Ultrastructure of <Emphasis Type="Italic">Medicago sativa</Emphasis> rhizodermis cells over their early development

Ultrastructure was investigated along the files of developing epidermal cells in the root tip of a model plant Medicago sativa, in which all rhizodermal cells are potential hair-forming trichoblasts. Differentiation at subcellular level was observed up to the stage of bulge initiation in the trichoblasts. Root hair initiation indicated by the emergence of bulges from trichoblasts was detected at various distances from the root tip and, it was independent of the trichoblast size. During rhizodermal cell differentiation, starch grains accumulated in the plastids. Nuclei located in the central part of the young, meristematic cells moved towards the inner periclinal wall as the central vacuole enlarged. The bulging region of the trichoblasts located opposite the nucleus and was rich in mitochondria, ER, ribosomes, and Golgi bodies, and contained also vesicles enclosing fibrillar material. This material responded positively to phosphotungstic acid, which was used for detection of cell wall polysaccharides. The cell wall thickness within the bulging domain was significantly lower than in other parts of trichoblasts. We suggest that internalization of cell wall polysaccharides occurs within the bulging area, contributing to local thinning of the cell wall and providing a source of osmotically active compounds for maintaining turgor in the trichoblast. Thus, the internalization process might be necessary for root hair outgrowth.     

TAXONOMY AND PHYLOGENY OF OSMUNDEA (RHODOMELACEAE, RHODOPHYTA) IN ATLANTIC EUROPE

Two species of Osmundea Stackhouse (Rhodomelaceae, Rhodophyta) that occur in Atlantic Europe have been confused under the names Osmundea ramosissima (Oeder) Athanasiadis and Osmundea truncata (Kützing) Nam et Maggs, regarded until now as a synonym of O. ramosissima. An epitype from its type locality (Stavanger, Norway) is selected for Osmundea ramosissima Athanasiadis, recognized here as a valid name for Fucus ramosissimus Oeder, nom. illeg. Details of vegetative and reproductive morphology of O. ramosissima are reported, based on material from France, the British Isles, and Helgoland. Osmundea ramosissima resembles other species of Osmundea in its vegetative axial segments with two pericentral cells and one trichoblast, spermatangial development from apical and epidermal cells (filament type), the formation of five pericentral cells in the procarp‐bearing segment of the female trichoblast, and tetrasporangial production from random epidermal cells. Among the species of Osmundea, O. ramosissima is most similar to O. truncata. Both species have discoid holdfasts, secondary pit connections between epidermal cells, and cup‐shaped spermatangial pits. They differ in that: (a) O. ramosissima lacks lenticular wall thickenings and refractive needle‐like inclusions in medullary cells, both of which are present in O. truncata; (b) O. ramosissima has branched spermatangial filaments that terminate in a cluster of several cells, whereas in O. truncata the unbranched spermatangial filaments have a single large terminal sterile cell; and (c) cystocarps of O. ramosissima lack protuberant ostioles but ostioles are remarkably protuberant in O. truncata. Phylogenetic analyses of rbcL sequences of Laurencia obtusa (Hudson) Lamouroux and all five Atlantic European species of Osmundea, including the type species, strongly support the generic status of Osmundea. Osmundea ramosissima and O. truncata are closely related (5.2% sequence divergence) and form a well‐supported clade sister to a clade consisting of O. pinnatifida (Hudson) Stackhouse, O. osmunda Stackhouse and O. hybrida (A. P. de Candolle) Nam. The formation of secondary pit connections between epidermal cells is a synapomorphy for the O. ramosissima+O. truncata clade. The close relationship between species with cup‐shaped spermatangial pits (Osmundea hybrida) and urn‐shaped pits (Osmundea pinnatifida and Osmundea osmunda) shows that spermatangial pit shape is not an important phylogenetic character. Parsimony analysis of a morphological data set also supports the genus Osmundea but conflicts with the molecular trees in infrageneric relationships, placing O. hybrida basal within the Osmundea clade and grouping O. osmunda and O. pinnatifida but not O. truncata and O. ramosissima. A key to Osmundea species is presented.     

