Ion Channels in the Xylem Parenchyma of Barley Roots (A Procedure to Isolate Protoplasts from This Tissue and a Patch-Clamp Exploration of Salt Passageways into Xylem Vessels

To identify mechanisms for the simultaneous release of anions and cations into the xylem sap in roots, we investigated voltage-dependent ion conductances in the plasmalemma of xylem parenchyma cells. We applied the patch-clamp technique to protoplasts isolated from the xylem parenchyma by differential enzymic digestion of steles of barley roots (Hordeum vulgare L. cv Apex). In the whole-cell configuration, three types of cation-selective rectifiers could be identified: (a) one activated at membrane potentials above about -50 mV; (b) a second type of outward current appeared at membrane potentials above +20 to +40 mV; (c) below a membrane potential of approximately -110 mV, an inward rectifier could be distinguished. In addition, an anion-specific conductance manifested itself in single-channel activity in a voltage range extending from about -100 to +30 mV, with remarkably slow gating. In excised patches, K+ channels activated at hyperpolarization as well as at depolarization. We suggest that salt is released from the xylem parenchyma into the xylem apoplast by simultaneous flow of cations and anions through channels, following electrochemical gradients set up by the ion uptake processes in the cortex and, possibly, the release and reabsorption of ions on their way to the xylem.     

Zostera capensis setchell: Root structure in relation to function

Root structure of the seagrass Zostera capensis Setchell was investigated by light and electron microscopy. Roots possess conspicuous root hairs which greatly increase the surface area available for absorption. Exodermal cells abutting root-hair bases possess transfer cell characteristics. The strategic location of these cells suggests that they participate in absorptive and/or transfer processes between the epidermis and cortex. Vascular parenchyma cells within the stele also possess transfer cell features. Wall ingrowths of these cells about xylem elements, sieve tubes, companion cells and other vascular parenchyma cells, suggesting that they play a role in absorptive and/or transfer processes between the stele and cortex. Apoplastic barriers in the form of suberin lamellae and Casparian bands occur in walls of both the exodermis and endodermis. However, plasmodesmata perforate the suberin lamellae in these walls, and a symplastic pathway can be traced from the root hairs to vascular parenchyma transfer cells contiguous with conducting elements of the stele. The occurrence of wall ingrowths adjacent to xylem elements implies that transfer processes occur between vascular transfer cells and xylem. Although reduced, xylem could therefore play a role in transport. Structural evidence obtained in this study supports the role of the roots in absorptive processes and shows pathways available for transport from the water column to the conducting tissues of the root.     

Ultrastructure of xylem parenchyma cells of barley roots in relation to ion transport to the xylem

Summary The structure of xylem parenchyma cells is examined in relation to transport of ions through the root. Measurement of uptake of ⁸⁶Rb⁺ and its transport through the root at different distances from the apex show that this is a general activity along the length of the root and not confined to a limited region. Thus transport through the root is not stopped by removal of that part of the root tip containing metaxylem vessels with living contents. The structure of xylem parenchyma appears to be suitable for involvement in ion transport from the stele to the xylem. At 1 cm behind the tip, where metaxylem vessels have no living contents but ion uptake and transport are going on at high rates, xylem parenchyma cells are rich in cytoplasm with extensive rough endoplasmic reticulum and well-developed mitochondria. Their cell walls contain numerous plasmodesmata, establishing the possibility of a symplastic pathway across the stele up to the vessels. The results are discussed in relation to regulation of ion transport to the xylem vessels in roots.Dedicated to Professor O. Stocker on the occasion of his 85th birthday.     

Ultrastructure of the haustorial interface and apoplastic continuum between host and the root hemiparasiteOlax phyllanthi (Labill.) R. Br. (Olacaceae)

Summary Structural features of haustorial interface parenchyma of the root hemiparasiteOlax phyllanthi are described. Walls contacting host xylem are thickened non-uniformly with polysaccharides, not lignin, and show only a thin protective wall layer when abutting pits in walls of host xylem vessels or tracheids. Lateral walls of interface parenchyma exhibit an expanded middle layer of open fibrillar appearance, sometimes with, but mostly lacking adjoining layers of dense wall material. Free ribosomes and rough endoplasmic reticulum are prominent and occasional wall ingrowths present. Experiments involving transpirational feeding of the apoplast tracers lanthanum nitrate or uranyl acetate to host roots cut below haustorial connections, indicate effective apoplastic transfer from host to parasite root via the haustorium. Deposits of the tracers suggest a major pathway for water flow through host xylem pits, across the thin protective wall layer, and thence into the haustorium via the electronopaque regions of the terminal and lateral walls of the contact parenchyma. Graniferous tracheary elements and walls of parenchyma cells of the body of the haustorium appear to participate in tracer flow as do walls of cortical cells, stele parenchyma and xylem conducting elements of the parasite root, suggesting that both vascular and non-vascular routes are involved in extracytoplasmic transfer of xylem sap from host to parasite. The Casparian strip of the endodermis and the suberin lamella of the exodermis of theOlax root act as barriers to flow within the system.     

