Organization of cortical microtubules and microfibril deposition in response to blue-light-induced apical swelling in a tip-growing Adiantum protonema cell

The arrangements of cortical microtubules (MTs) in a tip-growing protonemal cell of Adiantum capillus-veneris L. and of cellulose microfibrils (MFs) in its wall were examined during blue-light (BL)-induced apical swelling. In most protonemal cells which had been growing in the longitudinal direction under red light, apical swelling was induced within 2 h of the onset of BL irradiation, and swelling continued for at least 8 h. During the longitudinal growth under red light, the arrangement of MFs around the base of the apical hemisphere (the subapical region) was perpendicular to the cell axis, while a random arrangement of MFs was found at the very tip, and a roughly axial arrangement was observed in the cylindrical region of most cells. This orientation of MFs corresponds to that of the cortical MTs reported previously (Murata et al. 1987, Protoplasma 141, 135–138). In cells irradiated with BL, a random rather than transverse arrangement of both MTs and MFs was found in the subapical region. Time-course studies showed that this reorientation occurred within 1 h after the onset of the BL irradiation, i.e. it preceded the change in growth pattern. These results indicate that the orientation of cortical MTs and of cellulose MFs is involved in the regulation of cell diameter in a tip-growing Adiantum protonemal cell.Abbreviations BL blue light - MF(s) microfibril(s) - MT(s) microtubule(s)     

Chloroquine,a lysosomotropic agent,inhibits zygote formation in yeast

The effects of solar UV-B radiation on the green flagellate, Euglena gracilis, are measured under controlled conditions. Both photoorientation and motility are drastically impaired even after short exposure times of a few hours to sunlight not filtered by an ozone cuvette. Phototactic orientation starts to deteriorate after about 90 min and is completely lost after about 5 h. The percentage of motile cells in a population decreases likewise after an exposure of about 2 h and the velocity distributions shows a reduced speed of movement after an initial photokinetic increase. The damage is irreversible: in populations exposed for >2 h no living cell was found 24 h later. The UV-B sensitivity seems to be independent of the culture age at least over three weeks: While the percentage of motile cells changes with a peak at about 8 d, the relative UV-B induced inhibition is constant and depends only on the UV dose. DNA seems not to be the primary UV-B target since UV-B inhibition could not be repaired during subsequent dark or moderate light conditions even after low doses.     

Effects of Colchicine and Light on Cell Form in Fern Gametophytes. Implications for a Mechanism of Light-induced Cell Elongation

Spores of the fern, Onoclea sensihilis L., suffer a disruption of normal development when they are cultured on media containing colchicine. Cell division is inhibited, and the spores develop into giant spherical cells under continuous white fluorescent light. In darkness only slight cell expansion occurs. Spherical cell expansion in the light requires continuous irradiation. Photosynthesis does not seem to be involved, since variations in light intensity do not affect the final cell diameter; the addition of sucrose to the medium does not permit cell expansion in darkness; and the inhibitor DCMU does not block the light-induced cell expansion. Continuous irradiation of colchicine-treated spores with blue, red or far-red light produces different patterns of cell expansion. Blue light permits spherical growth, similar to that found under white light, whereas red and far-red light promote the reestablishment of polarized filamentous growth. Although ethylene is unable to induce polarized cell expansion in colchicine-treated spores in darkness or white and blue light, it enhances filamentous growth which already is established by red or far-red irradiation. Both red and far-red light increase the elongation of normal filaments (untreated with colchicine) above that of dark-grown plants, but under all 3 conditions the rates of volume growth are identical. Light, however, does cause a decrease in the cell diameters of irradiated filaments. These data are used to construct an hypothesis to explain the promotion of cell elongation in fern protonemata by red and far-red light. The model proposes light-mediated changes in microtubular orientation and cell wall structure which lead to restriction of lateral cell expansion and enhanced elongation growth.     

