Disruption of microtubules and retardation of development ofDictyostelium with ethylN-phenylcarbamate and thiabendazole

Summary The effects of ethylN-phenylcarbamate (EPC) and thiabendazole (TB) onDictyostelium discoideum andD. mucoroides cells were examined as a step toward purifying tubulin and clarifying the function of microtubules in cellular slime molds. EPC (1.5 × 10^–3M) or TB (5 × 10^–5M) inhibited the development ofDictyostelium, inducing the formation of aberrant fruiting bodies with stalks irregular in shape and sori containing spores of various sizes and shapes.EPC and TB inhibited cell division but not cell growth, resulting in the production of giant cells up to ten times larger than untreated cells. The giant cells either had a single huge nucleus of irregular shape or contained multiple nuclei. The effects of the inhibitors were reversible. After the removal of the inhibitors, the giant cells underwent successive cell divisions producing many daughter cells. Interestingly, most of the giant cells induced by EPC treatment contained gigantic secondary lysosomes probably produced by extensive lysosomophagy.Light microscopy using Nomarski optics revealed that these inhibitors caused the round-up of the cells resulting in the inhibition of cell locomotion, whereas non-Brownian movement of the cytoplasmic granules was not affected. Indirect immunofluorescence using anti--tubulin revealed that networks of microtubules were apparently destroyed by the EPC or TB treatment.These results show both EPC and TB are potent inhibitors of microtubules inDictyostelium and are effective tools for studying the function of microtubules either in cellular or multicellular organization throughout its life cycle.     

Expansins and cell wall expansion

The subject of this review is the discovery of expansins andtheir role in plant cell wall expansion. The review is introducedwith a summary of the importance of wall expansion in the controlof plant cell growth, and a brief discussion of the nature ofcell wall extension in plants. The role of expansins in wallextension and their mechanism of action will be reviewed, and,finally, the role of expansins in plant cell growth will bediscussed. Key words: Expansins, cell expansion, cell wall extension, plant growth     

Do microtubules orient plant cell wall microfibrils?

Cortical microtubules (MTs) allegedly orient nascent cellulose microfibrils (CMFs) in plant cells. The frequently observed parallelism between them, and the effect of MT-depolymerizing agents, are the bases for this hypothesis. Data have, however, accumulated about cells in which MTs and CMFs are not in parallel alignment. These data will be reviewed. MT orientation cannot be the only factor determining CMF orientation, but MTs could overrule other factors in cells where, for instance, they are more tightly attached to the plasma membrane than in other cells. MT and CMF orientations could, however, both be controlled by a third factor, and CMFs may even impose orientation on MTs.     

Cell expansion in the epidermis: microtubules, cellulose orientation and wall loosening enzymes

Two models of isolated epidermis were used to demonstrate that the net orientation of cellulose microfibrils in the cell wall is related to mechanical properties of the tissue, and can be used as an indicator for wall anisotropy. In the developing plant epidermis, cells expand in one or two directions in the plane of the plant surface. In epidermis cells actively expanding in one direction (elongation), the orientation of cortical microtubules closely matches the net cellulose orientation. In epidermis cells expanding in two directions, the orientation of the parallel microtubules does not coincide with the net cellulose orientation in the adjacent cell wall. The orientation of cortical microtubules is thus not always a reliable indicator of wall characteristics. In both types of epidermis, a high rate of expansion correlates with a high activity of xyloglucan endotransglycosylase (XET), as determinedin situ. This high activity alone cannot explain unidirectional wall expansion.     

Effects of reduction of auxin and destruction of microtubules on cell wall proteins and cell morphology of the BY-2 line of tobacco cells

Variations of cell wall proteins and proteins in the medium associated with changes in cell morphology were investigated in the BY-2 line of cultured cells. BY-2 cells cultured in LS medium grew as long chains of cells, with the plane of division perpendicular to the longitudinal axis. Reduction in the levels of auxin in the medium resulted in inhibition of cell division and promotion of cell elongation. Levels of cell wall proteins in cell walls decreased and relative levels of cell wall proteins and proteins in the medium changed. Upon treatment with the anti-microtubule drug, propyzamide, cells expanded laterally. Level of cell wall proteins and relative levels of individual cell wall proteins did not change very much, but levels of proteins in the culture medium increased. In both cases, levels of acid and basic peroxidases in cell walls increased and isozyme patterns of these changed.     

