Reprogramming of cells following wounding in pea (Pisum sativum L.) roots. II. The effects of caffeine and colchicine on the development of new vascular elements

Summary Interference with the normal progression of the cell cycle by the drugs caffeine and colchicine does not prevent parenchyma cells in the cortex of pea roots from being reprogrammed to become tracheary and sieve elements following severance of the vascular cylinder of the root. The pattern of secondary wall deposition of the newly differentiated tracheary elements is highly aberrant in the presence of colchicine but is of normal appearance in the caffeinetreated roots. In each case, the new sieve elements have sieve plates and lateral sieve areas with callose deposits. Induction and redifferentiation are achieved in the absence of cell division and microtubules in colchicine-treated roots.Bi- and multi-nucleate cells are produced by both drugs. Microtubules are still present in the caffeine-treated roots but cell plate formation is inhibited. The partitioning of the multinucleate cortical cells by the interdigitation of free-growing walls between the nuclei occurs in the presence of caffeine but not colchicine.     

Repetitive cellular patterns in the secondary phloem of conifer and dicot trees,and a hypothesis for their development

Abstract

The radial fusiform cell files of the secondary phloem of conifers and dicots are composed of different cell types?–?fibres, parenchyma and sieve cells (in conifers), or sieve elements plus companion cells (in dicots). These cell types are arranged in characteristic, species-specific sequences along the radii of the files. The sequences are replicated in adjacent files and this leads to tangential bands of similar cell type. Moreover, the sequences are developed repetitively so that a sequence found in one year's growth increment of phloem is repeated in the next increment. In some species, many repetitions of the same sequence occur within one annual increment. A general hypothesis has been developed to account for the radial sequences of cell types. It is proposed that there is a gradient of a phloem-promoting morphogen, a series of morphogen thresholds for the determination of each phloem cell type, and a particular spatio-temporal pattern of periclinal cell division in the phloem domain of the vascular cambium that generates a corresponding pattern of cell displacement through the morphogen gradient in the immediately post-mitotic zone of cell determination. The feasibility of the hypothesis was supported by means of simulation which, using a constant set of initial conditions, could reproduce very nearly all the radial sequences of cell types found in the secondary phloem of a range of species of conifers and woody dicots. The tangential banding of the various cell types suggests that cell production and cell determination are events which occur synchronously across the radial files. The repeating blocks of cell types may constitute functional modules of phloem tissue, and the constituent cells probably have particular patterns of symplasmic connections and mechano-structural properties.     

The extent and differences in mitotic activity of the root tip ofVicia faba L.

Mitotic activity does not stop for different meristematic cells of the root apex at the same distance from the initials. The differences are connected with the functional heterogeneity of the apical meristem of the root. The arrangement of vascular bundles,i.e. the alternation of independent xylem and phloem groups, is of major importance. In broad bean roots, the protophloem sieve elements stop dividing first. The centre of the stelei. e. late metaxylem elements stop dividing next. Division in the stele gradually ceases centrifugally, while it ceases centripetally in the peripheral part of the root. The cylindrical region with prolonged cell division includes internal layers of the cortex including endodermis, pericycle and adjoining cells of the stele. Proximally apical meristem is reduced to isolated strands of cells adjacent to the protoxylem poles. Pericycle cells stop dividing last at a distance of approx. 9–10 mm from the initials. The number of the division cycles is limited and is specific for individual cell types. Epidermal and cortical cells divide in broad bean roots transversely approximately seven times, cells of late metaxylem approximately five times. Root apical meristem is an asynchronous cell population with a different duration of the mitotic cycle. We determined local variations in the duration of the mitotic cycle in the apical meristem of broad bean root by means of colchicine-induced polyploidy. The cells of the quiescent centre had the longest mitotic cycle after colchicine treatment. The region of the proper root adjacent to the quiescent centre was mixoploid (2n and 4n). Isolated cells with a long cycle occurred also in the cortex and in the central cylinder. Cells with a division cycle of 18h were found in the root cap, in the epidermis, in the cortex and in the central cylinder. Relatively numerous cells with the shortest division cycle, approx. 12 h, occurred farther of the quiescent centre in the epidermis, in the cortex, in the pericycle, and in adjacent layers of the stele through-out the entire meristematic region. The results derived from the analysis of the apical meristem are discussed in connection with the ontogenesis of different types of cells taking part in the primary structure of the root.     

