Perception et réaction géotropiques de l'épicotyle d'Asparagus officinalis

Geoperception and georeaction in Asparagus officinalis epicotyls The location and the role of the statenchyma were investigated in Asparagus officinalis epicotyls 1, 2 or 3 cm in length. The statocytes represent a monocellular sheath which comprises the outer layer of the pericycle. Three phases of differentiation (S₁, S₂, S₃) of the statocytes may be distinguished by criteria of cell length, the state of vacuolisation and the sedimentation of amyloplasts and nuclei with respect to gravity. The statenchyma was found to be located only in the first 10 mm of the epicotyl 1, 2 or 3 cm in length; in effect the oldest statocytes (S₃) differentiate into sclerenchyma beneath this level. The geotropic curvature of the epicotyls begins with a positive (downward) curvature and the negative (upward) response is only realized after 15–30 min of geotropic stimulation. The upper limit of the bending is located in the zone containing the mature statocytes (S₃), while the lower limit is located much closer to the base of the epicotyl. Experiments on decapitation of the epicotyls have indicated that the S₁ and S₂ statocytes do not play an important role in the perception of gravity. However, with S₃ statocytes, it has been demonstrated that there is a statistical correlation between the initial rate of curvature of the epicotyls and the length of their sheaths. Thus, the longer the sheath, the more the epicotyls curve. From the results obtained it is proposed that the statocytes are responsible for the geotropic perception of Asparagus officinalis epicotyls.     

ARL2, ARG1 and PIN3 define a gravity signal transduction pathway in root statocytes

ALTERED RESPONSE TO GRAVITY1 (ARG1) and its paralog ARG1-LIKE2 (ARL2) are J-domain proteins that are required for normal root and hypocotyl gravitropism. In this paper, we show that both ARL2 and ARG1 function in a gravity signal transduction pathway with PIN3, an auxin efflux facilitator that is expressed in the statocytes. In gravi-stimulated roots, PIN3 relocalizes to the lower side of statocytes, a process that is thought to, in part, drive the asymmetrical redistribution of auxin toward the lower flank of the root. We show that ARL2 and ARG1 are required for PIN3 relocalization and asymmetrical distribution of auxin upon gravi-stimulation. ARL2 is expressed specifically in the root statocytes, where it localizes to the plasma membrane. Upon ectopic expression, ARL2 is also found at the cell plate of dividing cells during cytokinesis, an area of intense membrane dynamics. Mutations in ARL2 and ARG1 also result in auxin-related expansion of the root cap columella, consistent with a role for ARL2 and ARG1 in regulating auxin flux through the root tip. Together these data suggest that ARL2 and ARG1 functionally link gravity sensation in the statocytes to auxin redistribution through the root cap.     

Cytochalasin B affects the structural polarity of statocytes from cress roots (Lepidium sativum L.)

Summary The effect of cytochalasin B (CB; 25 ·ml^–1 in 1% dimethylsulfoxide, DMSO) upon the structural polarity of statocytes in cress roots is demonstrated. If normal, vertically grown roots are incubated in CB, the structural polarity of the statocytes is altered according to the developmental stage of the root. Statocytes from young roots (13 or 17 hours, additionally 7 hours CB) are characterized by proximal ER cisternae and a sparsely developed distal ER-complex. Statocytes from older roots (24 hours, additionally 7 hours CB) still accumulate distal ER, as in control roots, but at the proximal cell pole in the vicinity of the nucleus additional ER is found. These effects are reversed by washing out the drug in DMSO. Growth of the roots under a continuous supply of CB yields statocytes with sedimented nuclei, proximal ER and almost no distal ER. Together with quantitative data from morphometric studies, a dynamic model of the expression of inherent cell polarity in structural polarity is proposed.Abbreviations CB cytochalasin B - DMSO dimethylsulfoxide - ER endoplasmic reticulum Preliminary results were presented at the joint Annual Meeting of the Belgian and German Society for Cell Biology, Bonn, 18–22 March 1985; Eur. J. Cell Biol. 36 (Suppl. 7), 1985, 25.Dedicated to Professor Dr. A.Betz on the occasion of his 65th birthday.     

Cytodifferentiation of polar plant cells Use of anti-microtubular agents during the differentiation of statocytes from cress roots (Lepidium sativum L.)

