Endoplasmic microtubules configure the subapical cytoplasm and are required for fast growth of Medicago truncatula root hairs

To investigate the configuration and function of microtubules (MTs) in tip-growing Medicago truncatula root hairs, we used immunocytochemistry or in vivo decoration by a GFP linked to a MT-binding domain. The two approaches gave similar results and allowed the study of MTs during hair development. Cortical MTs (CMTs) are present in all developmental stages. During the transition from bulge to a tip-growing root hair, endoplasmic MTs (EMTs) appear at the tip of the young hair and remain there until growth arrest. EMTs are a specific feature of tip-growing hairs, forming a three-dimensional array throughout the subapical cytoplasmic dense region. During growth arrest, EMTs, together with the subapical cytoplasmic dense region, progressively disappear, whereas CMTs extend further toward the tip. In full-grown root hairs, CMTs, the only remaining population of MTs, converge at the tip and their density decreases over time. Upon treatment of growing hairs with 1 microM oryzalin, EMTs disappear, but CMTs remain present. The subapical cytoplasmic dense region becomes very short, the distance nucleus tip increases, growth slows down, and the nucleus still follows the advancing tip, though at a much larger distance. Taxol has no effect on the cytoarchitecture of growing hairs; the subapical cytoplasmic dense region remains intact, the nucleus keeps its distance from the tip, but growth rate drops to the same extent as in hairs treated with 1 microM oryzalin. The role of EMTs in growing root hairs is discussed.     

The distributional changes and role of microtubules in Nod factor-challenged<Emphasis Type="Italic"> Medicago sativa</Emphasis> root hairs

The normal tip-growing pattern exhibited by root hairs of legumes is disrupted when the hair is exposed to Nod factors generated by compatible bacteria capable of inducing nodule formation. Since microtubules (MTs) play an important role in regulating directionality and stability of apical growth in root hairs [T.N. Bibikova et al. (1999) Plant J 17:657–665], we examined the possibility that Nod factors might affect the MT distribution patterns in root hairs of Medicago sativa L. We observed that Nod factor application caused rapid changes in the pattern of MTs starting as early as 3 min after perfusion. Within 3 to 10 min after Nod factor application, first endoplasmic and then cortical MTs depolymerised, initially at the proximal ends of cells. Twenty minutes after exposure to Nod factors, a transverse band of microtubules was seen behind the tip, while almost all other MTs had depolymerised. By 30 min, very few MTs remained in the root hair and yet by 1 h the MT cytoskeleton re-formed. When Nod factors were applied in the presence of 10 M oryzalin or 5 M taxol, the MTs appeared disintegrated while the morphological effects, such as bulging and branching, became enhanced. Compared to the treatments with oryzalin or taxol alone, the combinatory treatments exhibited higher growth rates. Since microtubule reorganization is one of the earliest measurable events following Nod factor application we conclude that microtubules have an important role in the early phases of the signalling cascade. Microtubule involvement could be direct or a consequence of Nod factor-induced changes in ion levels.Electronic Supplementary Material Supplementary material is available in the online version of this article at http://dx.doi.org/10.1007/s00425-003-1097-1Abbreviations BNM buffered nodulation medium - CLSM confocal laser scanning microscopy - MT microtubule     

Reorganization and in vivo dynamics of microtubules during Arabidopsis root hair development

Root hairs emerge from epidermal root cells (trichoblasts) and differentiate by highly localized tip growth. Microtubules (MTs) are essential for establishing and maintaining the growth polarity of root hairs. The current knowledge about the configuration of the MT cytoskeleton during root hair development is largely based on experiments on fixed material, and reorganization and in vivo dynamics of MTs during root hair development is at present unclear. This in vivo study provides new insights into the mechanisms of MT (re)organization during root hair development in Arabidopsis (Arabidopsis thaliana). Expression of a binding site of the MT-associated protein-4 tagged with green fluorescent protein enabled imaging of MT nucleation, growth, and shortening and revealed distinct MT configurations. Depending on the dynamics of the different MT populations during root hair development, either repeated two-dimensional (x, y, t) or repeated three-dimensional (x, y, z, t) scanning was performed. Furthermore, a new image evaluation tool was developed to reveal important data on MT instability. The data show how MTs reorient after apparent contact with other MTs and support a model for MT alignment based on repeated reorientation of dynamic MT growth.     

