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.     

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.     

Ethylene Contamination of CO(2) Cylinders: Effects on Plant Growth in CO(2) Enrichment Studies

The mechanical extensibilities of stage IVb Phycomyces were measured before and after a humidified wind stimulus. We find that when the humidity of the wind is greater than that of the ambient air, there is an increase in the mechanical extensibility of the cell wall. We also find that a step decrease in wind humidity results in a decrease in the mechanical extensibility of the cell wall.     

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.     

Intracellular recordings from phycomyces

Intracellular recordings from the giant sporangiophore of Phycomyces stage II were obtained. The mean transmembrane potential for 30 observations was −119.9 millivolts (negative inside), and it did not change either as a result of a light stimulus or during dark adaptation. Injected depolarizing and hyperpolarizing step currents and steady currents did not produce any avidence of spike activity. We conclude that light transduction and dark adaptation in Phycomyces are not based on alterations of the transmembrane potential.     

Signal transduction in Phycomyces sporangiophores: columella as a novel sensory organelle mediating auxin-modulated growth rate and membrane potential

Protoplasma - The growing zone (GZ) of the unicellular coenocytic sporangiophore of Phycomyces blakesleeanus represents the site of stimulus reception (light, gravity, gas) and stimulus response,...     

Phycomyces: MODIFICATION OF LIGHT-INDUCED SPIRAL GROWTH AFTER MECHANICAL CONDITIONING OF THE CELL WALL

Mature stage IVb Phycomyces sporangiophores show left-hand spiral growth; that is, viewed from above, the sporangium rotates clockwise. It has been shown that mechanical conditioning (strain-hardening) of the cell wall by the Instron technique increases the ratio of rotation to the elongation growth rate compared to nonmechanically conditioned controls. It is reported that the addition of a saturating light stimulus to these sporangiophores causes a decrease in the ratio of rotation to elongation growth rate. This result is in agreement with the fibril slippage model, i.e. the counterclockwise rotation of stage IVa is a result of parallel fibrils lying in a right-handed spiral configuration slipping by one another. It is suggested that a light stimulus added to a mechanically conditioned stage IVb sporangiophore activates one or more cell wall-loosening enzymes which act by decreasing the number of intermolecular bonds between parallel fibrils causing fibril slippage, resulting in counterclockwise rotation. It is precisely this counterclockwise contribution that decreases the rotation to elongation growth ratio of mechanically conditioned and then light-stimulated stage IVb sporangiophores.     

Phytochrome control of maize coleoptile section elongation: the role of cell wall extensibility

Maize (Zea mays) coleoptile section cell wall extensibility was found to be stimulated by red light. This stimulation was largely removed by simultaneous or immediately subsequent far-red treatment. Qualitatively similar patterns of response occurred at 0 C and 20 C. Plastic extensibility responded more than elastic extensibility after red light treatment. Red-induced extensibility increases were detectable by 20 minutes after irradiation, and extensibility continued to increase up to at least 1 hour after irradiation. The kinetics of escape from far-red reversibility indicate that the initial events leading to this phenomenon are among the fastest known phytochrome responses.     

Phycomyces: growth responses of the sporangium

During the development of the sporangiophore of the fungus Phycomyces blakesleeanus there occurs a period of several hours when the sporangiophore does not elongate; instead, its “growth” is diverted into the formation of a sporangium at its top. This period of head formation is called stage II. Clearly, growth has not ceased but rather the geometry of the growing area has changed from that of a cylinder to a sphere. The growing sphere is found to have properties similar to the stage IV growing zone in that it functions as a sensory receptor and effector. The growing sporangium responds to both light (light head response) and humidity (wet head response). A model is presented giving a possible mechanism by which the ultimate size of the sporangium is regulated.     

The analysis of spiral growth in phycomyces using a novel optical method

A conical mirror was designed and used to measure simultaneously the elongational and rotational displacement of a number of markers on the growing zone of the sporangiophore of Phycomyces. The results obtained by this new optical method demonstrate that the rotational rate is roughly proportional to the elongational rate, except in the lower region of the growing zone where a significant amount of rotation occurs without measurable elongation. From the data presented in this report, we have constructed a model that appears to explain the mechanism responsible for the left-handed spiral growth of the developing sporangiophore.     

ON "REVERSAL" OF PHOTOTROPISM IN PHYCOMYCES

Alleged reversal of the phototropism of the sporangiophores of Phycomyces by high intensities of light does not occur if infra-red radiation is properly excluded. Phototropic "indifference" alone occurs at high intensities due to equal photic action on both sides of the sporangiophore. If heat radiation is not screened out, a gradual, negative thermotropic bending takes place.     