Analysis of early processes in wound-induced vascular regeneration using TED3 and ZeHB3 as molecular markers

Interruption of the vascular bundles of Zinnia internodes induced transdifferentiation of cells into tracheary elements (TEs) or sieve elements (SEs) within 4 d of wounding. The early stage of the regeneration processes was analyzed using two molecular marker genes, TED3 and ZeHB3, which are expressed specifically in TE precursor cells and immature phloem cells, respectively. An increase in the numbers of TED3 and ZeHB3 mRNA-expressing cells always preceded an increase in the numbers of TEs and SEs formed. The earliest sign of vascular differentiation was the appearance 24 h after wounding of a layer(s) of TED3 mRNA-expressing cells in the inter- and intrafascicular cambial-like regions along the severed vascular bundles. In contrast, the number of ZeHB3 mRNA-expressing cells decreased dramatically along the severed bundles 24 h after wounding, and increased again 36 h after wounding. These results clearly indicate that xylem and phloem differentiation are not synchronized during vascular regeneration. Treatment with 10(-3) M colchicine abolished the expression of ZeHB3 mRNA in pith parenchyma, but not TED3 mRNA; this suggests that cell division is a prerequisite for the transdifferentiation of pith parenchymal cells into immature phloem cells expressing ZeHB3. In contrast, transdifferentiation of pith parenchymal cells to TE precursor cells does not require preceding cell division. However, the inhibition of cell division prevented the formation of both radial files of TEs and the cambial-like layer(s) of TED3 mRNA-expressing cells, and, ultimately, vascular regeneration altogether. These results imply that wound-induced cambial-like activity in and between severed vascular bundles is essential for vascular regeneration.     

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.     

Multinucleate transfer cells induced in coleus roots by the root-knot nematode,Meloidogyne arenaria

Summary The occurrence and position of wall protuberances in giant cells induced in coleus roots by the root-knot nematodeMeloidogyne arenaria is described, and the structure and function of giant cells is compared with that of syncytia induced by cyst-nematodes.Extensive protuberance development occurs on walls of giant cells adjacent to xylem vessels. Protuberances are less well developed next to sieve elements, and almost absent next to parenchyma cells. On walls between giant cells they occur on both sides or only one side. The formation of protuberances indicates that giant cells are multinucleate transfer cells. The position of protuberances marks the wall area where solutes enter the cell. Solutes are obtained from xylem and phloem elements, and the position of protuberances at the junction between giant cells and vascular elements indicates an extensive flow of solutes along cell walls. The observations support the hypothesis that wall protuberances form as a result of selective solute flow across the plasmalemma.No cell wall dissolution was observed, although wall gaps may occur between giant cells as a result of breakage during rapid cell expansion.     

Cortical development in roots of the aquatic plant Pontederia cordata (Pontederiaceae)

Adventitious roots of marsh-grown Pontederia cordata were examined to determine cortical development and structure. The innermost layer of the ground meristem forms the endodermis and aerenchymatous cortex. The outermost layer of the early ground meristem undergoes a precise pattern of oblique and periclinal cell divisions to produce a single or double layer of prohypodermis with an anchor cell for each radial file of aerenchyma cells. At maturity, endodermal cell walls are modified only by narrow Casparian bands. The central regions of the ground meristem become proaerenchyma and exhibit asymmetric cell division and expansion. They produce an aerenchymatous zone with barrel-shaped large cells and irregularly shaped small cells traversing the aerenchyma horizontally along radii; some crystalliferous cells with raphides are present in the aerenchyma. The walls of the hypodermis are modified early by polyphenols. The outermost layer of the hypodermis later matures into an exodermis with Casparian bands that are impermeable to berberine, an apoplastic tracer dye. The nonexodermal layer(s) of the hypodermis has suberin-modified walls. Radial files of aerenchyma are usually connected by narrow protuberances near their midpoints, the aerenchyma lacunae having been produced by expansion of cells along walls lining intercellular spaces. We are terming this type of aerenchyma development, which is neither schizogenous nor lysigenous, "differential expansion."     