微管骨架在轮藻节间细胞伸长生长中的作用

利用免疫荧光定位及激光共聚焦扫描显微镜,结合细胞生长曲线的定量测定,对不同生长阶段的轮藻节间细胞微管骨架进行了观察研究,结果如下：轮藻顶端生长活跃的新生细胞中,与细胞长轴垂直的周质微管(cortical microtubules)占绝对优势,随着生长速率的减慢,周质微管由垂直于细胞长轴逐渐转为平行排列;基部生长基本停止的节间细胞中,胞内微管则以平行细胞长轴为主;不同生长阶段节间细胞的微管骨架,对微管特异解聚剂黄草消(oryzalin)处理的敏感性表现不相同。顶端生长活跃的节间细胞经oryzalin处理40min后,绝大多数周质微管发生解聚;而基部生长基本停止的老细胞中,即使延长处理时间,仍残留一些尚未完全解聚的微管片段;10μmol／L微管解聚剂oryzalin处理轮藻顶端新生细胞,在高精度的细胞伸长生长测定装置监测下,发现oryzalin对细胞的伸长生长速率有明显的抑制作用,去掉药剂后,伸长生长又有一定的恢复。并且发现,经oryzalin处理后,微管的解聚(40min左右)与顶端节间细胞伸长生长的停止(100min左右)两者间存在着时间上的差异,即微管解聚在先,细胞伸长停止在后。以上结果均说明微管骨架在轮藻节间细胞生长中具有重要作用。     

Complete disintegration of the microtubular cytoskeleton precedes its auxin-mediated reconstruction in postmitotic maize root cells

The inhibitory action of 0.1 microM auxin (IAA) on maize root growth was closely associated with a rapid and complete disintegration of the microtubular (MT) cytoskeleton, as visualized by indirect immunofluorescence of tubulin, throughout the growth region. After 30 min of this treatment, only fluorescent spots were present in root cells, accumulating either around nuclei or along cell walls. Six h later, in addition to some background fluorescence, dense but partially oriented oblique or longitudinal arrays of cortical MTs (CMTs) were found in most growing cells of the root apex. After 24 h of treatment, maize roots had adapted to the auxin, as inferred from the slowly recovering elongation rate and from the reassembly of a dense and well-ordered MT cytoskeleton which showed only slight deviations from that of the control root cells. Taxol pretreatment (100 microM, 24 h) prevented not only the rapid auxin-mediated disintegration of the MT cytoskeleton but also a reorientation of the CMT arrays, from transversal to longitudinal. The only tissue to show MTs in their cells throughout the auxin treatment was the epidermis. Significant resistance of transverse CMT arrays in these cells towards auxin was confirmed using a higher auxin concentration (100 microM, 24 h). The latter auxin dose also revealed inter-tissue-specific responses to auxin: outer cortical cell files reoriented their CMTs from the transversal to longitudinal orientation, whereas inner cortical cell files lost their MTs. This high auxin-mediated response, associated with the swelling of root apices, was abolished with the pretreatment of maize root with taxol.     

A nonsymbiotic root hair tip growth phenotype in NORK-mutated legumes: implications for nodulation factor-induced signaling and formation of a multifaceted root hair pocket for bacteria

The Medicago truncatula Does not Make Infections (DMI2) mutant is mutated in the nodulation receptor-like kinase, NORK. Here, we report that NORK-mutated legumes of three species show an enhanced touch response to experimental handling, which results in a nonsymbiotic root hair phenotype. When care is taken not to induce this response, DMI2 root hairs respond morphologically like the wild type to nodulation factor (NF). Global NF application results in root hair deformation, and NF spot application induces root hair reorientation or branching, depending on the position of application. In the presence of Sinorhizobium meliloti, DMI2 root hairs make two-dimensional 180 degrees curls but do not entrap bacteria in a three-dimensional pocket because curling stops when the root hair tip touches its own shank. Because DMI2 does not express the promoter of M. truncatula Early Nodulin11 (ENOD11) coupled to beta-glucuronidase upon NF application, we propose a split in NF-induced signaling, with one branch to root hair curling and the other to ENOD11 expression.     