Limited period of graviresponsiveness in germinating spores of Ceratopteris richardii

Rhizoids of the fern Ceratopteris richardii Brogn. usually emerge 40 h after germination is initiated by light, and more than 90% of them emerge growing in a downward direction. However, when the spores are germinated on a clinostat, the emerging rhizoids show no preferential orientation. This indicates that under normal 1 · g conditions the initial growth direction of rhizoids can be oriented by gravity. If the orientation of the spores is changed 3 h or less after the start of germination, the growth direction of most emerging rhizoids becomes downward relative to the new orientation. However, if the orientation of the spores is changed by 180° 8 h or more after germination is initiated by light, most rhizoids emerge growing upward; i.e., the same direction as if there had been no orientation change. Emerged rhizoids also do not change their direction of growth if their orientation is changed. These results indicate that the growth direction of emerging rhizoids is set by gravity prior to actual emergence, and that the time of full orientation responsiveness is limited to a period ranging from the initiation of germination to about 3–4 h after the start of germination. There is a gravity-oriented nuclear movement beginning at about 13 h after germination, and this movement appears to predict the initial growth direction of rhizoids.These studies were made possible by grant NAGW 1519 to S.J.R. and grant NGT-51065 to E.S.E., both from the National Aeronautics and Space Administration.     

How Chlamydomonas keeps track of the light once it has reached the right phototactic orientation.

By using a real-time assay that allows measurement of the phototactic orientation of the unicellular alga Chlamydomonas with millisecond time resolution, it can be shown that single photons not only induce transient direction changes but that fluence rates as low as 1 photon cell(-1) s(-1) can already lead to a persistent orientation. Orientation is a binary variable, i.e., in a partially oriented population some organisms are fully oriented while the rest are still at random. Action spectra reveal that the response to a pulsed stimulus follows the Dartnall-nomogram for a rhodopsin while the response to a persistent stimulus falls off more rapidly toward the red end of the spectrum. Thus light of 540 nm, for which chlamy-rhodopsin is equally sensitive as for 440-nm light, induces no measurable persistent orientation while 440-nm light does. A model is presented which explains not only this behavior, but also how Chlamydomonas can track the light direction and switches between a positive and negative phototaxis. According to the model the ability to detect the direction of light, to make the right turn and to stay oriented, is a direct consequence of the helical path of the organism, the orientation of its eyespot relative to the helix-axis, and the special shielding properties of eyespot and cell body. The model places particular emphasis on the fact that prolonged swimming into the correct direction not only requires making a correct turn initially, but also avoiding further turns once the right direction has been reached.     

The Adaptation of Plankton Algae

The various aspects of the adaptation of plankton algae lo light and temperature are discussed. The shape of a light intensity-photosynthesis curve is shown to be an important means of describing the physiological adjustment of an algal population. If the algae are not exposed to adverse influences such as poisons, pronounced nutrient deficiency or light shocks, the rate of real photosynthesis per mg chlorophyll a at 1 Klux (incandescent light) should be about 0.4–0.6 mg C/hour. Hence this rate presents an excellent means of judging the quality of experiments. Experiments are presented where Chlorella pyrenoidosa was adapted to light intensities between 0.32 klux and 21 Klux. This alga adapts to different light intensities by varying the amount of pigments per cell. Algae grown at 1 Klux have about 10 times more chlorophyll per cell than those grown at 21 klux. Other species of algae—but by no means all—are shown to behave in the same way. The problem of algal resistance to photo-oxidation at high light intensities is discussed. Adaplation is shown to he one of the mechanisms which make the algae resistent. “Chlorophyll inactivation” is another. Experiments with the diatom Skeletonema costatum concerning adaptation to different temperatures have been performed. The fact that the alga has essentially the same rate of photosynthesis per cell at all light intensities at 20°C and 7°C, may be attributed to an increase of all the enzymes at the low temperature. The amount of protein per cell was twice as high at 7°C as at 20°C.     

The patterns of cell division and chloroplast reproduction in young leaflets of Sphagnum

Abstract

The pattern of cell division in Sphagnum leaflets is reviewed. The review is supplemented by data on additional cell divisions, symmetry, and the pattern of chloroplast reproduction. The results are compared with Neuburg's (1960) results about Protosphagnum nervatum from the Upper Permian. The regular shift of the spindle orientation between subsequent mitoses (exceptions occur), the temporal pattern of asymmetrical and symmetrical cell divisions, and the handedness of cell triads are described and discussed. The pattern could hardly be more regular. Additional cell divisions modify the pattern but do not really change it.     