Pre-prophase bands of microtubules in all categories of formative and proliferative cell division in Azolla roots

Pre-prophase bands of microtubules were found in every category of cell division, symmetrical and asymmetrical, in the cell lineages of the root apex of Azolla pinnata R.Br. and A. filiculoides Lam., and in the transverse divisions in the cell files of the roots. They are also found in the asymmetrical cell division that gives rise to trichoblasts in roots of Hydrocharis dubia (B1). Backer. It is possible, in a variety of cell types in roots of Azolla, to predict within a fraction of a micrometre where a new cell wall will be located. In every such case the midline of the 1.5–3-m-wide pre-prophase band anticipates this location. Each of the daughter cells thus inherits approximately half of the former pre-prophase band site. Images interpreted as stages of formation of the band were obtained, its microtubules replacing the interphase cortical arrays. In one highly asymmetrical division, band formation precedes migration of the nucleus to the site of mitosis. The asymmetrical division that gives rise to root hairs passes acropetally along every cell in the dermatogen layer, and preprophase bands were seen up to 8 cells in advance of the last completed division. Here, and in the zone of formative divisions, the band is present for much longer than the duration of mitosis. The ubiquity of the band in the Azolla root tip is discussed in relation to the literature, and a working hypothesis is presented that takes into account current knowledge of occurrence, development and function of the band.     

STUDIES ON THE ROOT OF HORDEUM VULGARE L.– ULTRASTRUCTURE OF THE SEMINAL ROOT WITH SPECIAL REFERENCE TO THE PHLOEM

Seminal root tissue of Hordeum vulgare L. var. Barsoy was fixed in glutaraldehyde and osmium tetroxide and studied with the light and electron microscopes. The roots consist of an epidermis, 6–7 layers of cortical cells, a uniseriate endodermis and a central vascular cylinder. Cytologically, the cortical and endodermal cells are similar except for the presence of tubular-like invaginations of the plasmalemma, especially near the plasmodesmata, in the former. The vascular cylinder consists of a uniseriate pericycle surrounding 6–9 phloem strands occurring on alternating radii with an equal number of xylem bundles. The center of the root contains a single, late maturing metaxylem vessel element. Each phloem strand consists of one protophloem sieve element, two companion cells and 1–3 metaphloem sieve elements. The protophloem element and companion cells are contiguous with the pericycle. Metaphloem sieve elements are contiguous with companion cells and are separated from tracheary elements by xylem parenchyma cells. The protoplasts of contiguous cells of the root are joined by various numbers of cytoplasmic connections. With the exception of the pore-plasmodesmata connections between sieve-tube members and parenchymatic elements, the plasmodesmata between various cell types are similar in structure. The distribution of plasmodesmata supports a symplastic pathway for organic solute unloading and transport from the phloem to the cortex. Based on the arrangement of cell types and plasmodesmatal frequencies between various cell types of the root, the major symplastic pathway from sieve elements to cortex appears to be via the companion and xylem parenchyma cells.     

八角莲组织培养的器官发生途径研究

为探究小檗科植物八角莲组织培养的器官发生方式,该研究以八角莲离体叶片、叶柄在MS培养基上诱导产生的愈伤组织、不定芽、不定根为对象,用连续石蜡切片技术分析八角莲组织培养的器官发生途径。结果表明:八角莲愈伤组织形成的解剖学特征是靠近表皮的薄壁细胞经激素刺激恢复分裂能力,继续培养形成拟分生组织。拟分生组织可形成许多分化中心。通过对八角莲组织培养产生的不定芽细胞组织学观察发现芽原基起源于愈伤组织外侧的几层薄壁细胞,芽原基背离愈伤组织中央生长形成不定芽,故八角莲脱分化形成的芽起源方式为外起源。而八角莲的根原基起源于组织深处髓部薄壁细胞和部分维管形成层细胞,进而形成类似球形或楔形并朝韧皮部突起的根原基轮廓,根原基继续发育会突破表皮生成不定根,起源方式为内起源。八角莲离体再生途径为器官发生型,在组培苗生长过程中先诱导形成不定芽,再诱导形成不定根,在愈伤组织上形成维管组织将不定芽和不定根连接成完整植株。     