Effects of myosin ATPase inhibitor 2,3-butanedione 2-monoxime on distributions of myosins, F-actin, microtubules, and cortical endoplasmic reticulum in maize root apices

2,3-Butanedione 2-monoxime (BDM) is a general inhibitor of myosin ATPases of eukaryotic cells, and its effects on animal and yeast cells are well described. Using immunofluorescence and electron microscopy, we have analyzed the impacts of BDM on distributions of plant myosins, actin filaments (AFs), microtubules (MTs), and cortical endoplasmic reticulum (ER) elements in various cell types of maize root apices. Treatment of growing maize roots with BDM altered the typical distribution patterns of unconventional plant myosin VIII and of putative maize homologue(s) of myosin II. This pharmacological agent also induced a broad range of impacts on AFs and on cortical ER elements associated with plasmodesmata and pit fields. BDM-mediated effects on the actomyosin cytoskeleton were especially pronounced in cells of the root transition zone. Additionally, BDM elicited distinct reactions in the MT cytoskeleton; endoplasmic MTs vanished in all cells of the transition zone and cortical MTs assembled in increased amounts preferentially at plasmodesmata and pit-fields. Our data indicate that AFs and MTs interact together via BDM-sensitive plant myosins, which can be considered as putative integrators of the plant cytoskeleton. Morphometric analysis revealed that cell growth was prominently inhibited in the transition zone and the apical part, but not the central part, of the elongation region. Obviously, myosin-based contractility of the actin cytoskeleton is essential for the developmental progression of root cells through the transition zone.     

A mycorrhizal fungus changes microtubule orientation in tobacco root cells

Summary Cortical cells of mycorrhizal roots undergo drastic morphological changes, such as vacuole fragmentation, nucleus migration, and deposition of cell wall components at the plant-fungus interface. We hypothesized that the cytoskeleton is involved in these mechanisms leading to cell reorganization. We subjected longitudinal, meristem to basal zone, sections of uninfectedNicotiana tabacum roots to immunofluorescence methods to identify the microtubular (MT) structures associated with root cells. Similar sections were obtained from tobacco roots grown in the presence ofGigaspora margarita, an arbuscular mycorrhizal fungus which penetrates the root via the epidermal cells, but mostly develops in the inner cortical cells. While the usual MT structures were found in uninfected roots (e.g., MTs involved in mitosis in the meristem and cortical hoops in differentiated parenchyma cells), an increase in complexity of MT structures was observed in infected tissues. At least three new systems were identified: (i) MTs running along large intracellular hyphae, (ii) MTs linking hyphae, (iii) MTs binding the hyphae to the host nucleus. The experiments show that mycorrhizal infection causes reorganization of root MTs, suggesting their involvement in the drastic morphological changes shown by the cortical cells.     

Organization of cortical microtubules in graviresponding maize roots

Immunofluorescence labeling of cortical microtubules (MTs) was used to investigate the relationship between MT arrangement and changes in growth rate of the upper and lower sides of horizontally placed roots of maize (Zea mays L. cv. Merit). Cap cells and cells of the elongation zone of roots grown vertically in light or darkness showed MT arrangements that were transverse (perpendicular) to the growth direction. Microtubules of cells basal to the elongation zone typically showed oblique orientation. Two hours after horizontal reorientation, cap cells of gravicompetent, light-grown and curving roots contained MTs parallel to the gravity vector. The MT arrangement on the upper side of the elongation zone remained transverse but the MTs of the outer four to five layers of cortical cells along the lower side of the elongation zone showed reorientation parallel to the axis of the root. The MTs of the lower epidermis retained their transverse orientation. Dark-grown roots did not curve and did not show reorientation of MTs in cells of the root cap or elongation zone. The data indicate that MT depolymerization and reorientation is correlated with reduction in growth rate, and that MT reorientation is one of the steps of growth control of graviresponding roots.Abbreviations MT microtubule - QC quiescent center This work was supported by National Science Foundation grant IBN-9118094.     

The microtubule cytoskeleton does not integrate auxin transport and gravitropism in maize roots

The Cholodny-Went hypothesis of gravitropism suggests that the graviresponse is controlled by the distribution of auxin. However, the mechanism of auxin transport during the graviresponse of roots is still unresolved. To determine whether the microtubule (MT) cytoskeleton is participating in auxin transport, the cytoskeleton was examined and the movement of 3 H-IAA measured in intact and excised taxol, oryzalin, and naphthylphthalamic acid (NPA)-treated roots of Zea mays cv. Merit. Taxol and oryzalin did not inhibit the graviresponse of roots but the auxin transport inhibitor NPA greatly inhibited both auxin transport and graviresponse. NPA had no effect on MT organization in vertical roots, but caused MT reorientation in horizontally placed roots. Regardless of treatment, the organization of MTs in intact roots differed from that in root segments. The MT inhibitors, taxol and oryzalin had opposite effects on the MTs, namely, depolymerization (oryzalin) and stabilization and thickening (taxol), but both treatments caused swelling of the roots. The data indicate that the MT cytoskeleton does not directly interfere with auxin transport or auxin-mediated growth responses in maize roots.     

