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A Sievers

Publications and source records attributed to A Sievers.

69 records · Page 4Linked to original sources
PubMed1987

Gravitropic bending of cress roots without contact between amyloplasts and complexes of endoplasmic reticulum.

The polar arrangement of cell organelles in Lepidium root statocytes is persistently converted to a physical stratification during lateral centrifugation (the centrifugal force acts perpendicular to the root long axis) or by apically directed centrifugation combined with cytochalasin-treatment. Lateral centrifugation (10 min, 60 min at 10g or 50g) causes displacement of amyloplasts to the centrifugal anticlinal cell wall and shifting of the endoplasmic reticulum (ER) complex to the centripetal distal cell edge. After 60 min of lateral centrifugation at 10g or 50g all roots show a clear gravitropic curvature. The average angle of curvature is about 40 degrees and corresponds to that of roots stimulated gravitropically in the horizontal position at 1 g in spite of the fact that the gravistimulus is 10- or 50-fold higher. Apically directed centrifugation combined with cytochalasin B (25 micrograms ml-1) or cytochalasin D (2.5 micrograms ml-1) incubation yields statocytes with the amyloplasts sedimented close to the centrifugal periclinal cell wall and ER cisternae accumulated at the proximal cell pole. Gravitropic stimulation for 30 min in the horizontal position at 1 g and additional 3 h rotation on a clinostat result in gravicurvature of cytochalasin B-treated centrifuged (1 h at 50 g) roots, but because of retarded root growth the angle of curvature is lower than in control roots. Cytochalasin D-treatment during centrifugation (20 min at 50 g) does not affect either root growth or gravicurvature during 3 h horizontal exposure to 1-g relative to untreated roots. As lateral centrifugation enables only short-term contact between the amyloplasts and the distal ER complex at the onset of centrifugation and apically directed centrifugation combined with cytochalasin-treatment even exclude any contact the integrity of the distal cell pole need not necessarily be a prerequisite for graviperception in Lepidium root statocytes.

Brassicaceae↗
PubMed1987

An empirical function for the description of root growth.

Vertical growth of young cress roots (Lepidium sativum L.) which have not yet attained their steady state is analyzed in terms of the relative elemental rate of elongation (RELEL). To this end, an empirical model function is suggested which describes, by means of six parameters, the movement of marker particles on the root surface. These parameters are interpreted as morphological and physiological quantities. Their determination by independent measurements is in good agreement with that obtained by fitting the model function to the experimental data. The RELEL is then evaluated analytically, so that numerical smoothing and differentiating algorithms with all their problems are avoided. The result demonstrates that-apart from the root cap-nearly the whole root is elongating, including the root hair region and part of the basal portion beyond it. This explains the basal curvature component that is observed during the graviresponse of cress roots.

Journal Article↗
PubMed1984

Can a Ca2+ pump in the endoplasmic reticulum of the Lepidium root be the trigger for rapid changes in membrane potential after gravistimulation?

Since gravistimulation is followed by alterations in the external current symmetry (Behrens et al., 1982), the effect of gravistimulation on cellular membrane potential was investigated using conventional glass microelectrode techniques. The resting potential of statocytes in a vertically oriented root is approx. -118 mV. Upon gravistimulation, the membrane potential is temporarily depolarized (lag time = 2 s) to a potential of approx. -93 mV. This depolarization is only observed in statocytes located on the physically lower root flank while those on the corresponding upper flank become weakly hyperpolarized (approx. -13 mV). These results reflect altered ion fluxes across the plasma membrane. The perception of gravistimulus was suggested to result from a pressure of the amyloplasts on the distal endoplasmic reticulum (ER) of the statocytes (Sievers and Volkmann, 1972). A causal relationship between changes in ER-amyloplast interactions and the rapid alterations in plasma membrane potential described above is not known. A candidate for such an intracellular messenger is Ca2+. As a first step in establishing the validity of such an assumption, we have isolated ER membranes from roots. When incubated with micromolar concentrations of Ca2+, the vesicular membrane fraction accumulates Ca2+. The accumulation is ATP-dependent and -specific and is directly coupled to ATP hydrolysis since a protonophore shows no inhibitory effect. Thus, in analogy to the sarcoplasmic reticulum of muscle, regulation of an ER-localized Ca2+ compartment might be an important step in such complex processes as stimulus-transduction in gravitropism.

Adenosine Triphosphate↗
PubMed1983

The effect of centrifugal accelerations on the polarity of statocytes and on the graviperception of cress roots.

