Development of mestome sheath cells in leaves ofAegilops comosa var.thessalica

Summary The development of mestome sheath cells ofAegilops comosa var.thessalica was studied by electron microscopy. Anatomical and cytological observations show that this grass belongs to the C₃ or non-Kranz plants. In the asymmetrically thickened walls of mestome sheath cells a suberized lamella is present. This lamella is deposited asynchronously. In the midrib and the large lateral bundles it appears first in the outer and inner walls and usually later in the radial walls. In the small lateral bundles its appearance is delayed in the inner walls of those cells situated on the xylem side. At maturity the suberized lamella is observed in all cell walls; however, in the small lateral bundles it is partly or totally absent from the walls of some cells situated on the xylem side. Tertiary wall formation is asynchronous as well, for it generally follows the deposition pattern of the suberized lamella.During the development of the mestome sheath cells microtubules show marked changes in their number and orientation, being fewer and longitudinal during suberin deposition. Dictyosomes are very active and may be involved in primary and tertiary wall formation. Endoplasmic reticulum cisternae are abundant and partly smooth, while plasmalemmasomes may function to reduce the plasmalemma extension. However, cytoplasmic structures that are clearly involved in suberin synthesis could not be identified.Suberized lamellae react strongly with silver hexamine. This is probably due to post-fixation with osmium tetroxide.On the basis of structural characteristics the mestome sheath may be regarded as an endodermis (cf., alsoFahn 1974). The significance of this view for water and assimilate exchange between the mesophyll and the bundle is discussed.This report represents a portion of a doctoral dissertation.     

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.     

Inter- and intraplant C-banding polymorphism in one population ofAegilops comosa subsp.comosa var.comosa (Poaceae)

The Giemsa C-banding pattern of the chromosomes of the native self-pollinatedAegilops comosa subsp.comosa var.comosa was studied. Six of the seven chromosomes of the haploid genome were found to be polymorphic for C-banding patterns. Chromosome A had four variants, chromosome E three variants and each of the chromosomes B, D, and F two variants. Chromosomes E and G were polymorphic for arm length and arm ratio.This paper is part of the doctoral dissertation ofA. Georgiou.     

Evidence for initiation of microtubules in discrete regions of the cell cortex in Azolla root-tip cells,and an hypothesis on the development of cortical arrays of microtubules

Complexes of microtubules, vesicles, and (to varying degrees) dense matrix material around the microtubules were seen along the edges of cells in root apices of Azolla pinnata R.Br. (viewing the cells as polyhedra with faces, vertices and edges). They are best developed after cytokinesis has been completed, when the daughter cells are reinstating their interphase arrays of microtubules. They are not confined to edges made by the junction of new cell plates with parental walls, but occur also along older edges. Similar matrices and vesicles are seen amongst phragmoplast microtubules and where pre-prophase bands intersect the edges of cells. It is suggested that the complexes participate in the development of cortical arrays of microtubules. The observations are combined with others, made on pre-prophase bands and on the substructure of cortical arrays lying against the faces of cells, to develop an hypothesis on the development of cortical microtubules, summarised below: Microtubules are nucleated along the edges of cells, at first growing in unspecified orientations and then becoming bridged to the plasma membrane. Parallelism of microtubules in the arrays arises by inter-tubule cross-bridging. Lengths of microtubule are released from, or break off, the nucleating centres and are moved out onto the face of the cell by intertubule and tubule-membrane sliding, thus accounting for the presence there of short tubules with randomly placed terminations. The nucleating zones along cell edges might have vectorial properties, and thus be able to control the orientation of the microtubules on the different faces of the cell. Also, localised activation could generate localised arrays, especially pre-prophase bands in specified sites and planes. Two possible reasons for the spatial restriction of nucleation to cell edges are considered. One is that the geometry of an edge is itself important; the other is that along most cell edges there is a persistent specialised zone, inherited at cytokinesis by the daughter cells when the cell plate bisects the former pre-prophase-band zone.     

Abnormal structure of protophloem sieve-element cell wall in colchicine-treated roots of Triticum aestivum L.

