Positioning of Nuclei in Arabidopsis Root Hairs: An Actin-Regulated Process of Tip Growth (original) (raw)
Related papers
THE PLANT CELL ONLINE, 2003
Plant cells expand by exocytosis of wall material contained in Golgi-derived vesicles. We examined the role of local instability of the actin cytoskeleton in specifying the exocytosis site in Arabidopsis root hairs. During root hair growth, a specific actin cytoskeleton configuration is present in the cell's subapex, which consists of fine bundles of actin filaments that become more and more fine toward the apex, where they may be absent. Pulse application of low concentrations of the actin-depolymerizing drugs cytochalasin D and latrunculin A broadened growing root hair tips (i.e., they increased the area of cell expansion). Interestingly, recovery from cytochalasin D led to new growth in the original growth direction, whereas in the presence of oryzalin, a microtubule-depolymerizing drug, this direction was altered. Oryzalin alone, at the same concentration, had no influence on root hair elongation. These results represent an important step toward understanding the spatial and directional regulation of root hair growth.
1987
A prominent feature of tip growth in filamentous plant cells is that the nucleus often migrates in step with the tip as it extends. We have studied this longrecognized but unexplained relationship in root hairs of the legume Viciu hirsutu by a variety of microscopic techniques. Using rhodaminyl lysine phallotoxin, and antitubulin antibodies, root hairs are shown to contain axial bundles of F-actin and a complex microtubular system. To the basal side of the nucleus the microtubules are cortical and net axial but in the region between nucleus and tip the arrangement is more complicated. Electron microscopic thin sections demonstrate that internal bundles of microtubles exist in addition to the plasma membrane-associated kind. Computerized deblurring of through-focal series of antitubulin stained hairs clarifies the three-dimensional organization: bundles of endoplasmic microtubules progress from the nuclear region toward the apical dome where they can be seen to fountain out upon the cortex. The relationship between nucleus and tip can be uncoupled with antiniicrotubule herbicides. Time lapse video microscopy shows that these agents cause the nucleus to migrate toward the base. This contrary migration can be inhibited by adding cytochalasin D, which fragments the F-actin bundles. It is concluded that microtubules connect the nucleus to the tip but that F-actin is involved in basipetal migration as is known to occur when symbiotic bacteria uncouple the nucleus from the tip.
Reorganization and in Vivo Dynamics of Microtubules during Arabidopsis Root Hair Development
PLANT PHYSIOLOGY, 2004
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. * Corresponding author; e-mail nvanbruaen@cri.be; fax 32-9-264-62-19.
Microtubules regulate tip growth and orientation in root hairs ofArabidopsis thaliana
Plant Journal, 1999
The polarized growth of cells as diverse as fungal hyphae, pollen tubes, algal rhizoids and root hairs is characterized by a highly localized regulation of cell expansion confined to the growing tip. In apically growing plant cells, a tipfocused [Ca 2⍣ ] c gradient and the cytoskeleton have been associated with growth. Although actin has been established to be essential for the maintenance of elongation, the role of microtubules remains unclear. To address whether the microtubule cytoskeleton is involved in root hair growth and orientation, we applied microtubule antagonists to root hairs of Arabidopsis. In this report, we show that depolymerizing or stabilizing the microtubule cytoskeleton of these apically growing root hairs led to a loss of directionality of growth and the formation of multiple, independent growth points in a single root hair.
The Plant Journal, 1999
Golgi vesicles are targeted, released and inserted into the plasma membrane on one side of the cell. We studied the role of actin in vesicle delivery and retention by comparing the actin filament configuration during bulge formation, root hair initiation, sustained tip growth, growth termination, and in full-grown hairs. Lipochito-oligosaccharides (LCOs) were used to interfere with growth (De Ruijter et al., 1998, Plant J. 13, 341-350), and cytochalasin D (CD) was used to interfere with actin function. Actin filament bundles lie net-axially in cytoplasmic strands in the root hair tube. In the subapex of growing hairs, these bundles flare out into fine bundles. The apex is devoid of actin filament bundles. This subapical actin filament configuration is not present in full-grown hairs; instead, actin filament bundles loop through the tip. After LCO application, the tips of hairs that are terminating growth swell, and a new outgrowth appears from a site in the swelling. At the start of this outgrowth, net-axial fine bundles of actin filaments reappear, and the tip region of the outgrowth is devoid of actin filament bundles. CD at 1.0 µM, which does not affect cytoplasmic streaming, does not inhibit bulge formation and LCO-induced swelling, but inhibits initiation of polar growth from bulges, elongation of root hairs and LCOinduced outgrowth from swellings. We conclude that elongating net-axial fine bundles of actin filaments, which we call FB-actin, function in polar growth by targeting and releasing Golgi vesicles to the vesicle-rich region, while actin filament bundles looping through the tip impede vesicle retention.
Journal of Experimental Botany, 1997
Root hairs are excellent cells for the study of the exocytotic process that leads to growth in higher plants, because the exocytotic event takes place locally and because the cells are directly accessible for signals, drugs, fixatives, microinjection, and microscopic observation. Well-characterized lipochitooligosaccharides, signal molecules excreted by Rhizobium bacteria, induce root hair growth which can be recorded microscopically in a root hair deformation assay developed for Vicia sativa L. Root hair deformation is a morphogenetic process involving swelling of the hair tip and subsequent new hair outgrowth from that swelling. This response to the signal occurs at a specific developmental stage, namely when hairs are terminating growth. Thus, since polar growth can be triggered intentionally, the system allows the study of growth phenomena in higher plants at the cellular level. Furthermore, important advances are being made with molecular genetics that will allow the unravelling of the signal transduction pathways in root hair morphogenesis leading to growth. This paper first discusses cytological phenomena involved in the process of polar growth, such as cytoplasmic polarity, cytoplasmic streaming and the organization of actin filaments, the location of a spectrin-like antigen, the distribution of intracellular calcium, cortical microtubules and cell wall texture, endocytosis by means of coated pits, and physical aspects of the incorporation of exocytotic vesicles into the plasma membrane. In the second part, changes are discussed that occur in some of these phenomena when growth is influenced by growth regulators and mutations.
TheROOT HAIRLESS 1gene encodes a nuclear protein required for root hair initiation in Arabidopsis
Genes & Development, 1998
The epidermis of Arabidopsis wild-type primary roots, in which some cells grow hairs and others remain hairless in a position-dependent manner, has become an established model system to study cell differentiation. Here we present a molecular analysis of the RHL1 (ROOT HAIRLESS 1) gene that, if mutated, prevents the formation of hairs on primary roots and causes a seedling lethal phenotype. We have cloned the RHL1 gene by use of a T-DNA-tagged mutant and found that it encodes a protein that appears to be plant specific. The predicted RHL1 gene product is a small hydrophilic protein (38.9 kD) containing putative nuclear localization signals and shows no significant homology to any known amino acid sequence. We demonstrate that a 78-amino-acid sequence at its amino terminus is capable of directing an RHL1-GFP fusion protein to the nucleus. The RHL1 transcript is present throughout the wild-type plant and in suspension culture cells, but in very low amounts, suggesting a regulatory function for the RHL1 protein. Structural evidence suggests a role for the RHL1 gene product in the nucleolus. We have examined the genetic relationship between RHL1 and GL2, an inhibitor of root hair initiation in non-hair cells. Our molecular and genetic data with double mutants, together with the expression analysis of a GL2 promoter-GUS reporter gene construct, indicate that the RHL1 gene acts independently of GL2.