Cell wall extension behavior of Phycomyces sporangiophores during the pressure response.

The cylindrical, single-celled sporangiophore of Phycomyces blakesleeanus grows (enlarges) predominantly in the longitudinal direction during two stages of development; stage I and stage IVb. Cell enlargement (cell wall extension) occurs in a distinct region termed the "growing zone." It was previously reported that a large step-up or pulse-up in turgor pressure, greater than approximately 0.02 MPa, will elicit a transient decrease in longitudinal growth rate of the stage I and stage IVb sporangiophore. This transient decrease in longitudinal growth rate is termed the "pressure response." Both the magnitude and duration of the pressure response depend on the magnitude of the turgor pressure step-up or pulse-up. Qualitatively, the pressure response is similar to the stretch response, which is produced with the application of a longitudinal force (load) on the sporangiophore. In this investigation, the growth (extension) behavior of the cell wall in the growing zone is studied during the pressure response. It is found that both the extension rate of the cell wall in the growing zone and the length of the growing zone decrease during the pressure response, and that together they account for the observed decrease in longitudinal growth rate.     

The problem of handedness reversal during the spiral growth of Phycomyces

One may easily conclude that the mechanism of cell wall growth of the sporangiophore of Phycomyces is an extremely complex one since the sporangiophore not only grows vertically (stretches) but also rotates (twists) about its longitudinal axis during growth. The result is spiral growth. The spiraling changes direction during the sporangiophore's development going from an initial left-handed spiral to a right-handed one and finally returning to the left-handed form. We believe that these observations can be explained in the following way. The cell's turgor pressure causes both longitudinal and radial deformation in the soft, thin, plastic region of the growing cell wall thus causing the wall to stretch. The cell wall microfibrils, which are initially oriented in a near transverse direction in the upper region of the growing zone, are displaced toward the longitudinal axis as a result of vertical stretch. This fibril displacement, from a transverse to a longitudinal direction, causes a horizontal displacement of the cell wall. This horizontal displacement is coupled with the vertical stretch to generate a spiral effect, i.e. spiral growth. We are further proposing that interfibril slippage occurs as the cell wall softens between stages IVa and IVb and it is this slippage that accounts for the change in the direction of spiraling when the sporangiophore goes from the left-handed form to the right-handed one.     

Phycomyces: An Increase in Mechanical Extensibility during the Avoidance Growth Response

The sporangiophore of Phycomyces shows a transient response to a double barrier, the avoidance growth response. Tensile tests conducted on the stage IV sporangiophore demonstrate that an increase in mechanical extensibility occurs about a minute after a double barrier stimulus. This change in mechanical extensibility is similar to the one that occurs after a light stimulus. We have concluded that the avoidance stimulus occurs somewhere on the same pathway between the photoreceptor mechanism and the final growth response.     

Ethylene is involved in the autochemotropism of Phycomyces

The sporangiophore of the fungus Phycomyces blakesleeanus has the property of growing away from a barrier which is few mm from the growing zone of the sporangiophore (avoidance or autochemotropic response). A model has been published (Cohen, R.J., Jan, N.Y., Matricon, J., Delbrück, M.: J. Gen. Physiol. 66, 67–95 (1975)). To explain the avoidance response which postulates that the sporangiophore emits and readsorbs a volatile growth-promoting effector (gas X) and that the barrier modifies the effector distribution by acting as an aerodynamic obstacle, causing a higher concentration of gas X on the side of the sporangiophore closer to the barrier. From this model we deduced three properties of the gas X. Of the several gases tested (N₂, CO₂, CH₄, C₂H₂, C₂H₄, C₂H₆) only ethylene (C₂H₄) had all these three properties, a finding which suggests that it has a role in the avoidance response (autochemotropism).Abbreviation Spph Sporangiophore     

Avoidance of Phycomyces in a controlled environment.

The sporangiophore of the fungus Phycomyces bends away from nearby objects without ever touching them. It has been thought that these objects act as aerodynamic obstacles that damp random winds, thereby generating asymmetric distributions of a growth-promoting gas emitted by the growth zone. In the interest of testing this hypothesis, we studied avoidance in an environmental chamber in which convection was suppressed by a shallow thermal gradient. We also controlled pressure, temperature, and relative humidity of the air, electrostatic charge, and ambient light. A protocol was established that yielded avoidance rates constant from sporangiophore to sporangiophore to within +/- 10%. We found that avoidance occurred at normal rates in the complete absence of random winds. The rates were smaller at 100% than at lower values of relative humidity, but not by much. Remarkably, at a distance as great as 0.5 mm, avoidance from a 30-micron diam glass fiber (aligned parallel to the sporangiophore) was about the same as that from a planar glass sheet. However, the rate for the fiber fell more rapidly with distance. The rate for the sheet remained nearly constant out to approximately 4 mm. We conclude that avoidance depends either on adsorption by the barrier of a growth-inhibiting substance or emission by the barrier of a growth-promoting substance; it cannot occur by passive reflection. Models that can explain these effects are analyzed in the Appendix.     

