhizal association and to infect new hair roots as these emerge. The thick wall has a very specialized structure and composition and could have a number of roles, either acting as a substrate or protective coat or in controlling water status and uptake. Young hair-roots are surrounded by a mucilage sheath that is similar in appearance to that in Ericaceae and apparently produced by root cap cells, not the epidermis.

Background and aims Many wetland species form aerenchyma and a barrier to radial O(2) loss (ROL) in roots. These features enhance internal O(2) diffusion to the root apex. Barrier formation in rice is induced by growth in stagnant solution, but knowledge of the dynamics of barrier induction and early anatomical changes was lacking. Methods ROL barrier induction in short and long roots of rice (Oryza sativa L. 'Nipponbare') was assessed using cylindrical root-sleeving O(2) electrodes and methylene blue indicator dye for O(2) leakage. Aerenchyma formation was also monitored in root cross-sections. Microstructure of hypodermal/exodermal layers was observed by transmission electron microscopy (TEM). Key results In stagnant medium, barrier to ROL formation commenced in long adventitious roots within a few hours and the barrier was well formed within 24 h. By contrast, barrier formation took longer than 48 h in short roots. The timing of enhancement of aerenchyma formation was the same in short and long roots. Comparison of ROL data and subsequent methylene blue staining determined the apparent ROL threshold for the dye method, and the dye method confirmed that barrier induction was faster for long roots than for short roots. Barrier formation might be related to deposition of new electron-dense materials in the cell walls at the peripheral side of the exodermis. Histochemical staining indicated suberin depositions were enhanced prior to increases in lignin. Conclusions As root length affected formation of the barrier to ROL, but not aerenchyma, these two acclimations are differentially regulated in roots of rice. Moreover, ROL barrier induction occurred before histochemically detectable changes in putative suberin and lignin deposits could be seen, whereas TEM showed deposition of new electron-dense materials in exodermal cell walls, so structural changes required for barrier functioning appear to be more subtle than previously described.

• Background and Aims Free-hand sectioning of living plant tissues allows fast microscopic observation of internal structures. The aim of this study was to improve the quality of preparations from roots with suberized cell walls. A whole-mount procedure that enables visualization of exo- and endodermal cells along the root axis was also established. • Methods Free-hand sections were cleared with lactic acid saturated with chloral hydrate, and observed with or without post-staining in toluidine blue O or aniline blue. Both white light and UV light were used for observation. Lactic acid was also used as a solvent for berberine, and fluorol yellow for clearing and staining the samples used for suberin observation. This procedure was also applied to whole-mount roots with suberized celllayers. • Key Results Clearing of sections results in good image quality to observe the tissue structure and cell walls compared with non-cleared sections. The use of lactic acid as a solvent for fluorol yellow proved superior to previously used solvents such as polyethylene glycol–glycerol. Clearing and fluorescence staining of thin roots such as those of Arabidopsis thaliana were succesful for suberin visualization in endodermal cells within whole-mount roots. For thicker roots, such as those of maize, sorghum or tea, this procedure could be used for visualizing the exodermis in a longitudinal view. Clearing and staining of peeled maize root segments enabled observation of endodermal cell walls. • Conclusions The clearing procedure using lactic acid improves the quality of images from free-hand sections and clearings. This method enhaces the study of plant root anatomy, in particular the histological development and changes of cell walls, when used in combination with fluorescence microscopy.

The anatomical changes in the different tissues during root contraction were studied in two species of Eucomis (Liliaceae). It is evident that the shortening of the root is brought about by radial and tangential broadening and longitudinal shortening of the perivascular cortical cells. This tissue is hence called contractile parenchyma. As a result of this contraction the cell walls of the exodermis, endodermis, pericycle, phloem and pith become buckled longitudinally while the annular and spiral thickenings of the xylem are pressed together.

All the tissues in the contracted region of the contractile root of Narcissus become adjusted to the shortening of the root, which is brought about by vertical contraction and radial expansion of the inner cortical cells. Consequently the outer cortex including the exodermis is lifted passively in large wrinkles over the surface, while the vertical walls of cells in the stele become undulated or indented as in the case of the vessels. These phenomena are primarily brought about by the mechanical process of lifting and pulling due to the shortening and radial expansion of the inner cortical tissue. The possible mechanism of contraction of this cortical tissue is discussed.