Seismic communication between the burrows of kangaroo rats, Dipodomys spectabilis

Banner-tailed kangaroo rats, Dipodomys spectabilis, footdrum to produce substrate-borne and airborne acoustic energy. Previous studies show that they communicate territorial ownership via airborne footdrumming signals. The research reported here used simulated footdrum patterns generated by an artificial `thumper' to address the question of whether kangaroo rats communicate through seismic components of these acoustic signals. With microphones suspended in sealed burrows, we found that airborne sounds were attenuated by approximately 40 dB as they passed through the burrow wall into the burrow chamber. The substrate-borne vibrations from the thumper yielded sound approximately 40 dB greater in peak amplitude than the attenuated airborne sound. Thus, 99.9% of the peak power of the thumper was transmitted directly through the substrate into the burrow. The rats in sealed burrows timed their responses to playbacks of footdrums from the thumper and a loudspeaker so they did not initiate a drumming sequence during either the seismic or airborne signals. When these signals were masked by loud noise, the rats continued to drum to the seismic signal but drummed randomly during the airborne playback. These results suggest that the sealed burrow provides a quiet place in which D. spectabilis can listen for substrate-borne communications from conspecifics. Accepted: 13 May 1997     

TRICHOBLASTS IN HYDROCHARIS. I. ORIGIN. DIFFERENTIATION,DIMENSIONS AND GROWTH

In the root epidermis of Hydrocharis morsus-ranae L. distinctive trichoblasts, which later grow out as root hairs, are formed by the unequal division of protodermal (immature epidermal) cells. The trichoblast is the more proximal product of this division. Trichoblasts differ from adjacent epidermal cells in manner of growth, in size, amount of cytoplasm, degree of succinic dehydrogenase and cytochrome oxidase activity, and in the structure of their plastids. Plastids in the trichoblasts gradually become colorless and of less complex structure with increasing distance from the root tip, in contrast to those in adjacent epidermal cells. The trichoblasts do not divide, but they elongate to a considerable extent in the most distal 3000 μ of the root tip and less extensively in the next 3000 μ. By contrast the sister cells and their products divide, but the individual products do not become markedly longer than the mother cell until situated more than 3000 μ from the root tip, when they undergo extensive elongation. The trichoblasts are thus characterized by delayed maturation and inhibition of cytokinesis; it is suggested that delayed maturation is a necessary prerequisite for differentiation of root hairs in this and other species.     

Subcellular distribution and chemical forms of Cd in Bougainvillea spectabilis Willd. as an ornamental phytostabilizer: An integrated consideration

The effects of Cd stress on the growth and Cd accumulation of Bougainvillea spectabilis Willd. as an ornamental plant and the related mechanisms were investigated in the study. We studied the impact of Cd on the plant ultrastructure, examined the cellular distribution of Cd, explored the Cd chemical forms and transformation, and determined the organic acid secretion in the plants. The results showed that B. spectabilis could grow well in the Cd treatment groups, and the roots could accumulate high concentration of Cd. The soluble fraction (primarily in the vacuole) as the form of citrate in leaves of B. spectabilis was the major compartment for Cd storage. The citric acid secreted by B. spectabilis played an important role in the detoxification of Cd, as well as the growth of plants and Cd accumulation. As an ornamental plant, B. spectabilis has the potential to be used in the phytostabilization of Cd-contaminated soils and can beautify the environment at the same time.     

Echinophycus minutus (Rhodomelaceae, Ceramiales), a new red algal genus and species from north-western Australia

Echinophycus minutus gen. et sp. nov. (Ceramiales, Rhodophyta) is described for specimens collected from a deep‐water habitat in the Dampier Archipelago, northwestern Australia. Plants were dredged from a coarse sand/rubble habitat, where they were partially decumbent and attached to the substratum by numerous unicellular rhizoids, Thalli are to 25 mm in height, uniaxial, with four pericentral cells and a persistent pigmented trichoblast arising on each axial cell. Tricho‐blasts are arranged in a spiral pattern, with a 90° divergence between successive segments. Each trichoblast is composed of a basal cell bearing a cluster of unbranched filaments. Tetrasporangia are tetrahedral and formed in series in normal branches. Procarps have two sterile cell groups. Spermatangia are formed In heads that are terminal on trichoblasts. The new genus is placed in the tribe Brongniartelleae of the family Rhodomelaceae, differing from all other included genera in the morphology of the trichoblasts.     

