Arrangement of Cortical Microtubules in Avena Coleoptiles and Mesocotyls and Pisum Epicotyls

Cortical microtubules (MTs) in coleoptiles and mesocotyls ofAvena sativa and epicotyls of Pisum sativum were examined byimmunofluorescence. In elongating Avena coleoptiles whose elongationis less localized, the orientations of cortical MTs of parenchymaand adaxial epidermal cells, and abaxial epidermal cells aretransverse, and oblique or longitudinal, respectively, and doesnot differ between the upper, middle and lower parts. The transverseMTs in parenchyma and adaxial epidermal cells turns to obliqueor longitudinal ones after elongation stops. The obliquity ofMTs in abaxial epidermal cells also tends to become steeperas elongation comes to a stop. In Avena mesocotyls and Pisumepicotyls whose elongation is localized, the orientation ofcortical MTs of cortical cells in the elongating region is relativelytransverse. The epidermis has intermingling cells of transverseor oblique MTs. In the non-elongating region, MT orientationbecomes steeper both in the cortex and epidermis. The present results indicate that whatever the degree of localizationof the elongation, the obliquity of MTs in these organs is steeperin epidermal than in inner tissue cells and becomes steeperas elongation stops in both tissues. (Received October 26, 1987; Accepted April 19, 1988)     

Orientation of Wall Microfibrils in Avena Coleoptiles and Mesocotyls and in Pisum Epicotyls

Microfibrils (MFs) on the inner surface of the walls of Avenacoleoptile and mesocotyl cells and of Pisum epicotyl cells wereexamined by a replica method. In the elongating epidermis ofthese three organs, cells having MFs that were transverse, obliqueor longitudinal to the elongation axis were intermingled. Inthe elongating parenchymal tissues, all cells deposited MFstransversely. In non-elongating cells of Avena coleoptiles andPisum epicotyls, the orientation of MFs on the inner wall surfaceof both epidermal and parenchymal cells was more longitudinalthan in elongating cells. These observations on the orientationsof MFs are compatible with those our previously reported observationson the orientations of microtubules (MT) (Iwata and Hogetsu1988). Disruption of MTs of Avena coleoptiles by treatment withamiprophosmethyl caused changes in the orientation of depositionof MFs. These results support the idea that MFs are usuallyco-aligned with MTs in organ cells and that the orientationof MFs is controlled by MTs. The averaged direction of MFs, visualized under polarized light,showed a clear difference between the epidermal and inner-tissuecell walls in the elongating regions of the three organs. Inalmost all elongating and non-elongating epidermal cells, theaveraged direction of MFs was longitudinal, while it was transversein all inner-tissue cells. (Received December 16, 1988; Accepted April 28, 1989)     

Gibberellin A3 and Abscisic Acid Cause the Reorientation of Cortical Microtubules in Epicotyl Cells of the Decapitated Dwarf Pea

To determine whether or not the changes in the orientation ofmicrotubules (MTs) that are induced by GA₃ and ABA result fromchanges in the rate of epicotyl elongation caused by these hormones,we examined the effects of GA₃ and ABA on the orientation ofMTs in epidermal cells of decapitated epicotyls of the dwarfpea (Pisum sativum cv. Little Marvel), in which neither GA₃nor ABA causes changes in the rate of epicotyl elongation. Cuttings taken from GA₃-pretreated seedlings were decapitatedand treated with ABA. ABA eliminated the GA₃-induced predominanceof transverse MTs and treatment with ABA resulted in a predominanceof longitudinal MTs in the decapitated cuttings. However, ABAdid not reduce the rate of epicotyl elongation in these samples.Cuttings taken from ABA-pretreated seedlings were decapitatedand treated with GA₃. GA₃ caused the orientation of MTs to changefrom longitudinal to transverse in the decapitated cuttings.However, GA₃ had no promotive effect on elongation of theseepicotyls. The results indicate that both ABA and GA₃ have the abilityto change the orientation of MTs by mechanisms that do not involvechanges in the rate of cell elongation. (Received August 18, 1992; Accepted January 18, 1993)     

Immunofluorescence Microscopical Observation of Cortical Microtubule Arrangement as Affected by Gibberellin in d5 Mutant of Zea mays L.