The structural polarity of statocytes of Lepidium sativum L. is converted to a physical stratification by a root-tip-directed centrifugal acceleration. Sedimentation of amyloplasts and nucleus to the centrifugal (distal) cell pole and the lateral displacement of the distal endoplasmic reticulum (ER) complex occur after centrifugation for 20 min at an acceleration of 50 g. With higher doses (20 min, 100-2,000 g), smaller organelles become increasingly displaced. From the centrifugal to the centripetal cell pole, the following stratification is observed: 1) amyloplasts with mitochondria; 2) nucleus with mitochondria and a few dictyosomes, as well as laterally located ER; 3) dictyosomes with a few mitochondria; 4) vacuoles; and 5) lipid droplets. Within the first 7.5 min, after the roots have been returned to 1 g, the original arrangement of the amyloplasts sedimented on the underlying ER complex is reestablished in 66% of the statocytes. When roots previously centrifuged in an apical direction are exposed in a horizontal position to 1 g, the latent period of the graviresponse is increased by 7.5 min relative to the non-centrifuged controls. The kinetics of the response are identical to the controls. Roots centrifuged first in an apical direction and then for 2 h in a lateral direction (1,000 g) have statocytes with a physical stratification perpendicular to the root axis. A gravitropic curvature does not take place during the lateral centrifugation. These results support the hypothesis that the distal ER complex is necessary and sufficient for graviperception.

Brassicaceae↗
PubMed1982

Rapid Changes in the Pattern of Electric Current around the Root Tip of Lepidium sativum L. following Gravistimulation.

Using a highly sensitive vibrating electrode, the pattern of naturally occurring electric currents around 1-day-old primary roots of Lepidium sativum L. growing vertically downward and the current pattern following gravistimulation of the root has been examined. A more or less symmetrical pattern of current was found around vertically oriented, downward growing roots. Current entered the root at the root cap, the meristem, and the beginning of the elongation zone and left the root along most of the elongation zone and in the root hair zone. After the root was tilted to a horizontal position, we observed current flowing acropetally at the upper side of the root cap and basipetally at the lower side within about 30 seconds in most cases. After a delay of several minutes, acropetally oriented current was also found flowing along the upper side of the meristematic zone. The apparent density of the acropetal current in the root cap region increased and then decreased with time. Gravitropic curvature was first visible approximately 10 minutes after tilting of the root to the horizontal position. Since the change in the pattern of current in the root cap region precedes bending of the root and is different for the upper and lower side, a close connection is suggested between the current and the transduction of information from the root cap to the elongation zone following graviperception in the cap.

Journal Article↗
PubMed1981

Regulation of the position of statoliths in Chara rhizoids.

The behavior of statoliths in rhizoids differently oriented with respect to the gravity vector indicates that there are cytoskeleton elements which exert forces on the statoliths, mostly in the longitudinal directions. Compared to the sum of the forces acting on a statolith, the gravitational force is a relatively small component, i.e., less than 1/5 of the cytoskeleton force. The balance is disturbed by displacing the rhizoid from the normal vertical orientation. It is also reversibly disturbed by cytochalasin B such that some statoliths move against the gravity force. Phalloidin stabilizes the position of the statoliths against cytochalasin B. We infer that microfilaments are involved in controlling the position of statoliths, and that there is a considerable tension on these microfilaments. The vibration frequency of the microfilaments corresponding to this tension is in the ultrasonic range.

Actin Cytoskeleton↗
PubMed1980

Effects of prolonged omnilateral gravistimulation on the ultrastructure of statocytes and on the graviresponse of roots.

Statocytes of vertically growing roots of Lepidium sativum L. exhibit a strict polarity: The nucleus is positioned near the proximal periclinal cell wall, amyloplasts are sedimented on a complex of rough endoplasmic reticulum (ER) consisting of parallel cisternae near the distal periclinal cell wall. When 24 h old, vertically grown roots are rotated for an additional 20 h on a horizontal clinostat, this polarity is destroyed. Furthermore, the prolonged omnilateral stimulation leads to a damage of the statocytes, which in some cases ends in the self-destruction of the sensitive cells. The different components of the ultrastructural responses of the statocytes are: Displacement of the nucleus; changes in amount and distribution of the ER; loss of amyloplast starch; confluence of lipid droplets to large aggregates; a considerable increase of the lytic compartment. In addition, even anticlinal cell walls may be lysed up to small stumps. As all these effects are clearly restricted to the statocytes, only these cells are able to respond to the continuously changing direction of the gravity vector, thus perceiving gravity as such. After being exposed horizontally, the graviresponse of rotated roots is delayed as compared to the controls. About 20% of the rotated roots do not respond (curve) at all, but grow perpendicular in relation to the gravity vector. Perception of gravity is inevitably correlated with the polarity and the integrity of the statocytes.