Phycomyces: Control of transpiration and the anemotropic reversal

Transpiration rates of single sporangiophores of the fungusPhycomyces blakesleeanus were measured with a microbalance. Sporangiophores transpired at the same rate at both high and low humidities. The transpiration rate was independent of sporangiophore length in the major growth stage, IVb. This finding is consistent with transpiration occurring only in the growing zone, where the cuticular layer is less developed. Sporangiophores in a wind tunnel, which were bending into the applied wind, showed a temporary reversal in their direction of bending in response to a humidity decrease. These results further confirm that water plays a role in the anemotropic and avoidance responses.     

The Avoidance Response in Phycomyces

Phycomyces sporangiophores grow away from stationary objects, a phenomenon known as the avoidance response. Evidence is presented suggesting that a growth-stimulating gas is emitted from the sporangiophore and is then swept to the leeward side by air currents resulting in higher gas concentration on that side. The presence of a stationary barrier decreases the passive movement of the gas away from the leeward side. It is proposed that an increase of this gas on one side causes that side to grow faster. Indirect evidence suggests that the gas is water vapor.     

Avoidance response, house response, and wind responses of the sporangiophore of Phycomyces

Avoidance response: An object placed 1 mm from the growing zone of a Phycomyces sporangiophore elicits a tropic response away from the object. The dependence of this response on the size of the object and its distance from the specimen is described, as well as measurements which exclude electric fields, electromagnetic radiation, temperature, and humidity as avoidance-mediating signals. This response is independent of the composition and surface properties of the object and of ambient light. House Response: A house of 0.5- to 10-cm diameter put over a sporangiophore elicits a transient growth response. Avoidance responses inside closed houses are slightly smaller than those in the open. Wind responses: A transverse wind elicits a tropic response into the wind, increasing with wind speed. A longitudinal wind, up or down, elicits a transient negative growth response to a step-up in wind speed, and vice versa. It is proposed that all of the effects listed involve wind sensing. This proposal is supported by measurements of aerodynamic effects of barriers and houses on random winds. The wind sensing is discussed in terms of the hypothesis that a gas is emitted by the growing zone (not water or any normal constituent of air), the concentration of which is modified by the winds and monitored by a chemical sensor. This model puts severe constraints on the physical properties of the gas.     

Photo-reactions in Phycomyces; growth and tropic responses to the stimulation of narrow test areas

Sporangiophores of Phycomyces in stage IV b have been stimulated by parallel light in test areas 0.2 mm. wide. The growth responses to large stimuli are very large, owing probably to light scattered within the specimen. For medium stimuli the sensitive zone coincides with the growth response zone obtained previously and excludes the region of maximum stretch. Sustained stimulations were used to elicit tropic responses. The bends formed travel away from the sporangium at a speed equal to the growth speed. Thus they remain very close to the stimulus when this is held at a constant level relative to ground but separate from it for stimuli programmed differently. The existence of a protoplasmic structure, the "inner wall," with the following properties is postulated: it is attached to the lower, non-growing part of the sporangiophore and grows by addition above the sensitive zone. It neither stretches nor twists in the sensitive zone. It is the seat of the light receptors and gives growth and tropic responses. The cell wall follows its bends by elastic stretch.     

Pressure probe study of the water relations of Phycomyces blakesleeanus sporangiophores

The physical characteristics which govern the water relations of the giant-celled sporangiophore of Phycomyces blakesleeanus were measured with the pressure probe technique and with nanoliter osmometry. These properties are important because they govern water uptake associated with cell growth and because they may influence expansion of the sporangiophore wall. Turgor pressure ranged from 1.1 to 6.6 bars (mean = 4.1 bars), and was the same for stage I and stage IV sporangiophores. Sporangiophore osmotic pressure averaged 11.5 bars. From the difference between cell osmotic pressure and turgor pressure, the average water potential of the sporangiophore was calculated to be about -7.4 bars. When sporangiophores were submerged under water, turgor remained nearly constant. We propose that the low cell turgor pressure is due to solutes in the cell wall solution, i.e., between the cuticle and the plasma membrane. Membrane hydraulic conductivity averaged 4.6 x 10(-6) cm s-1 bar-1, and was significantly greater in stage I sporangiophores than in stage IV sporangiophores. Contrary to previous reports, the sporangiophore is separated from the supporting mycelium by septa which prevent bulk volume flow between the two regions. The presence of a wall compartment between the cuticle and the plasma membrane results in anomalous osmosis during pressure clamp measurements. This behavior arises because of changes in solute concentration as water moves into or out of the wall compartment surrounding the sporangiophore. Theoretical analysis shows how the equations governing transient water flow are altered by the characteristics of the cell wall compartment.     