Augmented growth equation for cell wall expansion

The Growth Equation representing the relative rate of irreversible wall expansion is augmented with an elastic expansion component. Some of the utility of this augmented Growth Equation is demonstrated through selected applications.     

Plant cell expansion: scaling the wall

The regulation of plant cell size and shape is poorly understood at the molecular level. Recently, two loci required for normal cell expansion in Arabidopsis were cloned. They both encode enzymes involved in the construction of the cell wall. These studies are the first promising examples of the use of Arabidopsis molecular genetics for the study of wall synthesis and assembly during plant cell elongation.     

Auxin-induced cell expansion in relation to cell wall extensibility

Decapitation of 30 mm oat coleoptiles, which are commonly usedfor growth tests, resulted in a decrease in their elastic extensibility(DE) but not in their plastic extensibility (DP). By auxin treatmentunder osmotic stress, old coleoptile (45 mm) cells showed noincrease in subsequent expansion in water, whereas RNA synthesisin these cells was stimulated just as in young ones. Auxin increasedthe DE of young coleoptile cell walls but not that of old ones.Significant increase of DE occurred in only 10 min, and themaximum level of DE was reached in 15 min of the auxin treatment.An antiauxin (2,4,6-trichlorophenoxyacetic acid), mitomycinC and cycloheximide inhibited auxin-induced increases in expansionand DE (or Rex, reversible extensibility) of young coleoptilecells. (Received July 23, 1968; )     

Intercourse between cell wall and cytoplasm exemplified by arabinogalactan proteins and cortical microtubules

How does a plant cell sense and respond to the status of its cell wall? Intercourse between cell wall and cytoplasm has long been supposed to involve arabinogalactan proteins, in part because many of them are anchored to the plasma membrane. Disrupting arabinogalactan proteins has recently been shown to disrupt the array of cortical microtubules present just inside the plasma membrane, implying that microtubules and arabinogalactan proteins interact. In this article, we assess possibilities for how this interaction might be mediated. First, we consider microdomains in the plasma membrane (lipid rafts), which have been alleged to link internal and external regions of the plasma membrane; however, the characteristics and even the existence of these domains remains controversial. Next, we point out that disrupting the synthesis of cellulose also can disrupt microtubules and consider whether arabinogalactan proteins are part of a network linking microtubules and nascent microfibrils. Finally, we outline several signaling cascades that could transmit information from arabinogalactan proteins to microtubules through channels of cellular communication. These diverse possibilities highlight the work that remains to be done before we can understand how plant cells communicate across their membranes.     

Electric fields affect the orientation of cortical microtubules and cell expansion in pea callus

Summary Cortical microtubules in callus derived fromPisum sativum roots form parallel arrays within cells but are randomly oriented across the tissue. These arrays align perpendicular to the direction of an applied electric field of 6 mV per cell. Application of a field of 6 mV per cell for 4 days resulted in the co-ordinated expansion of cells parallel to the field direction. Cortical microtubule arrays were still aligned perpendicular to the applied field 24 h after removal of the field. The imposition of a field to callus after the removal of cortical microtubules by oryzalin and in the presence of the herbicide resulted in the orientation of recovering microtubules perpendicular to the direction of the field, indicating that microtubules are not directly involved in the detection of the field.Abbreviations EGTA ethylene glycol-bis (-aminoethyl ether) N,N,N-tetraacetic acid - FITC fluorescein isothiocyanate - MSB microtubule stabilising buffer - PIPES piperazine-N,N-bis(2-ethanesulphonic acid) - oryzalin 3,5-dinitro-N⁴,N⁴ dipropylsulphanil-amide     