The relationship between the dwarf habit of growth and corticalmicrotubule (MT) orientation as affected by gibberellin wasexamined using a gibberellin responding dwarf mutant of Zeamays L. (d₅). The 4 mm portion of the mesocotyl below the coleoptilarnode of dark-grown seedlings was divided into four 1 mm segments.MTs were observed by means of immunofluorescence microscopyon the tangential surface of the epidermal cells (EP-t), theradial surfaces of epidermal (EP-r) and cortex cells (C-r) inboth normal and d₅ seedlings. MT arrangement in EP-t was transverse,oblique and/or longitudinal. In lower regions, cells with transverselyoriented MTs decreased, while those with obliquely/longitudinallyoriented MTs increased. The frequency of the occurrence of transverselyoriented MTs was much higher in normal than in d₅. In EP-r almostall the cells of the four regions had transversely orientedMTs. In d₅ seedlings only a few percent of cells had obliquelyoriented MTs. In C-r all the cells of the four regions of normalseedlings had transversely oriented MTs, while in d₅, 20% ofthe uppermost region cells showed oblique/longitudinal orientationand more in lower regions. By GA₃ (100µM) treatment for24 h, the growth of d₅ mesocotyls was stimulated, and the orientationof MTs in EP-t, EP-r and C-r of any regions became transverse. ¹Present address: Department of Forestry, Faculty of Agriculture,the University of Tokyo, Yayoi-1-Chome, Bunkyo-ku, Tokyo, 113Japan (Received August 16, 1990; Accepted January 22, 1991)     

Reorientation of microtubules at the outer epidermal wall of maize coleoptiles by phytochrome,blue-light photoreceptor,and auxin

Summary The effects of red and blue light on the orientation of cortical microtubules (MTs) underneath the outer epidermal wall of maize (Zea mays L.) coleoptiles were investigated with immunofluorescent techniques. The epidermal cells of dark-grown coleoptiles demonstrated an irregular pattern of regions of parallel MTs with a random distribution of orientations. This pattern could be changed into a uniformly transverse MT alignment with respect to the long cell axis by 1 h of irradiation with red light. This response was transient as the MTs spontaneously shifted into a longitudinal orientation after 1–2 h of continued irradiation. Induction/reversion experiments with short red and far-red light pulses demonstrated the involvement of phytochrome in this response. In contrast to red light, irradiation with blue light induced a stable longitudinal MT alignment which was established within 10 min. The blue-light response could not be affected by subsequent irradiations with red or far-red light indicating the involvement of a separate blue-light photoreceptor which antagonizes the effect of phytochrome. In mixed light treatments with red and blue light, the blue-light photoreceptor always dominated over phytochrome which exhibited an apparently less stable influence on MT orientation. Long-term irradiations with red or blue light up to 6 h did not reveal any rhythmic changes of MT orientation that could be related to the rhythmicity of helicoidal cell-wall structure. Subapical segments isolated from dark-grown coleoptiles maintained a longitudinal MT arrangement even in red light indicating that the responsiveness to phytochrome was lost upon isolation. Conversely auxin induced a transverse MT arrangement in isolated segments even in blue light, indicating that the responsiveness to blue-light photoreceptor was eliminated by the hormone. These complex interactions are discussed in the context of current hypotheses on the functional significance of MT reorientations for cell development.Abbreviations MT cortical microtubule - P_r, P_fr red and far-red absorbing form of phytochrome     

Effects of abscisic acid on the orientation and cold stability of cortical microtubules in epicotyl cells of the dwarf pea

Summary The effects of abscisic acid (ABA) on the orientation and cold stability of cortical microtubules (MTs) in epidermal cells of epicotyls of the dwarf pea,Pisum sativum L. cv. Little Marvel, were examined by immunofluorescence microscopy. The effect of ABA on the elongation of epicotyls and on the orientation of cortical MTs was opposite to that of gibberellin A₃ (GA₃). Treatment with ABA, which reduced the promotion of epicotyl elongation by GA₃, eliminated the GA₃-induced predominance of transverse MTs and resulted in a predominance of longitudinal MTs. The effect of ABA on the cold stability of cortical MTs was also opposite to that of GA₃. ABA increased the cold stability of MTs, while GA₃ decreased it. The predominance of longitudinal MTs brought about by ABA may have some relationship to ABA-induced inhibition of the elongation of the epicotyl. ABA may alter membrane proteins to stabilize cortical MTs and induce cold hardiness of plants.Abbreviations ABA abscisic acid - DMSO dimethylsulfoxide - FITC fluorescein isothiocyanate - GA₃ gibberellin A₃ - MT microtubule - PBS phosphate-buffered saline Dedicated to the memory of Professor Oswald Kiermayer     