Expression of the early nodulin, ENOD40, in soybean roots in response to various lipo-chitin signal molecules

The lipo-chitin (LCO) nodulation signal (nod signal) purified from Bradyrhizobium japonicum induced nodule primordia on soybean (i.e. Glycine soja) roots. These primordia were characterized by a bifurcated vascular connection, cortical cell division, and the accumulation of mRNA of the early nodulin gene, ENOD40. A chemically synthesized LCO identical in structure to the Nod signal purified from B. japonicum cultures showed the same activity when inoculated on to soybean roots. Surprisingly, synthetic LCO or chitin pentamer, inactive in inducing root hair curling (HAD) or cortical cell division (NOI) in G. soja, induced the transient accumulation of ENOD40 mRNA. In roots inoculated with such LCO, ENOD40 mRNA was abundant at 40 h after inoculation but decreased to the background levels 6 days after inoculation. In contrast, nod signals active in inducing HAD and NOI induced high levels of ENOD40 accumulation at 40 h and 6 days after inoculation. In situ hybridization analysis showed that ENOD40 mRNA accumulated in the pericycle of the vascular bundle at 24 h after root inoculation with nod signal. At 6 days post-inoculation with nod signal, ENOD40 expression was seen in dividing subepidermal cortical cells. These results provide morphological and molecular evidence that nodule induction in soybean in response to purified or synthetic nod signal is similar, if not identical, to nodule formation induced by bacterial inoculation. Surprisingly, ENOD40 mRNA accumulation occurs in response to non-specific chitin signals. This suggests that, in the case of ENOD40, nodulation specificity is not determined at the level of initial gene expression.     

discordia mutations specifically misorient asymmetric cell divisions during development of the maize leaf epidermis.

In plant cells, cytokinesis depends on a cytoskeletal structure called a phragmoplast, which directs the formation of a new cell wall between daughter nuclei after mitosis. The orientation of cell division depends on guidance of the phragmoplast during cytokinesis to a cortical site marked throughout prophase by another cytoskeletal structure called a preprophase band. Asymmetrically dividing cells become polarized and form asymmetric preprophase bands prior to mitosis; phragmoplasts are subsequently guided to these asymmetric cortical sites to form daughter cells of different shapes and/or sizes. Here we describe two new recessive mutations, discordia1 (dcd1) and discordia2 (dcd2), which disrupt the spatial regulation of cytokinesis during asymmetric cell divisions. Both mutations disrupt four classes of asymmetric cell divisions during the development of the maize leaf epidermis, without affecting the symmetric divisions through which most epidermal cells arise. The effects of dcd mutations on asymmetric cell division can be mimicked by cytochalasin D treatment, and divisions affected by dcd1 are hypersensitive to the effects of cytochalasin D. Analysis of actin and microtubule organization in these mutants showed no effect of either mutation on cell polarity, or on formation and localization of preprophase bands and spindles. In mutant cells, phragmoplasts in asymmetrically dividing cells are structurally normal and are initiated in the correct location, but often fail to move to the position formerly occupied by the preprophase band. We propose that dcd mutations disrupt an actin-dependent process necessary for the guidance of phragmoplasts during cytokinesis in asymmetrically dividing cells.     

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.     

Auxin and ethylene interactions control mitotic activity of the quiescent centre, root cap size, and pattern of cap cell differentiation in maize

Root caps (RCs) are the terminal tissues of higher plant roots. In the present study the factors controlling RC size, shape and structure were examined. It was found that this control involves interactions between the RC and an adjacent population of slowly dividing cells, the quiescent centre, QC. Using the polar auxin transport inhibitor 1-N-naphthylphthalamic acid (NPA), the effects of QC activation on RC gene expression and border cell release was characterized. Ethylene was found to regulate RC size and cell differentiation, since its addition, or the inhibition of its synthesis, affected RC development. The stimulation of cell division in the QC following NPA treatment was reversed by ethylene, and quiescence was re-established. Moreover, inhibition of both ethylene synthesis and auxin polar transport triggered a new pattern of cell division in the root epidermis and led to the appearance of supernumerary epidermal cell files with cap-like characteristics. The data suggest that the QC ensures an ordered internal distribution of auxin, and thereby regulates not only the planes of growth and division in both the root apex proper and the RC meristem, but also regulates cell fate in the RC. Ethylene appears to regulate the auxin redistribution system that resides in the RC. Experiments with Arabidopsis roots also reveal that ethylene plays an important role in regulating the auxin sink, and consequently cell fate in the RC.     