DARK ADAPTATION AND THE LIGHT-GROWTH RESPONSE OF PHYCOMYCES

1. A single-celled, elongating sporangiophore of Phycomyces responds to a sufficient increase in intensity of illumination by a brief increase in growth rate. This is the "light-growth response" of Blaauw. 2. The reaction time is compound, consisting of an exposure period and a latent period (this comprising both the true latent period resulting from photochemical action and any "action time" necessary for the response). During the latter period the plant may be in darkness, responding nevertheless at the end of the latent period. 3. Both light adaptation and dark adaptation occur in the sporangiophore. The kinetics of dark adaptation can be accounted for on the basis of a bimolecular reaction, perhaps modified by autocatalysis. Attention is called to the bimolecular nature of the "dark" reaction in all other photosensory systems that have been studied, in spite of the diversity of the photosensitive substances themselves and of the different forms of the responses to light.     

Phycomyces: distribution of growth velocities in the growing zone

Distribution of growth velocities in the growing zone of stage IVb Phycomyces sporangiophores was measured by photographing the growing zone after dusting it with starch grains. When the entire growing zone is fully dark-adapted to red light and then subjected to a saturating white light stimulus, the entire growing zone increases in growth rate. When the growing zone is partially light-adapted, again the entire growing zone responds when subjected to a saturating white light stimulus but to a lesser degree than the fully dark-adapted sporangiophore. Phototropic mutants of class 1 and class 2 show a distribution of growth in the growing zone similar to wild type sporangiophores both during steady-state growth and during light-stimulated growth.     

A quantitative model of Phycomyces phototropism

A quantitative model accounting for phototropism in the wild type and in behavioural mutants of Phycomyces is described.Photomecisms (changes in the sporangiophore's growth velocity in response to changes in light intensity) are produced by a system composed of two sets of linear transducers separated by an adaptation mechanism, the first transducer being the photoreceptor.Phototropism under asymmetrical light distributions is caused by the summation of local photomecisms in the distal half of the sporangiophore, where two bright bands are produced by refraction of the incident light. The photoreceptors turn around the sporangiophore axis; they are approximately adapted to local intensity everywhere except upon entrance to the first bright band. Thus, a continuous photomecism originates at this band while the rest of the sporangiophore remains practically unstimulated.The mutants suffer a reduction in the efficiency of transduction.The behaviour of the wild type and of the mutants has been quantitatively simulated by computer. The predictions from the model fit the experimental results.     

Physiological Studies on Pea Tendrils : XIV. Effects of Mechanical Perturbation, Light, and 2-Deoxy-d-Glucose on Callose Deposition and Tendril Coiling

When excised tendrils of pea (Pisum sativum L. cv Alaska) are mechanically perturbed there is an immediate and transient increase in callose deposition in the sieve cells. Mechanical perturbation (MP) results in a coiling response in light-grown tendrils and in dark-adapted tendrils, provided, in the latter case, that they receive adequate illumination within a limited period of time after MP. In nonperturbed tendrils the number of callose deposits decreases to some minimum with increasing time in the dark, and their ability to coil in the dark in response to MP diminishes with time in the dark. The transient increase of callose deposition due to MP, however, occurs whether or not tendrils are dark adapted, and whether they receive light or are retained in the dark after MP. This indicates that if callose is directly involved in tendril coiling, then it exerts its effect on the sensory perception of the mechanical stimulus. In the present investigation, there is never tendril coiling without the transient increase in callose, and the time after MP at which the peak of callose deposition occurs precedes the time of the peak amount of coiling.     

Hypergravity stimulus enhances primary xylem development and decreases mechanical properties of secondary cell walls in inflorescence stems of Arabidopsis thaliana

BACKGROUND AND AIMS: The xylem plays an important role in strengthening plant bodies. Past studies on xylem formation in tension woods in poplar and also in clinorotated Prunus tree stems lead to the suggestion that changes in the gravitational conditions affect morphology and mechanical properties of xylem vessels. The aim of this study was to examine effects of hypergravity stimulus on morphology and development of primary xylem vessels and on mechanical properties of isolated secondary wall preparations in inflorescence stems of arabidopsis. METHODS: Morphology of primary xylem was examined under a light microscope on cross-sections of inflorescence stems of arabidopsis plants, which had been grown for 3-5 d after exposure to hypergravity at 300 g for 24 h. Extensibility of secondary cell wall preparation, isolated from inflorescence stems by enzyme digestion of primary cell wall components (mainly composed of metaxylem elements), was examined. Plants were treated with gadolinium chloride, a blocker of mechanoreceptors, to test the involvement of mechanoreceptors in the responses to hypergravity. KEY RESULTS: Number of metaxylem elements per xylem, apparent thickness of the secondary thickenings, and cross-section area of metaxylem elements in inflorescence stems increased in response to hypergravity. Gadolinium chloride suppressed the effect of hypergravity on the increase both in the thickness of secondary thickenings and in the cross-section area of metaxylem elements, while it did not suppress the effect of hypergravity on the increase in the number of metaxylem elements. Extensibility of secondary cell wall preparation decreased in response to hypergravity. Gadolinium chloride suppressed the effect of hypergravity on cell wall extensibility. CONCLUSIONS: Hypergravity stimulus promotes metaxylem development and decreases extensibility of secondary cell walls, and mechanoreceptors were suggested to be involved in these processes.     