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.     

Anatomical Response of Grain Sorghum Roots to Meloidogyne incognita acrita

The cotton root-knot nematode, Meloidogyne incognita acrita, reproduced on the roots of grain sorghum, causing syncytia in the cortex or stele of lateral roots. Giant cells developed either singly with few nuclei or in groups with many nuclei. Giant cells that developed in groups appeared the same as those which developed singly. The pericycle and endodermis were interrupted at the site of nematode invasion. Large areas of these tissues were absent for one-third of the circumference of the stele and extended 1.5 mm longitudinally along the root. In the area where pericycle and endodernris were absent, the parenchyma of the cortex extended to the vascular elements, and abnormal xylem surrounding giant cells extended into the region of the cortex. Root-knot galls appeared on sorghum roots as elongate swellings, discrete knots, or swellings with root proliferation. Galls were not observed on brace roots.     

Radial salt transport in corn roots

Primary roots of solution-grown, 5-day-old or 6-day-old seedlings of corn (Zea mays L.) 10 to 14 cm in length were used to study radial salt transport. Measurements were made of the volume of root pressure exudation, salt concentration of the exudate, and rate of salt movement into the xylem exudate. The ³²P uptake, O₂ consumption, and dehydrogenase activity of the root cortex and stele also were studied.

These roots produced copious root pressure exudate containing 4 to 10 times the concentration of ³²P in the external solution. Freshly separated stele from 5-day-old roots accumulated ³²P as rapidly as the cortex from which it was separated and the stele of intact roots also accumulated ³²P. Separated stele has a higher oxygen uptake than cortex. It also shows strong dehydrogenase activity with the tetrazolium test. The high oxygen consumption, ³²P uptake and strong dehydrogenase activity indicate that the cells of the stele probably play a direct role in salt transport.

These data raise doubts concerning theories of radial salt transport into the xylem based on the assumption that the stele is unable to accumulate salt vigorously.

     

植物对硅的吸收转运机制研究进展

硅(Si)能缓解生物与非生物胁迫对植物的毒害作用,Si的吸收转运是由Si转运蛋白介导的.最近,多个Si转运蛋白(Lsi)基因相继在水稻、大麦和玉米中被克隆出来,并在Si的吸收转运机制方面取得了很大进展.水稻OsLsi在根组织中呈极性分布,OsLsi1定位在根外皮层和内皮层凯氏带细胞外侧质膜,负责将外部溶液中的单硅酸转运到皮层细胞内.OsLsi2定位在凯氏带细胞内侧质膜,在外皮层中负责将Si输出到通气组织质外体中,在内皮层与OsLsi1协同作用将Si转运到中柱中.导管中的Si通过蒸腾流转运到地上部,再由定位在叶鞘和叶片木质部薄壁细胞靠近导管一侧的OsLsi6负责木质部Si的卸载和分配.在大麦和玉米中,ZmLsi1/HvLsi1定位在根表皮和皮层细胞外侧质膜负责Si的吸收,然后Si通过共质体途径被转运到内皮层凯氏带细胞中,再由ZmLsi2/HvLsi2输出转运到中柱中.ZmLsi6在细胞中的定位和活性与OsLsi6相似,推测其可能具有类似的功能,但大麦Lsi6至今未见报道.所以,Si转运机制仍需要进一步研究.     

Exogenous zeatin accumulation in wheat-root cells and its role in regulation of cytokinin transport

It was shown that the cytokinin content in the xylem sap of a wheat plant treated with exogenous zeatin was about ten times lower than in the nutrient solution in 24 h. Cytokinins were accumulated in roots rather shoots of treated plants. These data demonstrate the existence of a barrier in the cytokinin pathway from the nutrient solution to plant shoots. The deposition of lignin and suberin in stele detected with Sudan @III is enlarged with an increase in the distance from the tip of the root. The augmented content of suberin and lignin coincided with reduced cytokinin immunolabeling in root cells revealed by monoclonal antibodies to cytokinin and secondary gold-labeled antibodies. The accumulation of exogenous cytokinin in root stele cells shows that Casparian bands are not the only barrier on the cytokinin pathway to plant shoots. Intensive cytokinin immunolabeling in parenchyma cells surrounding stele vessels indicates the accumulation of cytokinin by these cells and suggests that there are mechanisms that limit the hormone loading in xylem vessels during transport to the shoot. The role of cytokinin transporters in this process is discussed.     