An Increase in Mechanical Extensibility during the Period of Light-stimulated Growth

The sporangiophore of Phycomyces responds to a temporary increase in light intensity with a transient increase in growth rate that begins 2 to 3 minutes after the initiation of the stimulus and continues until approximately the 12th minute. Tensile tests conducted on the stage IVb sporangiophore demonstrate that an increase in mechanical extensibility of the cell wall occurs 2 minutes after the initiation of a light stimulus and continues until approximately the 15th minute. This finding supports the theory that light-stimulated plant cell expansion and rate of expansion is a function of the mechanical extensibility of the cell wall.     

Cell wall structure and mechanical properties of Phycomyces

The cell wall of the sporangiophore of the fungus Phycomyces is modeled assuming the primary microstructural elements consist of stiff chitin microfibrils embedded in a Newtonian viscous matrix. The structural parameters of the model are estimated from scanning electron micrographs taken of the inside of the growth zone of the sporangiophore, published X-ray data and published transmission electron micrographs of the cell wall. The plastic extension rate normalized with respect to imposed crosshead speed is calculated from the model and compared to measured rates obtained from a tensile test. Reasonable agreement is observed for the most natural choice of the microstructural parameters employed in the model.     

Spiral growth in the radially-expanding piloboloid mutants ofPhycomyces blakesleeanus

The growth zone of the sporangiophore of a piloboloid mutant,pil, ofPhycomyces expands radially at an increased rate until the growth zone becomes nearly spherical, in sharp contrast to that of the wild-type sporangiophore which exhibits longitudinal elongation only and is conical. The rotation of thepil sporangiophore reverses its direction from clockwise (CW) to counterclockwise (CCW) during the period of increased radial expansion, and the CCW rotation continues as long as does the radial expansion. The direction of rotation and the time of reversal are correlated with the relative rates of cell-wall expansion in the longitudinal and transverse directions. The CCW rotation of the sporangiophore of this mutant can be explained by the behavior of the microfibrils, as previously proposed to explain the rotation of the wild-type sporangiophore.Abbreviations CW clockwise - CCW counterclockwise — both as viewed from above     

A Quantitative Study of Cellular Rheotropism

Sparsely sowed, hence independent Botrytis spores, which are fixed to the wall of a laminar flow chamber, tend to germinate downstream. For a velocity, v_∞, one cell radius from the wall, of 0.1 to 1000 μ/second, this tendency, expressed as a percentage of perfect orientation, approximates 9 · log (10 v_∞). Indirect proof is given that this rheotropic response is mediated by the convection across each cell of a diffusible stimulator emitted by it. Analysis of the response indicates that this stimulator has a diffusion constant of the order of 10^-7 cm²/second and thus is macromolecular, and a half-life of the order of 10 seconds and thus a radius of action in a stagnant medium of about one cell diameter. The rate of germination falls so slowly with increasing flow rate as to indicate a much lower sensitivity of the rate than of the localization of growth to this stimulator's concentration.     

Relationships between branchial chloride cells and gas transfer in freshwater fish

The gill lamellar epithelium is composed of two predominant cell types, pavement cells and mitochondria-rich chloride cells. The chloride cells play a vital role in ionic regulation because they are the sites of Ca2+ and Cl- uptake from water. Consequently, lamellar chloride cell proliferation occurs in response to ionoregulatory challenges so as to increase the ion-transporting capacity of the gill. It has been argued that such chloride cell proliferation might increase the thickness of the blood-to-water diffusion barrier and thereby impede gas diffusion. This review focuses on the potential negative consequences of chloride cell proliferation on gas transfer and possible compensatory mechanisms that might minimise the extent of respiratory impairment. Two approaches were used to evoke chloride cell proliferation in rainbow trout, hormone treatment (growth hormone/cortisol) and exposure to soft water. In all cases, chloride cell proliferation was associated with a pronounced thickening of the lamellar diffusion barrier. The thickening of the diffusion barrier was associated with a significant impairment of gas transfer. Subsequent studies revealed that several compensatory physiological responses occurred concurrently with the chloride cell proliferation to alleviate or reduce the detrimental consequences of the thickened diffusion barrier. These included hyperventilation, an increased affinity of haemoglobin-oxygen binding and earlier onset of catecholamine release during acute hypoxia.     