Immunofluorescence Microscopy of Microtubule Arrangement in Root Cells of Pisum sativum L. var Alaska

The arrangement of wall microtubules (MTs) in Pisum sativumroots was viewed immunofluorescently using cryosectioning. Mostcells in the tip region of pea roots (0–2 mm from tip)had wall MTs arranged transversely to the root axis. In theregion elongating at a higher rate (2–4 mm), wall MTsof epidermal, cortical and stelar cells were all transverselyarranged. In the region of about 5 mm from the tip, in whichcell elongation had already ceased, wall MTs in cortical cellschanged from a transverse to an oblique arrangement in relationto the root axis. Some cells had a crossed arrangement of wallMTs, which was interpreted as representing two sets of unidirectional,oblique wall MTs in opposite cell cortices of a single cell.This change was completed within a region of 1-mm width. Sinceroots elongated at a rate of 0.6 mm h^–1, it means thatthe arrangement of wall MTs changed within 2 h. An oblique arrangementof wall MTs was also observed in stelar cells. As the cellsaged, the oblique arrangement tended to change to a steeperor even a longitudinal one. (Received January 24, 1986; Accepted May 15, 1986)     

Phenotypic Characterization and Molecular Mapping of an acaulis2 Mutant of Arabidopsis thaliana with Flower Stalks of Much Reduced Length

An acaulis2-1 (acl2-1) mutant of Arabidopsis thaliana (L.) Heynh.was isolated and characterized. The mutant had inflorescencesof much reduced length. The defect was severer in lower internodesand was visible preferentially in type 2 metamers. The defectin the elongation of internodal cells of inflorescences wasattributable to a defect in the mechanism for elongation ofthese cells. The cortical microtubules (MTs) were analyzed inthe acl2 mutant, in a comparison with those of an acl1 mutantwhich was described earlier, but no changes were detected inamounts of MTs. In the type 2 and type 3 metamers, the defectin elongation in acl mutants was loosely correlated with a changein the orientation of MTs. Thus, the abnormal arrangement ofMTs appears to be one of the pleiotropic effects of acaulismutations. Genetic mapping was performed using molecular markers and theACL2 gene was located at 100 map units on chromosome 1. ¹Present address: Institute of Molecular and Cellular Biosciences,The University of Tokyo, Yayoi, Bunkyo-ku, Tokyo, 113 Japan ²Present address: Division of Biological Sciences, GraduateSchool of Science, Hokkaido University, Kita-ku, Sapporo, 060Japan     

The cyclic reorientation of cortical microtubules in epidermal cells of azuki bean epicotyls: the role of actin filaments in the progression of the cycle

The orientation of microtubules (MTs) was examined in epidermal cells of azuki bean (Vigna angularis Ohwi et Ohashi) epicotyls. The orientation of MTs adjacent to the outer tangential wall of the cells, which has a crossed polylamellate structure with lamellae of longitudinal cellulose microfibrils alternating with lamellae of transverse cellulose microfibrils, differed from one cell to another. Treatment with an auxin-free solution caused the accumulation of cells with longitudinal MTs and subsequent treatment with a solution that contained auxin resulted in the accumulation of cells with transverse MTs, showing that sequential treatments with auxin-free and auxin-containing solutions can synchronize the reorientation of MTs. The MTs, once reoriented from longitudinal to transverse, returned to longitudinal and then back to transverse once again, the duration of the cycle being about 6 h. Gibberellic acid, known to increase the percentage of cells with transverse MTs, promoted reorientation of MTs from longitudinal to transverse and inhibited that from transverse to longitudinal. Cytochalasin D, an agent that disrupts actin filaments, speeded up the reorientation from transverse to longitudinal and slowed down that from longitudinal to transverse. It caused an increase in the percentage of cells with MTs in mixed orientation, and the percentage of such cells was highest when the percentage of cells with longitudinal MTs was decreasing and that of cells with transverse MTs was increasing. Received: 27 November 1997 / Accepted: 7 January 1998     

Microtubule orientation in pea stem cells: a change in orientation follows the initiation of growth rate decline

The orientation of microtubules in cells of redlight-grown pea plants (Pisum sativum L.) was examined by means of immunofluorescence. Microtubules (MTs) in rapidly elongating, subepidermal cells commonly form multiple, parallel strands that run transverse to the cell's axis of elongation. By contrast, MTs in nonelongating subepidermal cells form steeply pitched helical arrays; MTs in non-elongating epidermal cells are oriented parallel to the axis of elongation. This change in orientation occurs during the time interval in which growth rate is declining. The transition is abrupt rather than gradual and occurs in both epidermal and subepidermal cells at the same time. Plants irradiated for 2 h with a growth-inhibiting fluence of blue light did not undergo the same transition, indicating that factors other than changing elongation rates must be responsible for triggering the reorganization of MT arrays.     