Effects of tyrosine kinase and phosphatase inhibitors on microtubules in Arabidopsis root cells

To investigate the role of tyrosine phosphorylation/dephosphorylation processes in plant cells the morphology of Arabidopsis thaliana primary roots and the organization of cortical microtubules (MTs) were studied after inhibition of protein tyrosine kinases (PTKs) and tyrosine phosphatases (PTPs). It was found that all tested types of PTKs inhibitors (herbimycin A, genistein and tyrphostin AG 18) altered root hair growth and development, probably as a result of their significant influences on MTs organization in root hairs. The treatment also led to MTs reorientation and disruption in epidermis and cortex cells of both elongation and differentiation zones of primary roots. Enhanced tyrosine phosphorylation after treatment with a PTPs inhibitor (sodium orthovanadate) resulted in intense induction of root hair development and growth and caused a significant shortening of the elongation zone. It also led to changes of MTs orientation from transverse to longitudinal in epidermis and cortex cells of the elongation and differentiation zones of the root. From the data obtained we can suppose that tyrosine phosphorylation can be involved in the dynamics and organization of MTs in different types of plant cells.     

Growth and microtubule orientation of Zea mays roots subjected to osmotic stress

Previous work has shown that microtubule (MT) reorientation follows the onset of growth inhibition on the lower side of graviresponding roots, indicating that growth reduction can occur independently of MT reorientation. To test this observation further, we examined whether the reduction in growth in response to osmotic stress is correlated with MT reorientation. The distribution and rate of growth in maize roots exposed to 350 mOsm sorbitol and KCl or 5 mM Mes/Tris buffer were measured with a digitizer. After various times roots were processed for indirect immunofluorescence microscopy. Application of sorbitol or KCl had no effect on the organization of MTs in the apical 2 mm of the root but resulted in striking and different effects in the basal region of the root. Sorbitol treatment caused rapid appearance of oval to circular holes in the microtubular array that persisted for at least 9 h. Between 30 min and 4 h of submersion in KCl, MTs in cortical cells 4 mm and farther from the quiescent center began to reorient oblique to the longitudinal axis. After 9 h, the alignment of MTs had shifted to parallel to the root axis but MTs of the epidermal cells remained transverse. In KCl-treated roots MT reorientation appeared to follow a pattern of development similar to that in controls but without elongation. Our data provide additional evidence that MT reorientation is not the cause but a consequence of growth inhibition.     

Halogenated Auxins Affect Microtubules and Root Elongation in Lactuca sativa

We studied the effect of 4,4,4-trifluoro-3-(indole-3-)butyric acid (TFIBA), a recently described root growth stimulator, and 5,6-dichloro-indole-3-acetic acid (DCIAA) on growth and microtubule (MT) organization in roots of Lactuca sativa L. DCIAA and indole-3-butyric acid (IBA) inhibited root elongation and depolymerized MTs in the cortex of the elongation zone, inhibited the elongation of stele cells, and promoted xylem maturation. Both auxins caused the plane of cell division to shift from anticlinal to periclinal. In contrast, TFIBA (100 micromolar) promoted elongation of primary roots by 40% and stimulated the elongation of lateral roots, even in the presence of IBA, the microtubular inhibitors oryzalin and taxol, or the auxin transport inhibitor naphthylphthalamic acid. However, TFIBA inhibited the formation of lateral root primordia. Immunostaining showed that TFIBA stabilized MTs orientation perpendicular to the root axis, doubled the cortical cell length, but delayed xylem maturation. The data indicate that the auxin-induced inhibition of elongation and swelling of roots results from reoriented phragmoplasts, the destabilization of MTs in elongating cells, and promotion of vessel formation. In contrast, TFIBA induced promotion of root elongation by enhancing cell length, prolonging transverse MT orientation, delaying cell and xylem maturation.     