The structural aberrations of the cell walls of protophloem sieve elements (PSEs) in roots of wheat (Triticum aestivum L. cv. Maris Huntsman) caused by the anti-microtubule drug colchicine were investigated by electron microscopy. The initial effect of the drug on cell wall development was found to be an exceptionally rough wall surface, presumably caused by an uncontrolled fusion of Golgi vesicles with the plasma membrane. Cellulose microfibrils, which in normal PSEs are aligned transversely to the long axis and parallel to the cortical microtubules, in colchicine-treated PSEs display a predominant longitudinal orientation. The pattern of wall development is disturbed by deposition of wall material also within the sieve pores of the sieve-pore/plasmodesmata complexes, resulting in evenly thickened walls instead of the normal uneven layers, and in narrowing the sieve pores to the size of plasmodesmata. In prolonged and continuous colchicine treatment, PSEs develop unusual wall ingrowths projecting deeply into the cytoplasm, creating an extraordinary cell type not found in normal roots. The results confirm the view of microtubule involvement in the proper deposition and orientation of cellulose microfibrils, and in the normal patterning of the cell wall thickenings of differentiating PSEs.Abbreviations c colchicine-treated - PSE protophloem sieve element The author is grateful to Dr. B. Galatis, Dr. P. Apostolakos and Dr. C. Katsaros, Institute of General Botany, University of Athens, Greece, for helpful discussions and suggestions, and for the generous gift of the colchicine used here. This work was carried out in the Department of Botany, University of Thessaloniki, Greece, while observations were also made in the Lehrstuhl für Zellenlehre, University of Heidelberg, Germany, and in the Department of Botany, University of Georgia, USA. The author is thankful to Prof. E. Schnepf (Zellenlehre, Heidelberg, Germany) and Prof. B.A. Palevitz (Department of Botany, University of Athens, Ga., USA), for generously providing access to their equipment and facilities. The work was financially supported in part by the Stiftung Volkswagenwerk and by the Research Committee, University of Thessaloniki (No 7537).     

Developmental features of protophloem sieve elements in roots of wheat (Triticum aestivum L.)

Summary Protophloem sieve elements (PSEs) in roots of wheat (Triticum aestivum L.) are arranged in single vertical files. The number of PSEs within the files increases by symmetrical divisions, which take place after the completion of asymmetrical (formative) divisions and before the initiation of differentiation. The divisions are preceded by well defined pre-prophase bands (PPB) of microtubules, which surround the nucleus in an equatorial position. In the cytoplasmic region between the nuclear surface and the PPB, perinuclear and endoplasmic microtubules were observed. The perinuclear microtubules are considered as part of the developing spindle, while the endoplasmic ones interlink the perinuclear microtubules with the PPB. Dividing cells do not show any signs of incipient differentiation. The first and most reliable indication of a commencing differentiation is provided by the sieve-element plastids that begin to accumulate dense crystalloid inclusions in the very young PSEs. In mature PSEs plastids contain two kinds of crystalloid inclusions, dense and thin, in a translucent stroma. Depending on the plastid-inclusions criterion it was shown that: (a) the PSEs of a given root do not initiate differentiation at exactly the same stage, (b) the developmental sequence extends to a span of 7–9 actively differentiating PSEs arranged in a single vertical file, and (c) each PSE needs about 16–21 h to pass through the whole developmental sequence. In the last two differentiating PSEs of a file, mitochondria were found to be enveloped by single cisternae of ER. The association is temporary as it is lost in the first PSEs with an autolysed lumen. During differentiation, Golgi bodies were abundant and active in producing vesicles involved in cell wall development. Golgi vesicles were also found among the microtubules of the PPB, but no local thickening was observed. Golgi bodies disorganize in the last stages of autolysis and disappear in mature sieve elements.Abbreviations ER endoplasmic reticulum - MSE metaphloem sieve element - PPB pre-prophase band - PSE protophloem sieve element Dedicated to Prof. Dr. Dr. h.c. Eberhard Schnepf on the occasion of his retirement     

Some effects of colchicine on microtubules and cell division in roots ofAzolla pinnata