Effects of Blue Light on Cell Elongation and Microtubule Orientation in Dark-Grown Gametophytes of Ceratopteris richardii

Gametophytes of Ceratopteris richardii developed into strap-shapedprothalli in the dark. The prothalli had an apical meristem,a subapical elongation zone and a basal zone where no growthoccurred. Continuous irradiation of blue light inhibited cellgrowth of the elongation zone, although red or far-red lighthad no effect on elongation. Cortical microtubules (MTs) ofsubapical cells reoriented from transverse to oblique or parallelto the growing axis during blue light inhibition. Each regionof the strap-shaped prothallus was irradiated with a micro-beamof blue light. Cell elongation in the subapical region was notinhibited by irradiation at the apical meristem. Irradiationof the subapical region inhibited cell elongation to a similarextent as the whole irradiation control. When a single elongatingcell at one side of the subapical region was irradiated by ablue microbeam, elongation was inhibited in a cell at the irradiateside only, resulting in a bowing prothallus. Cortical MTs reorientedfrom transverse to longitudinal to the cell axis in irradiatedcells only. The results indicate that each cell in the subapicalregion perceives blue light by itself, reorients only its corticalMTs and ceases only its cell elongation, independent of surroundingcells. (Received October 17, 1996; Accepted December 9, 1996)     

Mesocotyl growth of etiolated seedlings ofAvena sativa andZea mays in relation to light and diffusible auxin

Diffusible auxin levels were measured in coleoptiles and mesocotyls of dark-grown seedlings ofavena sativa (cv. Spear) andZea mays (cv. Golden Cross Bantam) using theAvena curvature bioassay. The coleoptile tip was confirmed as the major auxin source in etiolated seedlings. Auxin levels were found to decrease basipetally in sequent sections of theAvena coleoptile but not to decrease in apical sections of increasing length. An inhibitor capable of inducing positive curvatures ofAvena test coleoptiles was discovered in diffusates from the mesocotyls of oat and corn seedlings. The amount of this inhibitor was correlated with the cessation of mesocotyl growth of oat seedlings grown in darkness, and with the inhibition of mesocotyl growth of corn seedlings exposed to red light.     

Mesocotyl growth of etiolated seedlings ofAvena sativa andZea mays in relation to light and diffusible auxin

Diffusible auxin levels were measured in coleoptiles and mesocotyls of dark-grown seedlings ofavena sativa (cv. Spear) andZea mays (cv. Golden Cross Bantam) using theAvena curvature bioassay. The coleoptile tip was confirmed as the major auxin source in etiolated seedlings. Auxin levels were found to decrease basipetally in sequent sections of theAvena coleoptile but not to decrease in apical sections of increasing length. An inhibitor capable of inducing positive curvatures ofAvena test coleoptiles was discovered in diffusates from the mesocotyls of oat and corn seedlings. The amount of this inhibitor was correlated with the cessation of mesocotyl growth of oat seedlings grown in darkness, and with the inhibition of mesocotyl growth of corn seedlings exposed to red light.     

Evidence Inconsistent with the Blaauw Model of Phototropism

The Blaauw model of phototropism equates the inhibition of growthat the illuminated side of a unilaterally illuminated organwith the blue light inhibition of overall organ extension evidentwhen some shoots are exposed to uniform blue light However,a study of the growth responses of Avena coleoptiles exposedto omnilateral, equal bilateral, unequal bilateral and unilateralblue light has revealed some light induced growth rate changeswhich cannot be explained by the Blaauw model. The growth responsesof cells at the illuminated and shaded sides of phototropicallystimulated coleoptiles seem to depend on the existence of alight gradient across the whole organ rather than the absolutelevels of light at either side. Key words: Phototropism, Avena coleoptile, Blaauw hypothesis, Blue light, Growth inhibition     

Auxin-dependent microtubule responses and seedling development are affected in a rice mutant resistant to EPC