植物钙素吸收和运转

近年来,钙素在植物体内的吸收和运输研究主要集中在细胞和分子水平,但整株水平上的研究也同样重要.整株水平上的钙吸收和运输包括根细胞的钙吸收、钙离子横向穿过根系并进入木质部、在木质部运输、从木质部移出并进入叶片或果实及在叶片或果实中运转分配等环节,既经过质外体也穿越共质体.钙离子通道、Ca2 -ATP酶和Ca2 /H 反向转运器等参与根细胞的钙吸收.在钙离子横向穿根进入木质部的过程中,需要穿越内皮层和木质部薄壁细胞组织.根系内皮层凯氏带阻挡了Ca2 沿质外体途径由内皮层外侧向内侧的移动,部分Ca2 由此通过离子通道流进内皮层细胞而转入共质体并到达木质部薄壁细胞组织,而由木质部薄壁细胞组织进入中柱质外体可能需要Ca2 -ATP酶驱动;还有一些Ca2 由内皮层细胞运出,沿内皮层内侧的质外体途径进入木质部导管,并通过导管运向枝干.钙离子以螯合态的形式在枝干导管运输;水流速率是影响钙离子沿导管运输的关键因子.钙离子在果实和叶片中的运输和分配不仅通过质外体途径也通过共质体途径.     

THE DEVELOPMENTAL ANATOMY OF OPUNTIA BASILARIS. I. EMBRYO,ROOT, AND TRANSITION ZONE

The large seeds of Opuntia basilaris Engelm. & Bigel. show an unusually high percentage of germination, followed by a rapid development of the seedling during the first 30 days of growth. The primary root has six xylem arms alternating with six phloem poles around a large central pith. Development of metaxylem opposite each of the primary phloem poles results in the formation of eight collateral bundles. Secondary and tertiary roots have four xylem and phloem poles with xylem developing to the center of the stele. The transition zone is characterized by a gradual disappearance of all but two of the primary xylem arms of the root. Metaxylem development in the central portion of the transition zone interconnects the protoxylem poles forming a primary xylem cylinder around the central pith. The collateral bundles pass through the transition zone essentially without change.     

Barriers to the radial diffusion of ions in maize roots

Summary The water-extractable and ion-exchangeable fractions of the free space of maize roots for sodium ions has been determined. The free space of whole roots, excised roots and isolated stelar and cortical tissues, has been compared and the results examined for any evidence of a barrier between the cortex and the stele. Similarly the free space of whole roots and excised roots, from which the epidermal and outer cortical cells have been removed by shaving, has been compared and the results examined for any evidence of an epidermal barrier.Whole roots gave a free space value some 20% lower than excised roots. It was calculated that this difference could be accounted for if the cortical tissues only were considered in estimating the whole root value, that is if the stele was considered as participating in the excised root but not in the whole root. Samples in which isolated cortical and stelar material were measured together, or separately and the value calculated, gave similar values to those obtained for excised roots. These results are interpreted as evidence that a barrier to free diffusion exists between the cortical and the stelar tissues at or near the endodermis. Shaving both whole and excised roots increased the free space by about 35%. However, as this value was similar for both, it was concluded that the increase was due to the contribution of damaged cortical cells and does not indicate that the epidermis is an effective barrier to the diffusive entry of sodium ions into the root.     

Development and Structure of the Haustorium of the Parasite Rhamphicarpa fistulosa (Scrophulariaceae)

Rhamphicarpa fistulosa (Hochst.) Benth. (Scrophulariaceae), a parasite of African cereals, develops secondary haustoria which penetrate the roots of the host plant. Light and electron microscopy have been used to study the structure and development of haustoria in this species, which, until now, have not been well characterized. Haustoria are initiated in the hypodermis of the parasite roots. A meristematic strand is developed between the parasite root stele and the host-parasite interface. From this strand, cells differentiate into xylem elements after penetration of the host root. Xylem differentiation follows an acropetal pattern. Mature haustoria are characterized by a continuous xylem bridge between water conducting elements of parasite and host. A detailed study of the hostparasite interface revealed the presence of collapsed and compressed host cells at the lateral interface (between parasite cells and host cortex), whereas the central interface between parasite cells and the host stele is almost devoid of host cell remnants. Implications of these observations for the penetration mechanisms are discussed.     