Spatial orientation of rainbow trout to plane-polarized light: The ontogeny of E-vector discrimination and spectral sensitivity characteristics

Summary The results of this study demonstrate that trout (Salmo gairdneri) are capable of orienting to polarized light fields. The spectral composition of the polarized light fields can significantly influence the orientation of trout. Rainbow trout exhibit ontogenetic losses in orientation to polarized light fields which appears coincident with the ontogenetic loss of the UV-sensitive cones. Trout were trained to swim to a refuge located at one end of the training tank under a polarized light field. The E-vector of the polarized light field was oriented parallel or perpendicular to the long axis of the training tank. Trained fish were released in a circular test tank and their angular response scored. Under a white plus ultraviolet polarized light field, trout oriented in the trained E-vector orientation. For instance, fish trained under a parallel E-vector orientation exhibited angular responses close to parallel in the test tank. However, when the spectral composition of the polarized light field was manipulated, the accuracy of spatial orientation of the trout varied. Trout weighing about 30 g exhibited accurate orientation to the white plus UV polarized light field. The trout were incapable of orientation at a body weight of 50 to 60 g.     

Shedding light on diatom photonics by means of digital holography

Diatoms are among the dominant phytoplankters in the world's oceans, and their external silica investments, resembling artificial photonic crystals, are expected to play an active role in light manipulation. Digital holography allowed studying the interaction with light of Coscinodiscus wailesii cell wall reconstructing the light confinement inside the cell cytoplasm, condition that is hardly accessible via standard microscopy. The full characterization of the propagated beam, in terms of quantitative phase and intensity, removed a long‐standing ambiguity about the origin of the light confinement. The data were discussed in the light of living cell behavior in response to their environment. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Changes in microtubule and microfibril arrangement during polarotropism inAdiantum protonemata

Polarotropism was induced inAdiantum (fern) protonemata grown under polarized red light by turning the electrical vector 45 or 70 degrees. One hour after the light treatment, tropic responses became apparent in many cells as a slight distortion of the apical dome. Changes in the position of the circumferentially-arranged cortical microtubule band (Mt-band) (Murataet al., 1987) and the arrangement of microfibrils around the subapical part of protonemata were investigated in relation to the polarotropic responses. Twenty minutes after turning the electrical vector, preceding the morphological change of cell shape, the Mt-band began to change its orientation from perpendicular to oblique to the initial growing axis. After 30 min, the Mt-band changed its orientation further under 45 degrees polarized light, but under light rotated 70 degrees, it began to disappear. In phototropic responses induced by local irradiation of a side of the subapical part of a protonema with a non-polarized red microbeam, the Mt-band on the irradiated side disappeared or became faint within 20 min, but neither disappearance nor a change of orientation of Mts occurred on the non-irradiated side. One hour after turning the electrical vector 45 degrees, in half of the cells tested, the innermost layer of microfibrils in the subapical part of the protonema changed its orientation from perpendicular to oblique to the growing axis, corresponding to the changes in the orientation of the Mt-band. After 2 hr, those changes were obvious in all cells examined. The same basic results on the orientation of microfibrils were obtained with protonemata cultured for 2 hr under 70 degrees polarized light. The role of the Mt-band in tropic responses is discussed.     

Golgi orientation and cell behaviour in the developing pattern of chondrogenic condensations in chick limb-bud mesenchyme

Summary Using a silver-impregnation method, the occurrence and significance of Golgi apparatus orientation has been studied in cells contributing to the cartilage condensations in the developing skeleton of the chick limb bud, both in normal embryos and in thetalpid ³ mutant, in which the pattern of condensationin situ, and cell behaviourin vitro, is abnormal. Analysis of photographed sections made up as photomontages with a final magnification of x 1000, indicates a sequence of changing Golgi orientation in the course of establishing cartilage condensations in the mesenchyme in normal limb buds, including at an early stage an orientation of one population of cells towards the condensation centre and of another population in the contrary direction, and a modification of the sequence in the mutant. The changing patterns of cell orientation has been further analysed in scanning electron microscope studies on the formation of cartilage condensationsin vitro.     

Effects of Calcium Ions and of Calcium Channel Blockers on Galvanotaxis of Chlamydomonas reinhardtii