Stabilization of cortical microtubules by the cell wall in cultured tobacco cells

Cortical microtubules (MTs) in protoplasts prepared from tobacco (Nicotiana tabacum L.) BY-2 cells were found to be sensitive to cold. However, as the protoplasts regenerated cell walls they became resistant to cold, indicating that the cell wall stabilizes cortical MTs against the effects of cold. Since poly-l-lysine was found to stabilize MTs in protoplasts, we examined extensin, an important polycationic component of the cell wall, and found it also to be effective in stabilizing the MTs of protoplasts. Both extensin isolated from culture filtrates of tobacco BY-2 cells and extensin isolated in a similar way from cultures of tobacco XD-6S cells rendered the cortical MTs in protoplasts resistant to cold. Extensin at 0.1 mg·ml⁻¹ was as effective as the cell wall in this respect. It is probable that extensin in the cell wall plays an important role in stabilizing cortical MTs in tobacco BY-2 cells.     

植物激素在植物细胞壁扩展中的作用

细胞壁不仅是植物细胞结构的重要组成部分,而且控制着细胞的大小、形状和生长。细胞经有丝分裂后,原生质体吸水膨胀,细胞壁重塑,新生壁物质合成,纤维素定向沉积等引发细胞壁生长。在这些过程中,乙烯（ethylene,ET）、生长素（auxin）、赤霉素（gibberellin,GA）、油菜素甾醇（brassinosteroids,BR）等植物激素调控细胞壁生长相关酶类如纤维素合酶复合体（cellulose synthase A,CESA）、扩展素（expansin,EXP）、木葡聚糖内糖基转移酶／水解酶（xyloglucan endotran glucosylase／hydrolase,XET／XTH）的表达活性,进而调控细胞壁扩展,促使细胞壁的生长。     

Cortical microtubules and microfibril deposition in the cell wall of root hairs ofEquisetum hyemale

Summary The cell wall of root hairs ofEquisetum hyemale is shown to be composed of three different cell wall textures. The growing cell wall at the tip of the hair is composed of a dispersed texture of microfibrils, which continues along the outside of the whole hair. With increasing distance from the tip an increasing number of helicoidally arranged lamellae is deposited. These findings correspond with the observed isotropism of young hairs in polarized light.Hairs of approximately 4 days old become positive birefringent, indicating that longitudinally oriented layers prevail over layers with a transverse direction. This phenomenon starts at the base of the hair. Full-grown hairs are positive birefringent up to the tip and concordantly show a thick additional inner cell wall layer which forms a helical pattern the length of the hair, with a mean microfibril angle of 25 with the cell axis.Cortical microtubules, subjacent to the dispersed, the helicoidal and the helical wall texture are axially aligned and, thus, not in coalignment with the last deposited microfibrils.Coated and smooth vesicles are present in the cortical cytoplasm of both growing and full-grown hairs. Electron-dense profiles (20 nm in diameter), surrounded by a halo (of 50 nm) were observed on the wall-plasmalemma interface in full-grown hairs only. A relation of these structures with microfibril deposition could not be demonstrated. They might represent channels transporting material to the wall, which, in full-grown hairs, is heavily impregnated with a tawny brown substance.The general hypothesis that cortical microtubule orientation directs microfibril deposition is disputed.     

Chemically mediated mechanical expansion of the pollen tube cell wall

Morphogenesis of plant cells is tantamount to the shaping of the stiff cell wall that surrounds them. To this end, these cells integrate two concomitant processes: 1), deposition of new material into the existing wall, and 2), mechanical deformation of this material by the turgor pressure. However, due to uncertainty regarding the mechanisms that coordinate these processes, existing models typically adopt a limiting case in which either one or the other dictates morphogenesis. In this report, we formulate a simple mechanism in pollen tubes by which deposition causes turnover of cell wall cross-links, thereby facilitating mechanical deformation. Accordingly, deposition and mechanics are coupled and are both integral aspects of the morphogenetic process. Among the key experimental qualifications of this model are: its ability to precisely reproduce the morphologies of pollen tubes; its prediction of the growth oscillations exhibited by rapidly growing pollen tubes; and its prediction of the observed phase relationships between variables such as wall thickness, cell morphology, and growth rate within oscillatory cells. In short, the model captures the rich phenomenology of pollen tube morphogenesis and has implications for other plant cell types.     