Permeability of Plant Young Root Endodermis to Cu Ions and Cu-Citrate Complexes in Corn and Soybean

The non-selective apoplastic passage of Cu and Cu-citrate complexes into the root stele of monocotyledonous corn and dicotyledonous soybean was investigated using an inorganic-salt-precipitation technique. Either Cu ions or Cu-citrate complexes were drawn into root through the apoplast from the root growth medium, and K₄[Fe(CN)₆] was subsequently perfused through xylem vessels or the entire root cross section. Based on microscopic identification of the reddish-brown precipitates of copper ferrocyanide in the cell walls of the xylem of corn and soybean roots, Cu²⁺ passed through the endodermal barrier into the xylem of both species. When the solution containing 200 μM CuSO₄ and 400 μM sodium citrate (containing 199.98 μM Cu-citrate, 0.02 μM Cu²⁺) was drawn via differential pressure gradients into the root xylem while being perfused with K₄[Fe(CN)₆] through the entire root cross-section, reddish-brown precipitates were observed in the walls of the stele of soybean, but not corn root. However, when a CuSO₄ solution containing 0.02 or 0.2 μM free Cu²⁺ was used, no reddish-brown precipitates were detected in the stele of either of the two plants. Results indicated that endodermis was permeable to Cu-citrate complexes in primary roots of soybean, but not corn. The permeability of the endodermal barrier to the Cu-citrate complex may vary between dicotyledonous and monocotyledonous plants, which has considerable implications for chelant-enhanced phytoextraction.     

白鲜根的发育解剖学研究

应用半薄切片、常规石蜡切片并结合离析法,对药用植物白鲜(Dictamnus dasycarpus Turcz.)根的发生发育过程进行了研究。结果表明:白鲜根的发生发育过程包括4个阶段,即原分生组织阶段、初生分生组织阶段、初生结构阶段以及次生结构阶段。原分生组织位于根冠内侧及初生分生组织之间,衍生细胞分化为初生分生组织。初生分生组织由原表皮、基本分生组织以及中柱原组成。原表皮分化为表皮,基本分生组织分化为皮层,中柱原分化为维管柱,共同组成根的初生结构;在初生结构中,部分表皮细胞外壁向外延伸形成根毛,皮层中分布有油细胞,内皮层有凯氏带,初生木质部为二原型或偶见三原型,外始式;根初生结构有髓或无。次生结构来源于原形成层起源的维管形成层的活动以及中柱鞘起源的木栓形成层的活动;白鲜次生韧皮部宽广,其中多年生根中可占根横切面积的85%,另外除基本组成分子外,还分布有油细胞;周皮发达,木栓层厚;初生皮层、次生木质部和次生韧皮部薄壁细胞中常充满丰富的淀粉粒。     

Mechanisms of Reduction of the Stelar Pattern along Barley Roots

The stelar pattern along the seminal and nodal roots of barley (Hordeum vulgare L.) is gradually simplified due to a decreasing frequency of longitudinal cell division in the apical meristem. The decrease involves the proportion of stelar parenchyma, the number of vascular strands on the periphery of the stele and, in nodal roots with a more complex structure, the number of central metaxylem files. In spite of the fact that the stelar parenchyma is reduced in distal parts of the roots to approximately one half, the discontinuity of central and peripheral metaxylem is preserved. Reduction of the number of central metaxylem files is due to fusion. In the reduction of peripheral xylem and phloem strands, the development of certain xylem strands is discontinued and they are terminated blindly. Two phloem strands that had alternated radially with them, approach each other, coalesce and a single phloem strand continues to develop. In this way the regular alternation of phloem and xylem is re-established. The importance of fusions ensuring reduction of the functional continuity in vascular tissue by formation of a network structure must be stressed. This reduction mechanism is involved not only in files of the wide central metaxylem but also in phloem strands which are thus preferred over blindly terminating peripheral xylem strands.     

ANATOMY OF MACROZAMIA COMMUNIS LATERAL ROOTS AND ROOT NODULES FORMED IN VITRO STUDIED WITH LIGHT AND SCANNING ELECTRON MICROSCOPY