ANALYSIS OF MUCILAGE SECRETION AND EXCRETION IN MICRASTERIAS (CHLOROPHYTA) BY MEANS OF IMMUNOELECTRON MICROSCOPY AND DIGITAL TIME LAPSE VIDEO MICROSCOPY1

Two different, independent, and alternative modes of mucilage excretion were found in the unicellular green alga Micrasterias denticulata Bréb. under constant culture conditions. The cells were capable of either excreting mucilage over all their cell surface or they extruded mucilage from one of their polar ends, which enabled directed movement such as photoorientation or escape from unfavorable environmental conditions. By means of a polyclonal antibody raised against Micrasterias mucilage, the secretory pathway of Golgi derived mucilage vesicles from their origin to their discharge was analyzed by means of conventional and energy filtering TEM. Depending on the stage of the cell cycle, mucilage vesicles were subjected to maturation processes. This may occur either after they have been pinched off from the dictyosomes (e.g. during cell growth) or when still connected to trans‐Golgi cisternae, as in the case of interphase cells. Only fully grown mature vesicles contained mucilage in its final composition as indicated by antibody labeling. After fusion of mucilage vesicles with vacuoles, no immunolabeling was found in vacuoles, indicating that the vesicle content was digested. Mucilage vesicles fused with the plasma membrane in areas of cell wall pores but were also able to excrete mucilage at any site directly through the respective cell wall layer. This result disproves earlier assumptions that the pore apparatus in desmids are the only mucilage excreting areas at the cell surface. Both mechanisms, excretion through the pores and through the cell wall, lead to formation of mucilage envelopes covering the entire cell surface.     

Tube formation by complex cellular processes in Ciona intestinalis notochord

In the course of embryogenesis multicellular structures and organs are assembled from constituent cells. One structural component common to many organs is the tube, which consists most simply of a luminal space surrounded by a single layer of epithelial cells. The notochord of ascidian Ciona forms a tube consisting of only 40 cells, and serves as a hydrostatic “skeleton” essential for swimming. While the early processes of convergent extension in ascidian notochord development have been extensively studied, the later phases of development, which include lumen formation, have not been well characterized. Here we used molecular markers and confocal imaging to describe tubulogenesis in the developing Ciona notochord. We found that during tubulogenesis each notochord cell established de novo apical domains, and underwent a mesenchymal–epithelial transition to become an unusual epithelial cell with two opposing apical domains. Concomitantly, extracellular luminal matrix was produced and deposited between notochord cells. Subsequently, each notochord cell simultaneously executed two types of crawling movements bi-directionally along the anterior/posterior axis on the inner surface of notochordal sheath. Lamellipodia-like protrusions resulted in cell lengthening along the anterior/posterior axis, while the retraction of trailing edges of the same cell led to the merging of the two apical domains. As a result, the notochord cells acquired endothelial-like shape and formed the wall of the central lumen. Inhibition of actin polymerization prevented the cell movement and tube formation. Ciona notochord tube formation utilized an assortment of common and fundamental cellular processes including cell shape change, apical membrane biogenesis, cell/cell adhesion remodeling, dynamic cell crawling, and lumen matrix secretion.     

Comparative development of heavily asymmetric-cordate gametophytes of <Emphasis Type="Italic">Anemia phyllitidis</Emphasis> (Anemiaceae) focusing on meristem behavior

Development of heavily asymmetric cordate gametophytes of Anemia phyllitidis (Anemiaceae), one of the schizaeoid ferns, was examined using a sequential observation technique; epi-illuminated light micrographs of the same growing gametophytes were taken approximately every 24 h. The apical cell-like wedge-shaped cell was produced once from the terminal cell of a germ filament, but it stopped dividing soon after production of one or two derivative cells. Without a functional apical cell, the gametophyte developed by intercalary growth until the early stage of wing formation, and then the multicellular (pluricellular) meristem arose from the lower lateral side of the gametophyte. This was in sharp contrast to the observation that the multicellular meristem forms in place of the apical cell in typical cordate gametophytes. Loss of the functional apical cell probably caused a site-shift in the multicellular meristem of the Anemia phyllitidis gametophyte during evolution from apical to lateral. The results suggest that apical cell-based and multicellular meristems are primarily independent of each other. The multicellular meristem produced cells equally in the distal and proximal directions to form wings in both directions but proximally produced cells divided much less frequently. As a result, a heavily asymmetric gametophyte was formed.     