Summary Removal and subsequent reformation of microtubules in cells of the root-tips ofAzolla pinnata R. Br. was achieved by short pulse treatments with the drug colchicine. Loss of microtubules led to the formation of multinucleate cells more frequently than to the arrest of mitosis at metaphase, and primary and secondary wall formation was also disrupted. Recovery of root development was limited. Growth of all roots ceased 5–6 days after the pulse treatment. Following the reappearance of microtubules, renewed deposition of normal wall thickenings occurred in developing xylem elements. Multinucleate cells became subdivided by walls in the apparent absence of a phragmoplast. The plane in which the new wall was formed was often located as it would have been in an untreated root, but in a number of cases abnormal or precious positioning of new walls was observed. Clusters of microtubules, matrix material, and vesicles or particles, taken to indicate microtubule initiation, were observed during the recovery from treatment.     

An unusual feature of developing protophloem sieve elements in roots ofTriticum aestivum L.

Summary Developing protophloem sieve elements in roots of wheat are arranged in single vertical files. In the last immature differentiating sieve element bearing ribosomes the proximal end of the cytoplasm displays a diluted appearance in contrast to the distal end where the cytoplasm exhibits a considerably increased electron density. Differences can also be observed in ribosome quantity, organelle ultrastructure and the time of initiation of cell component degradation, those at the proximal end disorganizing first, suggesting a nonsimultaneous disorganization of the cell components in the two areas. This phenomenon, termedheterochronic lysis, is presumably an expression of an existing polarity not detectable in younger stages, but it might also be the result of an asynchronous enzymatic activity.Abbreviations CW Cell wall - D dictyosome - ER endoplasmic reticulum - M mitochondrion - N nucleus - P plastid - SE sieve element - SP sieve plate     

Analysis of cytoplasmic microtubules and flagellar roots in the zoospores ofAllomyces macrogynus

Summary Cytoskeletal and flagellar microtubules in the zoospores of the aquatic fungusAllomyces macrogynus are resistant to microtubule depolymerizing drugs. Consequently, we have analyzed the partial composition and organization of microtubules (Mts) in the cytoplasm and flagellar apparatus in the zoospores ofA. macrogynus. Evidence from two-dimensional gel electrophoresis demonstrated the presence of two -tubulin isoforms in axonemal and cytoplasmic Mts. In addition, a monoclonal antibody specific for acetylated -tubulin was used on one-dimensional protein blots to show that acetylated -tubulins are present in isolated zoospore cell bodies and axonemes. Immunofluorescence microscopy observations using this monoclonal antibody demonstrated that flagellar, kinetosomal, and cytoplasmic Mts were labeled. The nature of Mts in the flagellar apparatus was studied ultrastructurally. InA. macrogynus, the flagellar apparatus consists of the kinetosome, rhizopolast (striated flagellar rootlet), axoneme, and 9 sets of triplet Mts which radiate anteriorly from the proximal end of the kinetosome (microtubular rootlet), Analysis of the rhizoplast indicated that this structure does not contain Mts. The rhizoplast, which connects the functional kinetosome with a single, large basal mitochrondrion, consists of four electron-opaque bands. Serial-sectioning indicated that the rhizoplast is always adjacent to kinetosome triplets 1, 2, and 9, and thus lies perpendicular to the plane of flagellar beat. These results suggest that the primary function of the rhizoplast is to organize the kinetosome and mitochondrion with respect to one another and to bias flagellar beat in the appropriate orientation for cell motility.Abbreviations BSA bovine serum albumin - BCA bicinchoninic acid - DS dilute salts - EGTA ethylene glycol-bis-(-aminoethyl ether)-N,N-tetracetic acid - EM electron microscopy - Mes 2-(N-morpholinomethane sulfonic acid - Mt microtubule - NP-40 Nonidet P-40 - 1-D PAGE one-dimensional polyacrylamide gel electrophoresis - PBS phosphate-buffered saline - PMSF phenylmethylsulfonyl fluoride - SDS sodium dodecyl sulfate - 2-D PAGE two-dimensional polyacrylamide gel electrophoresis - Tween-20 polyoxyethylenesorbitan monolaurate     

Formative and proliferative cell divisions,cell differentiation,and developmental changes in the meristem of Azolla roots