The anatomy of Macrozamia communis L. Johnson lateral roots and nodules was studied following axenic culture in light and darkness. Pointed lateral roots from dark cultures had an open apical organization similar to that of other cycads and gymnosperms. A distinct protoderm-derived epidermis was not observed. At the apex, the dermis was formed by the outer root capcortical cell layer. Subapically, the outer cortex formed the dermis. No evidence of an algal zone was observed in these roots. The stele was bounded by a distinct endodermis and contained an exarch, diarch xylem. Apogeotropic nodules which developed at the root-shoot junction in darkness, branched dichotomously and had rounded tips covered by tangentially-enlarged root cap cells. The root cap was reduced to a few cell layers and was confined to the extreme nodule apex. The central region of the apical meristem was enlarged, and meristematic cells contained differentiated amyloplasts. A presumptive algal zone was present in some but not all nodules and divided the cortex into inner and outer regions. Stelar anatomy was similar to that observed in pointed, dark-grown lateral roots, except that there was greater xylem differentiation. Nodules which developed in the light were similar to dark-formed nodules, except that root cap cells were radially enlarged and extended over the flanks of the nodule forming a persistent root cap. The heteromorphic lateral roots of M. communis formed a developmental continuum not a heterorhizic root system.     

Identification of Arabidopsis subtilisin-like serine protease specifically expressed in root stele by gene trapping

Xylem plays a role not only in the transport of water and nutrients but also in the regulation of growth and development through the transport of biologically active substances. In addition to mineral salts, xylem sap contains hormones, organic nutrients and proteins. However, the physiological functions of most of those substances remain unclear. To explore genes involved in xylem sap production, we identified Arabidopsis genes expressed in the root stele of the root hair zone from gene-trap lines by randomly inserting the β-glucuronidase gene into the genome. Among 26 000 gene-trap lines, we found that 10 lines had β-glucuronidase (GUS) staining predominantly in the root stele of the root hair zone and no GUS staining in the shoots. Of these 10 lines, 2 lines showed that gene-trap tags inserted into the promoter region of the same gene, denoted Arabidopsis thaliana subtilase 4.12( AtSBT4.12 ). Analysis of AtSBT4.12 promoter using an pAtSBT4.12 ::β-glucuronidase transgenic line showed that the AtSBT4.12 gene was expressed only in the root stele of the root hair zone. AtSBT4.12 expression in roots was increased by application of methyl jasmonate. Subtilase proteins are commonly detected in proteomic analyses of xylem sap from various plant species, including Brassica napus , a relative of Arabidopsis . These results suggest that AtSBT4.12 may be a protein localized in the apoplast of root stele including xylem vessel and involved in stress responses in Arabidopsis roots.     

A microautoradiographic study of Ca45 and S35 distribution in the intact bean root

Summary Microautoradiographic techniques were used to determine the distribution of Ca⁴⁵ and S³⁵ in regions of the bean root where anatomical features may influence the processes of ion uptake and translocation. Root tissue from intact plants was prepared by methods that preserve both soluble and insoluble Ca and S. Ca⁴⁵ distribution was determined after 1 hour and 15 min, of uptake, after 2 efflux periods, and after replacement by non-tracer Ca.S³⁵ distribution was determined after 1 hour and 15 min of uptake.The quantity of Ca⁴⁵ that entered the root was greater than the quantity of S³⁵. Ca⁴⁵ concentration within the root increased with linear distance from the 8-mm level behind the tip. The pathways of Ca and S across the cortex appeared to be different since Ca⁴⁵ was particularly associated with cell walls and S³⁵ was distributed more evenly through the cells. There was no evidence that the endodermis was a diffusion barrier for Ca; the small parenchyma cells associated with conducting elements acquired a high concentration of Ca⁴⁵ and thus appear to be implicated in absorption and perhaps in transfer to the xylem. The evidence suggests that the endodermis may have been a barrier for S, but if so, certain parenchyma cells inside the stele, especially at xylem poles, were equally involved. The region from 30 to 80 mm from the tip appeared to participate in Ca uptake and transfer to the xylem; because of tissue immaturity the 8-mm region, which contained the least Ca⁴⁵, was thought not to translocate to the shoot. Deposition of Ca⁴⁵ in oxalate crystals represented almost complete immobilization. Calcium oxalate metabolism was most active in the 30-mm region of secondary roots and in their small branches. S³⁵-labelled nuclei occurred in the cortex 2.5 to 3 mm behind the root tip.     

A histological study of lateral root initiation and development inZea mays

Summary A light microscopic study has been made of the origin and development of lateral roots inZea mays.The initiation of a lateral occurs adjacent to a xylem pole and involves an increase in cytoplasmic basophilia and subsequent divisions of cells of the pericycle and the parenchyma of the stele of the mother root.Cells derived from the parent pericycle form most of the young lateral but its epidermis and root cap are composed of cells of endodermal origin.Two different types of polysaccharides are secreted by cells of the young lateral root. One type which is PAS-positive and non-metachromatic, is produced by the epidermal cells, while the other type, metachromatic and only slightly PAS-positive, is secreted by the root cap cells.