Effect of oryzalin on root ultrastructure and respiration in various wheat cultivars subjected to cold hardening

The effect of anti-microtubular herbicide oryzalin (10 μM, 2–4 days) on the root ultrastructure and respiration in two cultivars of winter wheat (Triticum aestivum L.) contrasting in their frost-tolerance was studied during plant cold hardening (3°C, 3 days). The sensitivity of subcellular structures to oryzalin depended closely on cell metabolic activity and the extent of development of the cortical microtubule (MT) network. Most pronounced oryzalin-induced changes were related to enhanced cell vacuolation and the appearance of some signs of apoptosis (as judged from cytoplasm fragmentation) in some cells. In the root zone examined, cell heterogeneity increased, when, along with normally functioning cells, dramatically damaged and even completely destroyed cells appeared. Simultaneously, the activity of cyanide-resistant nonphosphorylating respiration pathway was activated, especially during cold hardening. In hardened cells, single cortical microtubules appeared in both wheat cultivars; this fact indicates that new cold-resistant subpopulations of MT were resistant to depolymerizing action of oryzalin.     

Microtubules, MAPs and plant directional cell expansion

Plant microtubules (MTs) polymerize and depolymerize in a process termed dynamic instability. This allows the assembly, reorganization, and disassembly of at least four MT arrays throughout the cell cycle. The cortical MT array lines the plasma membrane during interphase and plays a central role in directional cell expansion. Microtubule-associated proteins (MAPs) decorate cortical MTs with distinct patterns, regulating MT dynamic instability, MT severing, and other array-ordering processes. The Arabidopsis root has emerged as a highly useful system for identifying and studying cell-expansion-related MAPs. Here, we review how cortical MTs are thought to behave and become ordered in expanding root cells, and we discuss the emerging picture of how MAPs fundamentally govern MT ordering and directional growth processes.     

Effects of hexavalent chromium on microtubule organization,ER distribution and callose deposition in root tip cells of <Emphasis Type="Italic">Allium cepa</Emphasis> L.

The subcellular targets of hexavalent chromium [Cr(VI)] were examined in Allium cepa root tips with confocal laser scanning microscopy. Cr(VI) exerted dose- and time-dependent negative effects on root growth rate, the mitotic index and microtubule (MT) organization during cell division cycle. Interphase MTs were more resistant than the mitotic ones, but when affected they were shorter, sparse and disoriented. The preprophase band of MTs became poorly organized, branched or with fragmented MTs, whilst neither a perinuclear array nor a prophase spindle was formed. Metaphase spindles converged to eccentric mini poles or consisted of dissimilar halves and were unable to correctly orient the chromosomes. Anaphase spindles were less disturbed, but chromatids failed to separate; neither did they move to the poles. At telophase, projecting, lagging or bridging chromosomes and micronuclei also occurred. Phragmoplasts were unilaterally developed, split, located at unexpected sites and frequently dissociated from the branched and misaligned cell plates. Chromosomal aberrations were directly correlated with MT disturbance. The morphology and distribution of endoplasmic reticulum was severely perturbed and presumably contributed to MT disassembly. Heavy callose apposition was also induced by Cr(VI), maybe in the context of a cellular defence reaction. Results indicate that MTs are one of the main subcellular targets of Cr(VI), MT impairment underlies chromosomal and mitotic aberrations, and MTs may constitute a reliable biomonitoring system for Cr(VI) toxicity in plants.     

Reinstatement of Microtubule Arrays from Cortical Nucleating Sites in Stomatal Complexes of Lolium rigidum Following Depolymerization of Microtubules by Oryzalin and High Pressure

Immunofluorescence visualization of microtubule (MT) arraysin stomatal complexes of Lolium rigidum shows that disassemblyof the arrays can be successfully achieved using oryzalin orhigh pressure treatments. Under conditions allowing for MT recovery,MTs reappear within an hour after oryzalin or within 5 min afterhigh pressure treatment. During recovery guard mother cells(GMCs) nucleate MTs at sites distributed randomly in the cellcortex. Even after 22 h of recovery the MTs are not arrangedinto any configuration found in untreated tissue. This inabilityto reorganize their MTs after treatment makes GMCs more sensitiveto the loss of MTs than are other cells of the leaf. In guardcells (GCs) MTs reappear around the pore at the junction ofthe periclinal and ventral walls. They subsequently appear throughoutmost of the cell cortex and the majority of stomatal complexesrecover fully organized MT arrays indistinguishable from thosein untreated cells. The results support and extend ultrastructuraland immunofluorescence observations that suggest that MTs inGCs of developing stomata are nucleated in the cell cortex. ²Present address: Department of Biology, The University of SouthwesternLouisiana, Lafayette, Louisiana 70504-2451, U.S.A. (Received April 24, 1990; Accepted July 7, 1990)     