Cell-specific expression of two arabinogalactan protein epitopes recognized by monoclonal antobodies JIM8 and JIM13 in maize roots

Summary The cell-specific expression of two arabinogalactan protein (AGP) epitopes recognized by monoclonal antibodies JIM8 and JIM13 is reported in maize roots. Employing immunofluorescence and immunogold electron microscopy, the JIM8 antibody was shown to label exclusively protophloem sieve elements, while the JIM13 antibody labelled sieve elements very strongly and adjacent pericycle and companion cells, as well as sloughing root cap cells less strongly. Since the labelling of sieve elements with JIM8 antibody was specific and did not spread to other cell types during root development, it is concluded that this AGP epitope can serve as a specific marker of these specialized cells within the maize root. In the case of the AGP epitope recognized by JIM13 antibody, part of the immunofluorescence label was also found to be associated with cytoplasmic strands in the pericycle and sloughing root cap cells. Immunogold-labelling of sieve elements revealed the association of both AGP epitopes (JIM8 and JIM13) with cortical sieve element reticulum and plasma membranes. Labelling of sieve element reticulum was prominent at its domains of adhesion to the plasma membrane, P-type plastids, and mitochondria. Based on our subcellular studies, we propose a new function of AGP epitopes in endomembrane recognition and adhesion within the sieve elements of maize roots.Abbreviations AGP arabinogalactan protein - SER sieve element reticulum     

Actin filaments line up acrossTradescantia epidermal cells,anticipating wound-induced divison planes

Summary This paper describes the role of actin filaments in setting up the phragmosome — the transvacuolar device that anticipates the division plane — and in forming a supracellular system that seems to override cell boundaries. Tradescantia leaf epidermal cells were induced to divide by wounding the leaf. New division planes formed parallel to slits, and encircled puncture wounds — the new division planes lining up across cells, instead of the joints being off-set as in normal, unwounded tissue. Within 30 min after wounding, rhodamine phalloidin staining showed that a belt of fine, cortical actin filaments formed parallel to the wound. In the next stage, migration of nuclei to a wall adjacent to the wound, involved pronounced association of actin filaments with the nucleus. Migration could be inhibited with cytochalasin D, confirming the role of actin in traumatotaxis. Later still, actin strands were seen to line up from cell to cell, parallel to the wound, anticipating the future division plane. Next, actin filaments accumulated in this anticlinal plane, throughout the depth of the cell, thereby contributing to the formation of the phragmosome. The phragmosome has been shown in previous work (Flanders et al. 1990) to contain microtubules that bridge nucleus to cortex, and is now found to contain actin filaments. Actin filaments are therefore involved in the key stages of nuclear migration and division plane alignment. The supracellular basis of actin alignment is discussed.Dedicated to the memory of Professor Oswald Kiermayer     

The cortical cytoskeletal network and cell-wall dynamics in the unicellular charophycean green alga Penium margaritaceum

Background and Aims

Penium margaritaceum is a unicellular charophycean green alga with a unique bi-directional polar expansion mechanism that occurs at the central isthmus zone prior to cell division. This entails the focused deposition of cell-wall polymers coordinated by the activities of components of the endomembrane system and cytoskeletal networks. The goal of this study was to elucidate the structural organization of the cortical cytoskeletal network during the cell cycle and identify its specific functional roles during key cell-wall developmental events: pre-division expansion and cell division.

Methods

Microtubules and actin filaments were labelled during various cell cycle phases with an anti-tubulin antibody and rhodamine phalloidin, respectively. Chemically induced disruption of the cytoskeleton was used to elucidate specific functional roles of microtubules and actin during cell expansion and division. Correlation of cytoskeletal dynamics with cell-wall development included live cell labelling with wall polymer-specific antibodies and electron microscopy.