Brassicaceae↗
PubMed1979

Responses of roots to simulated weightlessness on the fast-rotating clinostat.

Sedimentable cell particles are distributed randomly along the horizontal axis of the fast-rotating clinostat. They neither sediment in the direction of gravity, nor in the direction of the centrifugal force, nor in the direction of the resultant force of both. The effect of this "weightlessness" and that of very small centrifugal forces on the perception of mass acceleration was examined using young primary roots of Lepidium sativum L. (Cruciferae) during their early development on the fast-rotating clinostat. The results of the experiments show: 1) there is no response of the roots in the direction of gravity, 2) at small centrifugal forces (< 2.2 x 10(-2) g) a curvature response occurs in the direction of the stimulus, 3) the threshold value for the perception of mass acceleration lies at 4.3 x 10(-3) g, and 4) below the threshold value the existence of an autonomous root curvature has been proved for the first time, which is not caused by mass acceleration.

Gravitation↗
PubMed1979

[Proof of the subapical differential growth of the flanks in the Chara rhizoid during graviresponse].

The displacement of resin spheres attached to the tip of the gravitropically bending Chara rhizoid was measured by means of time-lapse photography. The relative growth of the flanks which at the beginning of stimulation were opposite to each other, was calculated during the earliest response time. The physically upper subapical flank section continues growing after stimulation at a decreasing rate. The growth of the opposite lower flank section is inhibited immediately after stimulation by the position of the statoliths. As soon as the statoliths are displaced in a basal direction, the growth of this section is transiently promoted. The gravitropic downward bending of the Chara rhizoid is by bowing, not by bulging.

Chlorophyta↗
PubMed1977

Ultrastructure of gravity-perceiving cells in plant roots.

The root cap is the gravity-perceiving organ of plant roots. The central statenchyma consists of polarly organized statocytes which are characterized by sedimentable amyloplasts and by a striking distal endoplasmic reticulum (e.r. complex). During the normal downwards orientated growth of the root, the amyloplasts are sedimented onto the e.r. complex. Some observations indicate that the amyloplasts stress the e.r. complex by their sedimentation. The stress possibly influences the structural and functional state of the e.r. membranes. Therefore, graviperception is probably a function of differential stress on the e.r. complexes. The amyloplasts are partly or totally separated from the e.r. complex under experimental conditions such as deviations of the roots from the perpendicular between 30 and 180 degrees, rotation of the roots on the horizontal klinostat at 2 rev/min and at 55-120 rev/min, and placing the roots into an electric field at 2000-3000 V/cm. These are important facts for an understanding of the nature of graviperception. Spacelab experiments may help to verify the interpretation of results obtained in the experiments in simulated weightlessness.

Brassicaceae↗
PubMed1977

[Preparative approach to mannobiose and laminaribiose (author's transl)].

For synthetic purposes mannobiose is isolated from the polysaccharide mannan of ivory nut flower. The chain length of mannan A is determined to approximately 15 mannose units on average with a definite distribution from 5 to 25 mannose units. Acetolysis is optimized and yields 41% mannobiose. The separation on Sephadex G-15 yields pure mannobiose, and in addition its higher homologues in gram amounts. Detection of the eluates is performed using a differential refractometer. Correspondingly laminaribiose can be obtained from the polysaccharides laminaran (brown algae Laminaria hyperborea) and pachyman (fungus Poria cocos Wolf, Bukuryo) by selectively optimized acetolysis in 30% yield. Depending on the starting material separations are favourably performed on silica gel or on Sephadex G-15. Laminaribiose and its higher homologues are obtained in gram amounts.

Chemical Phenomena↗
PubMedDate not supplied

An inhibitor of the Ca(2+)-ATPases in the sarcoplasmic and endoplasmic reticula inhibits transduction of the gravity stimulus in cress roots.

Cress (Lepidium sativum L.) roots were treated with 20 microM cyclopiazonic acid (CPA), an inhibitor of the Ca(2+)-transporting ATPases present in the sarcoplasmic/endoplasmic reticulum of animals and the endoplasmic reticulum of plants, in order to investigate its effect on the gravitropic response. Root growth was not significantly reduced by the applied dose of CPA, but the gravitropic response (curvature) was drastically inhibited. We hypothesize that the ER Ca(2+)-ATPase of statocytes is involved in transduction of the gravity stimulus and that CPA disturbs a cytosolic Ca2+ signal necessary for graviperception.

Brassicaceae↗