Egg release and germling development in Sargassum horneri (Fucales, Phaeophyceae)

The mature female conceptacle of Sargassum horneri (Turner) C. Agardh has an ostiole filled with a gelatinous plug. The oogonium in the conceptacle has cell walls that can be differentiated into a dense outer and a less dense inner microfibrillar layer. Just prior to egg release, stalk material is produced inside the outer layer and the inner layer disappears. At this stage the gelatinous plug is extruded and mucilage is released through the ostiole. The released eggs are retained on the receptacle by the stalk and are surrounded by a large amount of the mucilage. Three-celled germlings form a primary wall with a polylamellated structure of microfibril layers. In multicellular germlings that have differentiated into thallus and rhizoids, the peripheral thallus cells have an outer cell wall consisting of a microfibril layer under the primary wall, while the cell wall of the rhizoid tip has an amorphous structure. The germlings are released from the stalk and become attached to the substratum by an adhesive substance secreted from rhizoidal cells.     

OBSERVATIONS ON FILAMENT FORMATION IN MICRASTERIAS FOLIACEA (DESMIDIACEAE,CHLOROPHYTA)1

Filaments of Micrasterias foliacea Bailey were isolated from a sample of a Texas lake. Cultures were established and examined by light and scanning electron microscopy (SEM). Enzymatic removal of mucilage proved necessary to obtain well preserved cells for SEM investigation. The development of the overlapping polar lobes and the formation of the interlocking apical teeth are described. The importance of these, of mucilage and the primary wall for filament formation is discussed.     

Leishmanicidal activity of the crude extract,fractions and major piperidine alkaloids from the flowers of Senna spectabilis

Senna spectabilis (sin. Cassia excelsa, C. spectabilis) is an endemic tree of South America and Africa, very common in Brazil, where it is known as “canafistula-de-besouro” and “cassia-do-nordeste”. In folk medicine, this plant is indicated for the treatment of constipation, insomnia, anxiety, epilepsy, malaria, dysentery and headache. Phytopharmacological studies have also confirmed anticonvulsive, sedative, anti-malarial, antimicrobial and cytotoxic properties of many parts of S. spectabilis. In this communication, we present a comparative study of the leishmanicidal activity of the crude ethanolic extract, its fractions and also the two major alkaloidal metabolites (−)-cassine/(−)-spectaline, trying to establish a relationship between the presence of piperidine alkaloidal constituents and leishmanicidal activity. The growth inhibitory effect of promastigote forms of Leishmania major was determined for the crude extract, fractions of the flowers of S. spectabilis and a mixture of (−)-cassine/(−)-spectaline in comparison to pentamidine used as standard drug. The cytotoxic effects were assessed on macrophage strain J774 by lactate dehydrogenase assay. Fractions dichloromethane (FL-DCM) and n-butanol (FL-Bu) and a mixture of (−)-cassine/(−)-spectaline (∼7:3) exhibited significant activity against the parasite Leishmania major (IC₅₀ values of 0.6 ± 0.1 μg/ml, 1.6 ± 0.9 μg/ml and 24.9 ± 1.4 μg/ml, respectively), without toxic effects on murine macrophages. Due to the promising results elicited, further studies in vivo need to be performed to confirm the therapeutic potential of Senna spectabilis.     

Neosiphonia flavimarina gen. et sp. nov. with a taxonomic reassessment of the genus Polysiphonia (Rhodomelaceae, Rhodophyta)

Vegetative and reproductive development of Neosiphonia flavimarina gen. et sp. nov. (Rhodomelaceae, Ceramiales) from Bangpo on the western coast of Korea was investigated. This species is superficially similar to Polysiphonia, but differs distinctly from the latter in vegetative and reproductive structures. The plants attach by a solid disk composed of a dense cluster of rhizoids cut off from the pericentral cell wall, and bear erect indeterminate branches producing the lateral-branch initials from successive segments in a spiral arrangement. The procarps have a three-celled carpo-gonial branch. Spermatangial branches are formed on a primary branch of the trichoblasts, terminating in a single or occasionally two large, sterile cells. Tetra-sporangia are produced from the second pericentral cell adjacent to the trichoblast basal cell on indeterminate branches, and arranged spirally. Comparing several taxonomic characters among related genera, Neosiphonia occupies an independent phylogenetic position from Polysiphonia and leads to the conclusion that the genus may have a strong link with Fernandosiphonia which has a unilateral branching system. Relevant nomenclatural changes for several Polysiphonia species are also proposed.     