Key Results

The cortical cytoplasm of Penium is highlighted by a band of microtubules found at the cell isthmus, i.e. the site of pre-division wall expansion. This band, along with an associated, transient band of actin filaments, probably acts to direct the deposition of new wall material and to mark the plane of the future cell division. Two additional bands of microtubules, which we identify as satellite bands, arise from the isthmus microtubular band at the onset of expansion and displace toward the poles during expansion, ultimately marking the isthmus of future daughter cells. Treatment with microtubule and actin perturbation agents reversibly stops cell division.

Conclusions

The cortical cytoplasm of Penium contains distinct bands of microtubules and actin filaments that persist through the cell cycle. One of these bands, termed the isthmus microtubule band, or IMB, marks the site of both pre-division wall expansion and the zone where a cross wall will form during cytokinesis. This suggests that prior to the evolution of land plants, a dynamic, cortical cytoskeletal array similar to a pre-prophase band had evolved in the charophytes. However, an interesting variation on the cortical band theme is present in Penium, where two satellite microtubule bands are produced at the onset of cell expansion, each of which is destined to become an IMB in the two daughter cells after cytokinesis. These unique cytoskeletal components demonstrate the close temporal control and highly coordinated cytoskeletal dynamics of cellular development in Penium.     

A Preliminary Study on the Structure and Function of the Vascular Transfer Cells in Garlic Scape

The vascular transfer cells in garlic scape havebeen examined with electron microscope. Their structure, distributive feature and adenosine triphosphatase (ATPase) activity are studied. The mature vascular transfer cells exhibit the characteristic cell wall ingrowths. The cell contents include a large nucleus, dense cytoplasm and various normal organelles. It is notable that there are numerous mitochondria with well developed, cristae. Plasmodesmata are extensively present in the wall, and transfer cells are connected to adjacent cells by them. The senescing transfer cells become more vacuolated and have a large central vacuole and dense parietal cytoplasm. Their wall ingrowths seem to degenerate and finally disappear. The transfer cells show a particular pattern of distribution in the vascular bundle of the garlic scape. Some of them are present between the vessels of xylem and the sieve tubes of phloem. However, more abundant cell wall ingrowths occur on those walls which abut on, or are close to the vessel of xylem. The other transfer cells are located between the sieve tubes and parenehyma cells. The phloem transfer cell which is adjacent to sieve tube has developed from companion cell. All the transfer cells are mainly concerned with the loading and unloading of sieve tubes. And they may play an important role in facilitating intensive material transfer between two independent systems (i.e. the vessels and sieve tubes, the symplast and apoplast). The results of the cytochemical localization of ATPase using a lead precipitation technique exhibit strong enzyme activity on the plasmalemma of the transfer cells. It is suggested that the transfer cells are especially active in solute movement through them to which cellular energy metabolism coupled.     

Life Cycle of <Emphasis Type="Italic">Plasmodiophora brassicae</Emphasis>

Plasmodiphora brassicae is a soil-borne obligate parasite. The pathogen has three stages in its life cycle: survival in soil, root hair infection, and cortical infection. Resting spores of P. brassicae have a great ability to survive in soil. These resting spores release primary zoospores. When a zoospore reaches the surface of a root hair, it penetrates through the cell wall. This stage is termed the root hair infection stage. Inside root hairs the pathogen forms primary plasmodia. A number of nuclear divisions occur synchronously in the plasmodia, followed by cleavage into zoosporangia. Later, 4–16 secondary zoospores are formed in each zoosporangium and released into the soil. Secondary zoospores penetrate the cortical tissues of the main roots, a process called cortical infection. Inside invaded roots cells, the pathogen develops into secondary plasmodia which are associated with cellular hypertrophy, followed by gall formation in the tissues. The plasmodia finally develop into a new generation of resting spores, followed by their release back into soil as survival structures. In vitro dual cultures of P. brassicae with hairy root culture and suspension cultures have been developed to provide a way to nondestructively observe the growth of this pathogen within host cells. The development of P. brassicae in the hairy roots was similar to that found in intact plants. The observations of the cortical infection stage suggest that swelling of P. brassicae-infected cells and abnormal cell division of P. brassicae-infected and adjacent cells will induce hypertrophy and that movement of plasmodia by cytoplasmic streaming increases the number of P. brassicae-infected cells during cell division.