Mutants in rice (Oryza sativa L. cv. japonica) were used to study the role of the cytoskeleton in signal-dependent morphogenesis. Mutants obtained by gamma ray irradiation were selected that failed to show inhibition of coleoptile elongation by the antimicrotubular drug ethyl-N-phenylcarbamate (EPC). The mutation EPC-Resistant 31 (ER31), isolated from such a screen, caused lethality in putatively homozygous embryos. Heterozygotes exhibited drug resistance, impaired development of crown roots, and characteristic changes in the pattern of cell elongation: cell elongation was enhanced in mesocotyls and leaf sheaths, but inhibited in coleoptiles. The orientation of cortical microtubules changed correspondingly: for etiolated seedlings, compared with the wild-type, they were more transverse with respect to the long cell axis in mesocotyls and leaf sheaths, but more longitudinal in coleoptiles. In mutant coleoptiles, in contrast to wild-type, microtubules did not reorient in response to auxin, and their response to microtubule-eliminating and microtubule-stabilizing drugs was conspicuously reduced. In contrast, they responded normally to other stimuli such as gibberellins or red light. Auxin sensitivity as assayed by the dose-response for callus induction did not show any significant differences between wild-type and mutant. The mutant phenotype is interpreted in terms of an interrupted link between auxin-triggered signal transduction and microtubule reorientation.     

Stress relaxation properties of the cell wall of tissue segments under different growth conditions

Stress-relaxation parameters were compared under different experimentalconditions using 5th internode segments of light-grown pea seedlingsand coleoptile segments of dark-grown Avena seedlings. The followingresults were obtained. 1. In a short incubation period at 25?C, IAA caused a decreasein the minimum relaxation time, To, of the epidermal cell wallof pea internodes when it induced elongation; the optimum concentrationof IAA for decreasing To was 10 mg/liter. 2. At all concentrations of IAA used, 0.1–1000 mg/liter,the relationship between the To value of the epidermal cellwall peeled from segments incubated for 2 hr and the subsequentelongation rate in 2–3 hr incubation was linear, indicatingthat the To value of the cell wall at a certain time regulatesthe rate of the following elongation. 3. When segments of pea epicotyls or Avena coleoptiles wereincubated in mannitol solution of various concentrations inthe presence and absence of IAA and then allowed to grow inthe absence of both mannitol and IAA, the segments extendeddifferently depending upon the mannitol concentration, whichwas less than 0.3 M, given during preincubation. 4. The T_o and b (relaxation rate, S/log t) values were smallerin the cell wall of segments which extended more, than in thosewhich extended less. In this case, 0.2 M mannitol solution wasmost effective, since it inhibited IAA-induced elongation duringpre-incubation and the segments thus incubated extended themost afterward. 5. Extensibility, mm/gr, seemed to parallel the elongation whichhad occurred during pre-incubation, indicating that this value,contrary to To, represented at least partly the result of elongation. From these results we concluded that the growth rate to followis regulated by the minimum stress relaxation time, To, andpossibly by the relaxation rate, b, of the cell wall beforeextension, and these parameters may represent certain biochemicalmodifications of the cell wall components needed for cell extension. (Received August 12, 1974; )     

Response of actin microfilaments during phytochrome-controlled growth of maize seedlings

Summary In seedlings of maize (Zea mays L. cv. Percival), growth is controlled by the plant photoreceptor phytochrome. Whereas coleoptile growth is promoted by continuous far-red light, a dramatic block of mesocotyl elongation is observed. The response of the coleoptile is based entirely upon light-induced stimulation of cell elongation, whereas the response of the mesocotyl involves light-induced inhibition of cell elongation. The light response of actin microfilaments was followed over time in the epidermis by staining with fluorescence-labelled phalloidin. In contrast to the underlying tissue, epidermal cells are characterized by dense longitudinal bundles of microfilaments. These bundles become loosened during phases of rapid elongation (between 2–3 days in irradiated coleoptiles, between 5–6 days in dark-grown coleoptiles). The condensed bundles re-form when growth gradually ceases. The response of actin to light is fast. If etiolated mesocotyls are transferred to far-red light, condensation of microfilaments can be clearly seen 1 h after the onset of stimulation together with an almost complete block of mesocotyl elongation. The observations are discussed in relation to a possible role of actin microfilaments in the signal-dependent control of cell elongation.     

Auxin-like Growth Activity of 3-Phenylpropionitrile from Water-Cress (Nasturtium officinale R. Br.)