The root of the water fern Azolla is a compact higher-plant organ, advantageous for studies of cell division, cell differentiation, and morphogenesis. The cell complement of A. filiculoides Lam. and A. pinnata R.Br. roots is described, and the lineages of the cell types, all derived ultimately from a tetrahedral apical cell, are characterised in terms of sites and planes of cell division within the formative zone, where the initial cells of the cell files are generated. Subsequent proliferation of the initial cells is highly specific, each cell type having its own programme of divisions prior to terminal differentiation. Both formative and proliferative divisions (but especially the former) occur in regular sequences. Two enantiomorphic forms of root develop, with the dispositions of certain types of cell correlating with the direction, dextrorse or sinistrorse, of the cell-division sequence in the apical cells. Root growth is determinate, the apical cell dividing about 55 times, and its cell-cycle duration decreasing from an initial 10 h to about 4 h during the major phase of root development. Sites of proliferation progress acropetally during aging, but do not penetrate into the zone of formative divisions. The detailed portrait of root development that was obtained is discussed with respect to genetic and epigenetic influences; quantal and non-quantal cell cycles; variation in cell-cycle durations; relationships between cell expansion and cell division: the role of the apical cell; and the limitation of the total number of mitotic cycles during root formation.     

Microtubules in statocytes from roots of cress (Lepidium sativum L.)

Summary Statocytes in root caps ofLepidium sativum L. were examined by means of ultrathin serial sections to evaluate the amount and distribution of cortical microtubules. The microtubules encircle the cell, oriented normal to the root length axis. In the distal cell edges, microtubules form a network, separating the distal complex of endoplasmic reticulum from the plasmalemma. Preprophase bands in meristem cells are observable rarely, structures which can be regarded as nucleating sites for microtubules are lacking. During ageing of the root cap cells, the number of microtubules increases in combination with a decrease of microtubule length. Development of the roots on a horizontal clinostat preserves a younger developmental stage of the microtubule system regarding amount and length of the individual microtubules. Evidence for an involvement of microtubules in graviperception is low, whereas their role in orienting cellulose microfibrils cannot be ruled out. Compression of the distal network of microtubules after centrifugation of the roots indicates that microtubules in statocytes ofLepidium sativum L. roots might function in stabilizing the distal complex of endoplasmic reticulum.     

Re-organization of microtubules during preprophase band development inAdiantum protonemata

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

The organization of microtubules during generative-cell division inConvallaria majalis

Summary The organization of the microtubule cytoskeleton in the generative cell ofConvallaria majalis has been studied during migration of the cell through the pollen tube and its division into the two sperm cells. Analysis by conventional or confocal laser scanning microscopy after tubulin staining was used to investigate changes of the microtubule cytoskeleton during generative-cell migration and division in the pollen tube. Staining of DNA with 4,6-diamidino-2-phenylindole was used to correlate the rearrangement of microtubules with nuclear division during sperm cell formation. Before pollen germination the generative cell is spindle-shaped, with microtubules organized in bundles and distributed in the cell cortex to form a basketlike structure beneath the generative-cell plasma membrane. During generative-cell migration through the pollen tube, the organization of the microtubule bundles changes following nuclear division. A typical metaphase plate is not usually formed. The generative-cell division is characterized by the extension of microtubules concomitant with a significant cell elongation. After karyokinesis, microtubule bundles reorganize to form a phragmoplast between the two sperm nuclei. The microtubule organization during generative-cell division inConvallaria majalis shows some similarities but also differences to that in other members of the Liliaceae.Abbreviations CLSM confocal laser scanning microscopy - EM electron microscopy - GC generative cell - GN generative nucleus - MT microtubule - SC sperm cell - SN sperm nucleus - VN vegetative nucleus     

Preprophase bands of microtubules and the cell cycle: Kinetics and experimental uncoupling of their formation from the nuclear cycle in onion root-tip cells