The effects of calcium ions and of the calcium channel blockers verapamil, diltiazem and nifedipine on galvanotaxis in Chlamydomonas have been investigated using a fully automated and computerized population system. Galvanotaxis is a function of the voltage applied to the cell population. However, the galvanotactic orientation also depends on the external calcium concentration. In a calcium-deprived nutrient medium which still contains 6 × 10^?7M calcium, galvanotactic orientation is about 20% of orientation at optimal calcium concentration of 10^?4 M at 9 V. The higher the external calcium concentration is, the lower is the voltage necessary for optimal galvanotactic orientation. The calcium channel blockers diltiazem and nifedipine likewise inhibit galvanotaxis of Chlamydomonas very specifically without impairing motility. Verapamil is effective, but also inhibits motility by causing detachment or shortening of the flagella. Nevertheless, inhibition of galvanotaxis by verapamil is not the only result of decreased motility, because the galvanotactic orientation is impaired to a greater extent than motility. The effectiveness of the three blockers tested in inhibiting galvanotaxis depends on the concentration and on the voltage applied. At 10^?5 M, verapamil causes maximal inhibition of galvanotaxis at 9 V. At increasing concentrations up to 10^?4 M, diltiazem inhibits galvanotaxis more strongly than the other blockers. If the voltage is varied at a constant blocker concentration of 2 × 10^?5 M, nifedipine causes maximal inhibition at 3 V–6 V, diltiazem at 9 V and verapamil above 12 V.     

Possible involvement of the membrane potential in the gravitactic orientation of Euglena gracilis

Euglena gracilis, a unicellular photosynthetic flagellate, uses light and gravity as environmental hints to reach and stay in regions optimal for growth and reproduction. The current model of gravitaxis (the orientation with respect to the earth's gravitational field) is based on the specific density difference between cell body and medium. The resulting sedimentation of the cell body applies a force to the lower membrane. This force activates mechano-sensitive ion channels. The resulting ion flux changes the membrane potential, which in turn triggers reorientational movements of the trailing flagellum. One possibility for recording the predicted membrane potential changes during reorientation is the use of potential-sensitive dyes, such as Oxonol VI. The absorption changes of the dye indicating potential changes were recorded with a custom-made photometer, which allows a high precision measurement with a high temporal resolution. After a gravitactic stimulation, a short period of hyperpolarization was detected, followed by a massive depolarization of the cell. The membrane potential returned to initial values after a period of approximately 200 s. Parallel measurements of the precision of orientation and the membrane potential showed a close relationship between both phenomena. The obtained results support the current model of gravitaxis of Euglena gracilis     

Investigations on the phototactic orientation of Anabaena variabilis

Phototaxis of the blue-green alga Anabaena variabilis was studied using both population method and observation of single trichomes by microscope. The trichomes react positively at low and negatively at high illuminance. The inversion point lies at about 1000 1x. The action spectrum of positive phototaxis indicates that the photosynthetic pigments chlorophyll a, C-phycocyanin and allo-phycocyanin are involved in the absorption of the active light. The same range of wavelengths is active in negative phototaxis, but in addition, wavelengths between 500 and 560 nm and between 700 and 750 nm are also effective. Obviously pigments of unknown chemical nature are sharing in light absorption. Two alternatives are discussed. Since inhibitors of photosynthesis such as DCMU and DBMIB do not affect phototactic orientation, a direct coupling of phototaxis with photosynthesis can be excluded.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DBMIB Dibromothymoquinone (2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone) Presented in part at the International Symposium on Photosynthetic Prokaryotes: August 22–28, 1976, Dundee, Scotland     

Photocontrol of the orientation of cell division in Adiantum

Summary Two-dimensional prothallia of Adiantum capillus-veneris always expanded in a plane which was at a right angle to any given direction of irradiation with continuous white light. The expansion began with a longitudinal division of the apical cell, in the filamentous protonema, and the orientation of the mitotic cell plate of this first longitudinal division as well as the subsequent divisions was always parallel to the direction of the incident light. When three irradiations with white light, interrupted by periods of darkness, were given, two transverse and one subsequent longitudinal division were induced. When the last two irradiations were given from the same direction, the cell plate of the first longitudinal division in most protonemata was oriented parallel to the direction of light. However, when the direction of light during the third irradiation was at right angle to that during the second, the frequency of the longitudinal division greatly decreased but that of the third transverse division increased. Thus, the orientation of the first longitudinal division appeared to be controlled in some way not only by the irradiation which actually induced the third division but also by that inducing the preceding transverse division, while the direction of light for the first transverse division had little effect on the orientation of the third division.     