Solutions of 3-phenylpropionitrile (PPN) and 3-phenylpropionicacid (PPA) exhibited auxin-like plant growth activity by stimulatingelongation of sections cut from wheat coleoptiles and garden-cresshypocotyls. Elongation of sugar-beet hypocotyl sections wasstimulated by PPA but not by PPN. Although PPA stimulated elongationof pea-epicotyl sections after 19 h incubation, most concentrationsof PPN did not show activity until 70 h. PPN generally stimulatedadventitious root formation on hypocotyls of garden-cress andsugar-beet, and on epicotyls of pea seedlings whereas dilutesolutions of PPA were inactive and concentrated solutions weretoxic. Steam distillates of water-cress plants contained a substancewith auxin-like growth activity and chromatographic propertiessimilar to PPN. Nasturtium officinale R. Br. water-cress, 3-phenylpropionitrile, 3-phenylpropionic acid, auxins     

Mechanism for formation of the secondary wall thickening in tracheary elements: Microtubules and microfibrils of tracheary elements of Pisum sativum L. and Commelina communis L. and the effects of amiprophosmethyl

Arrangements of microfibrils (MFs) and microtubules (MTs) were examined in tracheary elements (TEs) of Pisum sativum L. and Commelina communis L. by production of replicas of cryo-sections, and by immunofluorescence microscopy, respectively. The secondary wall thickenings of TEs of Pisum and Commelina roots have pitted and latticed patterns, respectively. Most MFs in the pitted thickening of Pisum TEs retain a parallel alignment as they pass around the periphery of pits. However, some groups of MFs grow into the pits but then terminate at the edge of the thickening, indicating that cellulose-synthase complexes are inactivated in the plasma membrane under the pit. Microtubules of TEs of both Pisum and Commelina are localized under the secondary thickening and few MTs are detected in the areas between wall thickenings. In the presence of the MT-disrupting agent, amiprophosmethyl, cellulose and hemicellulose, which is specific to secondary thickening, are deposited in deformed patterns in TEs of Pisum roots, Pisum epicotyls and Commelina roots. This indicates that the localized deposition of hemicellulose as well as cellulose involves MTs. The deformed, but heterogeneous pattern of secondary thickening is still visible, indicating that MTs are involved in determining and maintaining the regular patterns of the secondary thickening but not the spatial heterogeneous pattern of the wall deposition. A working hypothesis for the formation of the secondary thickening is proposed.Abbreviations APM amiprophosmethyl - DMSO dimethyl sulfoxide - F-WGA fluorescein-conjugated wheat-germ agglutinin - M F microfibril - MT microtubule - PEG polyethyleneglycol - TE tracheary element I thank Ms. Aiko Hirata (Institute of Applied Microbiology, University of Tokyo, Japan) for help in taking stereomicrographs. This work was supported in part by a Grant-in-Aid from the Ministry of Education, Science and Culture of Japan.     

Possible involvement of 65 kda MAP in elongation growth of azuki bean epicotyls

Although regulation of the dynamics of plant microtubules (MTs) by microtubule-associated proteins (MAPs) has been suggested, the mechanism has not yet been elucidated. As one candidate, a MAP composed of a 65 kDa polypeptide (65 kDa MAP) has been isolated from tobacco cultured cells [Jiang and Sonobe (1993), J. Cell Sci 105: 8911. To investigate the physiological role of the 65 kDa MAP in situ, we analyzed the changes in content and colocalization of this MAP with cortical MTs in relation to elongation growth, using azuki bean epicotyls (Vigna angularis Ohwi et Ohashi). All apical, intermediate, and basal segments prepared from 6 d seedlings showed high growth activity. In 12 d seedlings, growth activity of intermediate and basal segments was low, although that of apical segments was high. The relationship between the growth activity and the orientation of cortical MTs in the epidermal cells was analyzed. Cells could be classified into four types with respect to orientation of cortical MTs: transverse (T), oblique (O), longitudinal (L) to the vertical axis of cells, and random (R). In rapidly growing segments, three types of cells, T, O, L, were observed at similar ratios. In such segments, significant amounts of the 65 kDa MAP were expressed, and it colocalized well with cortical MTs. In segments showing low growth activity, most of the cells showed oblique and longitudinal orientation of cortical MTs. In such segments, the content of the 65 kDa MAP was low. These results suggested involvement of this 65 kDa MAP in regulation of the elongation growth of this epicotyl.