Expansin(细胞壁松弛蛋白)的发展

Expansin是一种体外诱导分离的植物细胞壁伸展的蛋白,在修饰细胞壁基础上使细胞膨胀。Expansin的功能众多,除了促进细胞生长,还包括影响营养生长、形态发生、授粉受精、果实软化等,并表现出高度的组织、器官和细胞特异性。目前已经在多种植物及其他一些生物范围内对expansin及类expansin序列和蛋白质进行了研究,并对它们的作用机制进行了探索。     

Brassinosteroids, microtubules and cell elongation in Arabidopsis thaliana. II. Effects of brassinosteroids on microtubules and cell elongation in the bul1 mutant

In order to elucidate the involvement of brassinosteroids in the cell elongation process leading to normal plant morphology, indirect immunofluorescence and molecular techniques were use to study the expression of tubulin genes in the bul1-1 dwarf mutant of Arabidopsis thaliana (L.) Heynh., the characteristics of which are reported in this issue (M. Catterou et al., 2001). Microtubules were studied specifically in the regions of the mutant plant where the elongation zone is suppressed (hypocotyls and petioles), making the reduction in cell elongation evident. Indirect immunofluorescence of α-tubulin revealed that very few microtubules were present in mutant cells, resulting in the total lack of the parallel microtubule organization that is typical of elongating cells in the wild type. After brassinosteroid treatment, microtubules reorganized and became correctly oriented, suggesting the involvement of brassinosteroids in microtubule organization. Molecular analyses showed that the microtubule reorganization observed in brassinosteroid-treated bul1-1 plants did not result either from an activation of tubulin gene expression, or from an increase in tubulin content, suggesting that a brassinosteroid-responsive pathway exists which allows microtubule nucleation/organization and cell elongation without activation of tubulin gene expression. Received: 28 April 2000 / Accepted: 6 October 2000     

A model of cell wall expansion based on thermodynamics of polymer networks.

A theory of cell wall extension is proposed. It is shown that macroscopic properties of cell walls can be explained through the microscopic properties of interpenetrating networks of cellulose and hemicellulose. The qualitative conclusions of the theory agree with the existing experimental data. The dependence of the cell wall yield threshold on the secretion of the wall components is discussed.     

BOTERO1 is required for normal orientation of cortical microtubules and anisotropic cell expansion in Arabidopsis

Mutants at the BOTERO1 locus are affected in anisotropic growth in all non-tip-growing cell types examined. Mutant cells are shorter and broader than those of the wild type. Mutant inflorescence stems show a dramatically reduced bending modulus and maximum stress at yield. Our observations of root epidermis cells show that the cell expansion defect in bot1 is correlated with a defect in the orientation of the cortical microtubules. We found that in cells within the apical portion of the root, which roughly corresponds to the meristem, microtubules were loosely organized and became much more highly aligned in transverse arrays with increasing distance from the tip. Such a transition was not observed in bot1. No defect in microtubule organization was observed in kor-1, another mutant with a radial cell expansion defect. We also found that in wild-type root epidermal cells, cessation of radial expansion precedes the increased alignment of cortical microtubules into transverse arrays. Bot1 roots still show a gravitropic response, which indicates that ordered cortical microtubules are not required for differential growth during gravitropism. Interestingly, the fact that in the mutant, these major changes in microtubule organization cause relatively subtle changes in cell morphology, suggest that other levels of control of growth anisotropy remain to be discovered. Together, these observations suggest that BOT1 is required for organizing cortical microtubules into transverse arrays in interphase cells, and that this organization is required for consolidating, rather than initiating, changes in the direction of cell expansion.