We have studied the timing of preprophase band (PPB) development in the division cycle of onion (Allium cepa L.) root-tip cells by combinations of immunofluorescence microscopy of microtubules, microspectrophotometry of nuclear DNA, and autoradiography of [³H]thymidine incorporation during pulse-chase experiments. In normally grown onion root tips, every cell with a PPB had the G2 level of nuclear DNA. Some were in interphase, prior to chromatin condensation, and some had varying degrees of chromatin condensation, up to the stage of prophase at which the PPB-prophase spindle transition occurs. In addition, autoradiography showed that PPBs can be formed in cells which have just finished their S phase, and microspectrophotometry enabled us to detect a population of cells in G2 which had no PPBs, these presumably including cells which had left the division cycle. The effects of inhibitors of DNA synthesis showed that the formation of PPBs is not fully coupled to events of the nuclear cycle. Although the mitotic index decreased 6-10-fold to less than 0.5% when roots were kept in 20 g·ml^-1 aphidicolin for more than 8 h, the percentage of cells containing PPBs did not decrease in proportion: the number of cells in interphase with PPBs increased while the number in prophase decreased. Almost the same phenomena were observed in the presence of 100 g·ml^-1 5-aminouracil and 40 g·ml^-1 hydroxyurea. In controls, all cells with PPBs were in G2 or prophase, but in the presence of aphidicolin, 5-aminouracil or hydroxyurea, some of the interphase cells with PPBs were in the S phase or even in the G1 phase. We conclude that PPB formation normally occurs in G2 (in at least some cases very early in G2) and that this timing can be experimentally uncoupled from the timing of DNA duplication in the cell-division cycle. The result accords with other evidence indicating that the cytoplasmic events of cytokinesis are controlled in parallel to the nuclear cycle, rather than in an obligatorily coupled sequence.Abbreviations APC aphidicolin - 5-AU 5-aminouracil - DAPI 4, 6-diamidino-2phenylindole - HU hydroxyurea - MI mitotic index - MT microtubule - PMSF phenylmethyl-sulfonyl fluoride - PPB preprophase band - %PPB percentage of cells with PPBs     

Microscopic studies of cell divisions induced in alfalfa roots by Rhizobium meliloti

We have used spot-inoculation and new cytological procedures to observe the earliest events stimulated in alfalfa (Medicago sativa L.) roots by Rhizobium meliloti. Roots were inoculated with 1–10 nl of concentrated bacteria, fixed in paraformaldehyde, and after embedding and sectioning stained with a combination of acridine orange and DAPI (4-6-diamidino-2-phenylindole hydrochloride). Normal R. meliloti provoke cell dedifferentiation and mitosis in the inner cortex of the root within 21–24 h after inoculation. This activation of root cells spreads progressively, leading to nodule formation. In contrast, the R. meliloti nodA and nodC mutants do not stimulate any activation or mitosis. Thus the primary and earliest effect of Rhizobium nod gene action is plant cellular activation. A rapid, whole-mount visualization by lactic acid shows that the pattern of nodule form varies widely. Some R. meliloti strains were found to be capable of stimulating on alfalfa roots both normal nodules and a hybrid structure intermediate between a nodule and a lateral root.     

Prophase bands of microtubules occur in protoplast cultures of Vicia hajastana Grossh

Circumnuclear bands of microtubules (MT) have been found in the prophase of mitoses in cultured protoplasts of Vicia hajastana. The timing of the appearance and disappearance of the prophase band of MT (PB) relative to the stage of mitosis was studied using simultaneous staining of MT by immunofluorescence and DNA by Hoechst 33258. These protoplasts regenerate into unorganized tissue. Pre-prophase bands of MT have previously been found only in highly organized tissues of higher plants. The role of PB in cell division is discussed.Abbreviations MT microtubule(s) - PB prophase band(s) - FPB pre-prophase band(s) - PNF perinuclear fluorescence     

Reorganization of cortical microtubules and cellulose deposition during leaf formation in Graptopetalum paraguayense