Phototaxis in the flagellates,Euglena gracilis and Ochromonas danica

Oriented movement with respect to laterally impinging white light of the flagellates Euglena gracilis and Ochromonas danica has been analyzed in an individual cell study with a microvideographic technique. Using the deviation of track segments (in given time intervals of 1 s) from the light direction as raw data allowed a computer based analysis of the direction distribution. A number of statistical methods employed to test the significance of the obtained results demonstrated an obvious phototactic orientation in Ochromonas which was positive (toward the light source) in low illuminance (1.25 lx=5.3×10^-3 Wm^-2) and negative in higher illuminance (>12.5 lx=5.3×10^-2 Wm^-2). Since in this flagellate the threshold for negative phototaxis is much lower than that for the step-up photophobic response, the hypothesis that negative phototaxis may be brought about by repetitive step-up phobic responses can be rejected for at least this organism. In Euglena positive phototaxis was observed in 50 lx (=0.21 Wm^-2), while an illuminance of 500 lx (=2.1 Wm^-2) caused a negative phototaxis.The experiments were carried out in this laboratory     

Cell Elongation and Microtubule Behavior in the Arabidopsis Hypocotyl: Responses to Ethylene and Auxin

During elongation of the Arabidopsis hypocotyl, each cell reacts to light and hormones in a time- and position-dependent manner. Growth in darkness results in the maximal length a wild-type cell can reach. Elongation starts at the base and proceeds in the acropetal direction. Cells in the upper half of the hypocotyl can become the longest of the whole organ. Light strongly inhibits cell elongation all along the hypocotyl, but proportionally more in the upper half. The ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) is known to stimulate hypocotyl elongation in the light. Here we show that this stimulation only occurs in cells of the apical half of the hypocotyl. Moreover, ACC application can partially overcome light inhibition, whereas indole-3-acetic acid (IAA) cannot. On low-nutrient medium (LNM) in the light, elongation is severely reduced as compared to growth on rich medium, and both ACC and IAA can stimulate elongation to the levels reached on a nutrient-rich medium. Furthermore, microtubule orientation was studied in vivo. During elongation in darkness, transverse and longitudinal patterns are clearly related with rates of elongation. In other conditions, except for the association of longitudinally orientated microtubules with growth arrest, microtubule orientation is merely an indicator of developmental age, not of elongation activity. A hypothesis on the relation between microtubules and elongation rate is discussed.     

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     

Scanning motion,ocellar morphology and orientation mechanisms in the phototaxis of the nematode Mermis nigrescens

Summary The putative ocellus of Mermis females consists of a hollow cylinder of dense hemoglobin pigmentation located in the anterior tip. The exact location of the photoreceptive nerve endings, however, is unknown. During phototaxis a continual bending or scanning motion of the head (anterior 2 mm) causes the orientation of the tip to swing about the direction of the source. By turning off (shuttering) the light source whenever the tip orientation was to one side of the source direction, the average orientation of the base of the head, and eventually the body orientation, was caused to be biased about 28° to the opposite side. Because the shuttering was synchronized with the scanning motion, the scanning motion must be involved in the maintenance of orientation to light. The direction of the bias rules out a two-signal comparison mechanism of orientation and demonstrates that a deviation of the tip from the source direction must decrease, rather than increase, the illumination of the photoreceptors. These findings, and the ocellar morphology, require that the photoreceptors be located inside the hollow tube of pigmentation where they can be shadowed by the pigment during deviations of the tip. Focusing by the curved anterior end should cause a similar modulation of the illumination at this location. The occasional episodes of transverse phototaxis can be explained by the leakiness of the pigment walls to transverse illumination. Analysis of the motion of the anterior in the presence and absence of shuttering indicates that the orientation of the base of the head, due to the motion of the neck, is controlled by the signals generated during one or more cycles of the scanning motion of the head. The orientation may be regulated by the phase relationship between the photoreceptor signal and putative proprioceptive signals that indicate the bending in the head.     

ROLE OF LIGHT AND THE CELL CYCLE ON THE INDUCTION OF SPERMATOGENESIS IN A CENTRIC DIATOM1

The centric diatom, Thalassiosira weissflogii Grun., can be induced to undergo spermatogenesis by exposing cells maintained at saturating levels of continuous light to either dim light or darkness. Using flow cytometry to determine the relative DNA and chlorophyll content per cell, the number of cells within a population that responded to and induction signal was measured. From 0 to over 90% of a population differentiated into male gametes depending upon both the induction trigger and the population examined, regardless of the average cell size of the population. Through the use of synchromized cultures, we demonstrated that responsiveness to an induction trigger was a function of cell cycle stage; cells in early G₁ were not yet committed to complete mitosis and were induced to form male gametes, whereas cells further along in their cell cycle were unresponsive to these same cues. A simple model combining the influence of light on the mitotic cell cycle and on the induction of spermatogenesis is proposed to explain the observed diversity in population responses to changes in light conditions.