Impact of taxol-mediated stabilization of microtubules on nuclear morphology,ploidy levels and cell growth in maize roots

Direct contact of the radiating perinuclear microtubules (MTs) with the nuclear envelope was visualized with an immunogold technique using specific monoclonal tubulin antibody. The possibility that these perinuclear MT arrays are involved in establishing and maintaining nuclear organization during the interphase of cycling cells in maize root meristems was tested using taxol, a MT-stabilizing agent. Taxol not only stabilized all MTs against the action of the MT-disrupters colchicine and oryzalin but also prevented these agents from their usual induction of nuclear enlargement and decondensation of nuclear chromatin. On the contrary, nuclear size decreased and the chromatin became more compact in mitotically cycling cells of the taxol-treated root apices. Moreover, taxol prevented the stimulation, by colchicine and oryzalin, of the onset of the S phase in cells of the quiescent centre and proximal root meristem. Exposure of maize roots to taxol strongly decreased final cell volumes, suggesting that the more condensed nuclear chromatin is less efficient in genome expression and that this accounts for the restriction of cellular growth. All these findings support the hypothesis that MT arrays, radiating from the nuclear surface, are an essential part of an integrated plant ‘cell body’ consisting of nucleus and the MT cytoskeleton, and that they regulate, perhaps via their impact on chromatin condensation and activity, progress through the plant cell cycle.     

A generalized method for transfecting root epidermis uncovers endosomal dynamics in Arabidopsis root hairs

Progress in analysing the cellular functions of many structural proteins has accelerated through the use of confocal microscopy together with transient gene expression. Several methods for transient expression have been developed in the past few years, but their application has seen limited success beyond a few tractable species and tissues. We have developed a simple and efficient method to visualize fluorescent proteins in Arabidopsis root epidermis using co-cultivation of seedlings with Agrobacterium rhizogenes. The method is equally suitable for transient gene expression in other species, including Thellungiella, and can be combined with supporting molecular and biochemical analyses. The method promises significant advantages for study of membrane dynamics, cellular development and polar growth in root hairs without interference in the development of the plant. Since the method targets specifically the root epidermis, it also offers a powerful tool to approach issues of root-rhizosphere interactions, such as ion transport and nutrient acquisition. As a proof of principle, we carried out transfections with fluorescent markers for the plasma membrane (NpPMA2-GFP, Nicotiana plumbaginifolia L. Plasma Membrane H(+)-ATPase 2), the endoplasmic reticulum (YFP-HDEL), and the Golgi apparatus (sialyl transferase-GFP) to trace their distribution in growing Arabidopsis root hairs and epidermis. The results demonstrate that, in Arabidopsis root hairs, movement of the Golgi is faster than previously reported for tobacco leaf epidermal cells, consistent with the high secretory dynamics of the tip growing cell; they show a pattern to the endoplasmic reticulum within the cytoplasm that is more diffuse than found in tobacco leaf epidermis, and they confirm previous findings of a polarized distribution of the endoplasmic reticulum at the tip of growing root hairs.     

Endoplasmic reticulum-targeted GFP reveals ER remodeling in Mesorhizobium-treated Lotus japonicus root hairs during root hair curling and infection thread formation

The endoplasmic reticulum (ER) of the model legume Lotus japonicus was visualized using green fluorescent protein (GFP) fused with the KDEL sequence to investigate the changes in the root hair cortical ER in the presence or absence of Mesorhizobium loti using live fluorescence imaging. Uninoculated root hairs displayed dynamic forms of ER, ranging from a highly condensed form to an open reticulum. In the presence of M. loti, a highly dynamic condensed form of the ER linked with the nucleus was found in deformed, curled, and infected root hairs, similar to that in uninoculated and inoculated growing zone I and II root hairs. An open reticulum was primarily found in mature inoculated zone III root hairs, similar to that found in inactive deformed/curled root hairs and infected root hairs with aborted infection threads. Co-imaging of GFP-labeled ER with light transmission demonstrated a correlation between the mobility of the ER and other organelles and the directionality of the cytoplasmic streaming in root hairs in the early stages of infection thread formation and growth. ER remodeling in root hair cells is discussed in terms of possible biological significance during root hair growth, deformation/curling, and infection in the Mesorhizobium–L. japonicus symbiosis.