The development of the structural polarity of statocytes from cress roots (Lepidium sativum L.) was studied in a time- and stage-dependent manner. Outgrowing radicles had statocytes with abundant lipid droplets, sparsely developed endoplasmic reticulum (ER) and nuclei located at the proximal cell poles. During differentiation, coincidentally the lipid droplets disappeared, while rough ER increased in length. The ER was translocated into the distal cell pole to establish a complex of stacked ER. Microtubules occurred first at the distal cell edges. As a second step, ER was produced in the vicinity of the nucleus and was also translocated distally. By application of the antimicrotubular agents heavy water (90%), colchicine (10^-4 mol·l^-1) and triethyl lead chloride (20 mol·l^-1), the involvement of microtubules in these events was studied. Triethyl lead chloride led to a complete cessation of differentiation; root-cap cells remained at a stage without polar arrangement of the ER. Colchicine affected the development of structural polarity slightly, as shown by a higher density of cortical ER cisternae. Heavy water inhibited the translocation of ER almost completely and yielded ER located also in the cell center. All anti-microtubular agents inhibited cell division and the differentiation of the distal cell layer of the dermatocalyptrogen into statocytes. It is hypothesized that microtubules serve as anchoring sites for microfilaments, which actually mediate the translocation of the ER. Hence, an intact system of microtubules and microfilaments is necessary for the expression of structural polarity.Abbreviations DC dermatocalyptrogen - ER endoplasmic reticulum - M meristem cell layer - MT microtubule - pI prospective story I - TrEl triethyl lead chloride     

A role of microtubules in the polarity of statocytes from roots of Lepidium sativum L.

When roots of Lepidium sativum L. are immersed in a colchicine solution (10^-4 mol l^-1), the cortical microtubules of statocytes are affected such that the dense network ofmicrotubules at the distal cell edges, between the endoplasmic reticulum and the plasma membrane, disappears almost completely, whereas the microtubules, lining the anticlinal cell walls are reduced only to a limited extent. Upon inversion of colchicine-pretreated roots, the distal complex of endoplasmic reticulum sinks into the interior of the statocyte. Germination of seeds in the cold (3–4°C) leads to a retardation of statocyte development; the elaborated system of endoplasmic reticulum is lacking, and only a few microtubules are observable, lining the plasma membrane along the anticlinal cell walls. During an additional 4 h at 24°C, groups of microtubules develop near the plasma membrane in the distal one-third of the statocytes, coaligning with newly synthesized cisternae of the endoplasmic reticulum. It is proposed that, particularly at the distal statocyte pole, microtubules in coordination with cross-bridging structures, act in stabilizing the polar arrangement of the distal endoplasmic reticulum and, in turn, facilitate an integrated function of amyloplasts, endoplasmic reticulum and plasma membrane in graviperception.Abbreviations ER endoplasmic reticulum - MT microtubule     

Ultrastructure and movements of cell organelles in the root cap of agravitropic mutants and normal seedlings of Arabidopsis thaliana

The root anatomy and ultrastructure of the agravitropic Arabidopsis thaliana L. mutants Dwf and aux-1 were compared with the gravitropic mutant aux-2 and the wild type (WT) in an attempt to find an explanation for the lack of response to gravity. No differences were found in the organization of the root cap. The central part of the cap (columella) contains 5 storeys of developing, functioning and degenerating statocytes. Their ultrastructure is very similar in all four types of plant. Particular attention was paid to the distribution of rough endoplasmie reticulum (ER). Both in the WT and the mutants the ER is concentrated in the distal part at the "floor" of the cell.
Light micrographs were used to compare the sedimentation rates of movable cell structures in normal and agravitropic root statocytes. A longitudinal movement of amyloplasts and nuclei was observed when the roots were inverted. In WT and aux-2 the rates were on average 6.3 μm h⁻¹ (amyloplasts) and 2.1 μm h⁻¹ (nucleus). In aux-1 the sedimentation rates were significantly lower: 2.4 and 0.6 μm h⁻¹, respectively. Based on magnified electron micrographs of normal and inverted statocytes a morphometrical analysis of the distribution and redistribution of amyloplasts, nuclei, mitochondria, vacuoles and ER was made. The only significant difference was found in the redistribution of amyloplasts between aux-1 and the gravitropical normal types.