DIFFERENTIAL ENZYME ACTIVITY DURING TRICHOBLAST DIFFERENTIATION IN ELODEA CANADENSIS

Enzymes active in the developing root epidermis of Elodea canadensis Michx. were demonstrated by histochemical techniques. The future root-hair-forming cells (trichoblasts) showed a period of elevated activity more extensive than the one previously reported in trichoblasts of another species for dehydrogenases (glucose-6-phosphate, pyruvate, lactate, succinic, isocitrate, glutamate), phosphatases (acid, ATPase, 5-nucleotidase), cytochrome oxidase, and peroxidase. This elevated activity extended from the time of trichoblast formation up to the point of root hair outgrowth, even for enzymes not previously demonstrated in trichoblasts: alkaline phosphatase, NADH diaphorase, NADPH diaphorase, esterase, and leucine aminopeptidase. Glucose-6-phosphatase and aryl sulfatase were not detected. The single exception to this pattern was phosphorylase activity, which intensified only just prior to and during root hair outgrowth. The more generalized activity pattern is considered to indicate the so-called meristematic character of these cells in terms of both macromolecular synthesis and lack of specialization. It is suggested that specific root hair development begins just prior to initiation, at the point marked by elevated phosphorylase activity.     

Root hairs: Specialized tubular cells extending root surfaces

Root hairs are tubular extensions of epidermal cells that have their origin either in any protoderm cell or in specialized protoderm cells called trichoblasts. These latter cells are the result of an asymmetric cytokinesis determined by the positioning of a pre-prophase band of microtubules. The smaller sibling cell is the trichoblast and specializes physiologically and structurally prior to root hair outgrowth. Several genes are involved in the initiation and outgrowth of root hairs. Elongation of root hairs is by tip growth, and, correlated with this, cytoplasmic organelles and cytoskeletal elements show a polarized distribution; the apical dome consists of numerous vesicles, many associated with cell wall synthesis. The relationship between cellulose microfibril deposition and the pattern of cortical microtubules has received considerable attention, as has the role of the cytoskeleton and calcium in controlling cytoplasmic streaming. Root hairs extend the absorbing surface of the root and therefore have been studied in terms both of physiological characteristics of the plasma membrane and uptake of water and of various ions in the soil solution. Many plant species develop soil sheaths (rhizosheaths) which protect the root surface from desiccation and harbour various microorganisms; root hairs are intimately involved in these sheaths. Various growth regulators have been studied in terms of their effect on the structure and function of root hairs. Root hairs play a significant role in the interaction between plants and nitrogen-fixing microorganisms (e.g.,Rhizobium, Frankia) and symbiotic mycorrhizal fungi.     

The role of COBRA‐LIKE 2 function,as part of the complex network of interacting pathways regulating Arabidopsis seed mucilage polysaccharide matrix organization

The production of hydrophilic mucilage along the course of seed coat epidermal cell differentiation is a common adaptation in angiosperms. Previous studies have identified COBRA‐LIKE 2 (COBL2), a member of the COBRA‐LIKE gene family, as a novel component required for crystalline cellulose deposition in seed coat epidermal cells. In recent years, Arabidopsis seed coat epidermal cells (SCEs), also called mucilage secretory cells, have emerged as a powerful model system for the study of plant cell wall components biosynthesis, secretion, assembly and de muro modification. Despite accumulating data, the molecular mechanism of COBL function remains largely unknown. In the current research, we utilized genetic interactions to study the role of COBL2 as part of the protein network required for seed mucilage production. Using correlative phenotyping of structural and biochemical characteristics, unique features of the cobl2 extruded mucilage are revealed, including: ‘unraveled’ ray morphology, loss of primary cell wall ‘pyramidal’ organization, reduced Ruthenium red staining intensity of the adherent mucilage layer, and increased levels of the monosaccharides arabinose and galactose. Examination of the cobl2cesa5 double mutant provides insight into the interface between COBL function and cellulose deposition. Additionally, genetic interactions between cobl2 and fei1fei2 as well as between each of these mutants to mucilage‐modified 2 (mum2) suggest that COBL2 functions independently of the FEI‐SOS pathway. Altogether, the presented data place COBL2 within the complex protein network required for cell wall deposition in the context of seed mucilage and introduce new methodology expending the seed mucilage phenotyping toolbox.     