In the regeneration of a shoot from a leaf of the succulent, Graptopetalum paraguayense E. Walther the first new organs are leaf primordia. The original arrangement of cellulose microfibrils and of microtubules (MTs) in the epidermis of the leaf-forming site is one of parallel, straight lines. In the new primordium both structures still have a congruent arrangement but it is roughly in the form of concentric circles that surround the new cylindrical organ. The regions which undergo the greatest shift in orientation (90°) were studied in detail. Departures from the original cellulose alignment are detected in changes in the polarized-light image. Departures from the original cortical MT arrangement are detected using electron microscopy. The over-all reorganization of the MT pattern is followed by the tally of MT profiles, the various regions being studied in two perpendicular planes of section. This corrects for the difference in efficiency in counting transverse versus longitudinal profiles of MTs. Reorientation takes place sporadically, cell by cell, for both the cellulose microfibrils and the MTs, indicating a coordinated reorientation of the two structures. That MTs and cellulose microfibrils reorient jointly in individual cells was shown by reconstruction of the arrays of cortical MTs in paradermal sections of individual cells whose recent change in the orientation of cellulose deposition had been detected with polarized light. Closeness of the two alignments was also indicated by images where the MT and microfibril alignments co-varied within a single cell. The change-over in alignment of the MTs appears to involve stages where arrays of contrasting orientation co-exist to give a criss-cross image. During this critical reorganization, the frequency of the MTs is high. It falls during subsequent enlargement of the organ. It was found that the rearrangement of the cortical MTs to approximate a series of concentric circles on the residual meristem occurred before the emergence of leaf primordia. Through their apparent influence on microfibril alignments, the changes in MT disposition, described here, have the potential to generate major biophysical changes that accompany organogenesis.Abbreviation MT(s) microtubule(s)     

New arrays of cytoplasmic microtubules in the fission yeastSchizosaccharomyces pombe

Summary New arrays of microtubules in the fission yeastSchizosaccharomyces pombe, which distribute in the cell in a cell cycle-dependent manner, were characterized using conventional and confocal laser scanning immunofluorescence microscopy. During the interphase and prophase, we observed abundant cytoplasmic microtubules between cell poles, a peripheral network of randomly and helically distributed cortical microtubules, and perinuclear microtubules surrounding the nucleus. At the anaphase and telophase, an equatorial ring containing tubulin was visualized. This ring colocalized with an actin contractile ring, suggesting that they may control the plane of cell division cooperatively.Abbreviations MT(s) microtubule(s) - cMT(s) cytoplasmic microtubule(s) - CLSM confocal laser scanning microscopy - DAPI 4,6-diamidino-2-phenylindole     

Abundance of microtubules in preprophase bands of some Triticum species

Root tip procambial cells of Triticum speltoides, T. tauschii, T. turgidum and T. aestivum have been investigated ultrastructurally for the detection of preprophase microtubule bands (PMBs) and to estimate the number of microtubules comprising the bands. The species selected are phylogenetically related but differ in the ploidy level. It was found that all species develop well-defined PMBs prior to mitosis. Estimations of microtubule abundance in the PMBs was carried out in midpreprophase cells, a stage judged by a feature of the nucleus in which electron-transparent canals are formed around the initial condensations of the chromatin material and the nucleoli. Triticum speltoides bears the smaller average number of microtubules per PMB and T. aestivum the greater. The results indicate that the increase follows the upgrade of the number of chromosome sets. It is suggested that the average number of microtubules of PMBs is related to the ploidy level.Abbreviation PMB preprophase microtubule band     

Twisted growth and organization of cortical microtubules

In plants, directional cell expansion greatly contributes to the final shape of mature cells, and thus to organ architecture. A particularly interesting mode of cell expansion is helical growth in which the growth axis is continuously tilted either to the right or to the left as the cell grows. Fixed handedness of helical growth raises fundamental questions on the possible origin of left–right asymmetry. Twisting mutants of Arabidopsis thaliana offer unique opportunities to study the cellular basis of helical growth. Most of the twisting mutants with fixed handedness have been shown to have defects in microtubule functions, whereas mutants that twist in non-fixed directions appear to be defective in auxin response or transport. Good correlations have been found between the tilted growth direction and alignment of cortical microtubule arrays in twisting mutants with compromised microtubule functions. The present challenge is to understand how particular array patterns are organized during progression of the interphase in rapidly expanding cells. Molecular and cell biological studies on twisting mutants will lead to better understanding on how wild-type plant cells utilize the microtubule cytoskeleton to initiate and rigorously maintain straight growth. An erratum to this article can be found at