The Lens Effect and Phototropism of Phycomyces

Normally, the dioptrics in air of the cylindrical sporangiophore of Phycomyces blakesleeanus confer on the distal side a focusing advantage of about 30 per cent for unilateral stimuli of parallel light. This advantage can be nullified or reversed to produce negative curvatures by means of diverging light stimuli. A thin cylindrical glass lens was positioned 0.15 mm from the light-adapted growing zone with its long axis parallel to the long axis of the sporangiophore. A 3 minute blue stimulus was given and the lens removed. Reproducible negative curvatures were observed with a maximum of 13 degrees occurring within 8 minutes after the beginning of the stimulus. Experiments in air were done in a water-saturated atmosphere to minimize avoidance responses due to the proximity of the lens. The data support Buder's conclusion that the focusing advantage is the principal mechanism which produces the response differential necessary for phototropism. When the lens advantage is small, the attenuation becomes important in determining the direction of the response. Data obtained from sporangiophores immersed in inert liquids indicate that the attenuation is about 14 per cent. Therefore, whenever the focusing advantage is less than 14 per cent, negative curvatures are produced by unilateral stimuli.     

Cellulose orientation determines mechanical anisotropy in onion epidermis cell walls

The role of cellulose microfibril orientation in determining cell wall mechanical anisotropy and in the control of the wall plastic versus elastic properties was studied in the adaxial epidermis of onion bulb scales using the constant-load (creep) test. The mean or net cellulose orientation in the outer periclinal wall of the epidermis was parallel to the long axis of the cells. In vitro cell wall extensibility was 30-90% higher in the direction perpendicular to the net microfibril orientation than parallel to it. This was the case for the size of the initial deformation occurring just after the load application and for the rate of time-dependent creep. Loading/unloading experiments confirmed the presence of a real irreversible component in cell wall extension. The plastic component of the time-dependent deformation was higher perpendicular to the net cellulose orientation than parallel to it. An acid buffer (pH 4.5) increased the creep rate by 25-30% but this response was not related to cellulose orientation. The present data provide direct evidence that the net orientation of cellulose microfibrils confers mechanical anisotropy to the walls of seed plants, a characteristic that may be relevant to understanding anisotropic cell growth.     

Phycomyces: Mechanical Analysis of the Living Cell Wall

Stress relaxation measurements were conducted on stage IVb Phycomycessporangiophores in order to correlate the effect of imposedstress on cell wall growth. It was found that the cell wallshowed maximum growth when subjected to maximum stress. Growthunder stress decreased as the stress decreased. This techniquewas used to measure the response of the sporangiophore to alight stimulus; the response is measured directly from the stressrelaxation curve. Stress/strain measurements were also conducted on the stageIVb Phycomyces sporangiophores in order to further characterizethe mechanical properties of the growing zone. It was foundthat the stress/strain ratio was invariant to the strain ratewithin the ranges tested but the stress/strain ratio did increasewith larger loads, i.e., the stress/strain ratio shows non-linearbehaviour.     

Development of Sporangiophores of Peronospora parasitica (Pers. ex Fr.) Fr.

The morphology of developing sporangiophores of Peronosporaparasitica from wallflower is described, and morphogenesis maybe divided into the following five stages: the sporangiophoreprimordium, unbranched sporangiophore, branched sporangiophore,spore formation and maturation, and formation of the cross wall.The growth of individual sporangiophores in a humidity chamberwas followed under the microscope, and increase in height andincrease in volume measured. The greatest increase in volumewas during spore formation, when the sporangiophore volume mightquadruple within an hour.     

Initial permeability of the 19-day foetus to nickel

The administration of 63NiCl2 to 19-day pregnant rats produced a limited transfer of nickel to the foetus. An experimental model based on the incubation of the conceptuses still connected to the maternal organism resulted in an appreciable net transfer of nickel through the amniotic membranes. The placenta retained a significant amount of radioactive nickel as did the lining of the uterine wall. The circulation of nickel through the foetus following the direction maternal blood----placenta----foetus----amniotic membranes----endometrium----maternal venous blood, is postulated. This transfer could provide an additional protection to placental barrier against metal toxicity.