Ansavaricin J,a New Heterocyclic Ring‐Fused Streptovaricin from Gene stvP5‐Deleted Mutant of Streptomyces spectabilis CCTCC M2017417

A new ring‐fused streptovaricin analogue, named ansavaricin J, was unprecedently isolated from the culture of the genetically modified strains ΔstvP5 which derived from Streptomyces spectabilis CCTCC M2017417. Its structure was elucidated via comprehensive spectroscopic analyses, including 1D‐ and 2D‐NMR tests, and HR‐ESI‐MS data analysis. Notably, ansavaricin J and E represent the only two reported examples of heterocyclic ring‐fused streptovaricins thus far, however, it only showed insignificant antibacterial activities against Staphylococcus aureus.     

Gametangial interaction and oogenesis in the phycomycete Ciliomyces spectabilis (Fungi,Lagenidiales)

Gametangial interaction and oospore formation were studied in Ciliomyces spectabilis, a Lagenidiaceous fungus which is parasitic on ciliate cysts. Electron dense and granular vesicles of the antheridium are engaged in formation of the copulation porus between adjacent thalli. The oosphere is delimited by Golgi-derived cisternae which give rise to the membranes of the oosphere and the periplasm. The contents of the antheridium and the periplasm degenerate. The outer oospore wall is formed by wall vesicles originating from the endoplasmic reticulum. No vesicles are involved in the development of the thick inner oospore wall. Vacuoles with electron dense spherical contents fuse and form the central reserve globule. Lipid bodies aggregate first and disintegrate later into numerous small ones. The number of cytoplasmic organelles decreases. The possibility of wall formation via secretion of soluble wall material is discussed.     

ORIGIN,DEVELOPMENT, AND GROWTH OF DIFFERENTIATING TRICHOBLASTS IN ELODEA CANADENSIS

Root hairs of Elodea canadensis develop only from cells which undergo a particular series of developmental steps. These cells, the trichoblasts, are formed as the smaller, proximal product of an asymmetric division, and immediately enter a prolonged phase of synthesis. Histochemical tests show that large amounts of RNA and protein accumulate in the vastly enlarged nucleolus and cytoplasm, while histone increases in the enlarging nucleus. Cytophotometry shows that DNA in the nucleus reaches polyploid levels. Throughout the synthetic phase, almost to the point of root hair initiation at 9.5 mm proximal to the meristem initials, vacuolation is delayed and the trichoblasts elongate less extensively. All results suggest that this synthesis is the type which normally follows cell division, but is greatly enhanced in the trichoblast. In contrast, the initially larger atrichoblasts only accumulate RNA, DNA, and protein in the region from 1 mm to 2 mm proximal to the meristem tip, and they then enter a phase of extensive vacuolation and elongation.     

The reb1-1 mutation of Arabidopsis alters the morphology of trichoblasts,the expression of arabinogalactan-proteins and the organization of cortical microtubules

The root epidermal bulger 1 ( reb1) mutant of Arabidopsis thaliana (L.) Heynh. is characterized by a reduced elongation rate of the primary root and by the bulging of many, but not all, root epidermal cells. In this study, we investigated cell wall structure of root epidermal cells in reb1-1 by using serial sectioning, and light and electron microscopy in combination with immuno-cytochemistry and polysaccharide staining. We found that: (i) Cell bulging in the mutant was initiated in the zone of elongation of the root, and occurred exclusively in trichoblasts. (ii) reb1-1 and wild-type root cells stained identically with anti-pectin antibodies, such as JIM5. In contrast, the anti-arabinogalactan-protein antibodies, JIM14 and LM2, stained all epidermal cells in the wild type and trichoblasts preferentially, but in reb1-1 they stained the atrichoblasts only. (iii) Compared to the wild type, mutant trichoblasts had a thinner outer epidermal cell wall, which presented abnormal periodic acid-thio carbohydrazide silver proteinate (PATAg) staining. In addition, we investigated the organization of cortical microtubules in a reb1-1 mutant line expressing a green-fluorescent protein fused to a microtubule-binding domain from human microtubule-associated protein 4. Microtubules in the swollen trichoblasts of reb1-1 were either disordered or absent entirely. Together our findings indicate that the reb1-1 mutation results in an abnormal trichoblast cell wall, and suggest that cell surface arabinogalactan-proteins are required for anisotropic expansion and for orienting cortical microtubules.