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.     

Avoidance and rheotropic responses in phycomyces. Evidence for an 'avoidance gas" mechanism

If a mature sporangiophore is placed next to a barrier that is moving in a clockwise direction, it grows both away from the barrier and into the wind; the wind is generated by the moving barrier itself. When the barrier is moving in a counterclockwise direction, the sporangiophore grows towards both the barrier and the wind. The net direction of growth appears to be the vector sum of the rheotropic response and the avoidance aiming error and does not involve the classic stationary- barrier avoidance response. Our experiments all support the suggestion that the avoidance response, the rheotropic response and the variety of reported wind responses can be explained by the presence of a self- emitted, growth-simulating avoidance gas. We present data that suggest that it is the direction of the net flux (mass transfer) of this gas that determines both the direction and the magnitude of the sporangiophore growth. We further suggest that the region of the cell wall showing maximum mass transfer will show a minimum growth rate, i.e., the direction of growth will always be in the direction of maximum transfer. If water is the avoidance gas, then it would follow that the total hydration of the cell wall in an aqueous salt solution should result in cell wall softening; cell wall softening has been correlated directly to cell wall growth. Using the Instron technique, we now show that submerging the entire sporangiophore in an aqueous salt solution for 4 min causes an increase in cell wall extensibility.     

Salinity Stress Inhibits Bean Leaf Expansion by Reducing Turgor, Not Wall Extensibility

Treatment of bean (Phaseolus vulgaris L.) seedlings with low levels of salinity (50 or 100 millimolar NaCl) decreased the rate of light-induced leaf cell expansion in the primary leaves over a 3 day period. This decrease could be due to a reduction in one or both of the primary cellular growth parameters: wall extensibility and cell turgor. Wall extensibility was assessed by the Instron technique. Salinity did not decrease extensibility and caused small increases relative to the controls after 72 hours. On the other hand, 50 millimolar NaCl caused a significant reduction in leaf bulk turgor at 24 hours; adaptive decreases in leaf osmotic potential (osmotic adjustment) were more than compensated by parallel decreases in the xylem tension potential and the leaf apoplastic solute potential, resulting in a decreased leaf water potential. It is concluded that in bean seedlings, mild salinity initially affects leaf growth rate by a decrease in turgor rather than by a reduction in wall extensibility. Moreover, longterm salinization (10 days) resulted in an apparent mechanical adjustment, i.e. an increase in wall extensibility, which may help counteract reductions in turgor and maintain leaf growth rates.     

Differential effect of auxin on in vivo extensibility of cortical cylinder and epidermis in pea internodes

The effect of auxin indole-3-acetic acid (IAA) on growth and in vivo extensibility of third internode sections from red light grown pea seedlings (Pisum sativum L. cv Alaska) and the isolated tissues (cortex plus vascular tissue = cortical cylinder, and epidermis) was investigated. Living tissue was stretched at constant force (creep test) in a custom-built extensiometer. In the intact section, IAA-induced increase in total (E_tot), elastic (E_el), and plastic (E_pl) extensibility is closely related to the growth rate. The extensibility of the cortical cylinder, measured immediately after peeling of intact sections incubated for 4 hours in IAA, is not increased by IAA. Epidermal strips, peeled from growing sections incubated in IAA, show a E_pl increase, which is correlated to the growth rate of the intact segments. The isolated cortical cylinder expands in water; IAA has only a small growth-promoting effect. The extensibility of the cortical cylinder is not increased by IAA. Epidermal strips contract about 10% on isolation. When incubated in IAA, they do not elongate, but respond with an E_pl increase. The amount of expansion of the cortical cylinder and contraction of the epidermis (tissue tension), measured immediately following excision and peeling, stays constant during IAA-induced growth of intact sections. The results support the hypothesis that IAA induces growth of the intact section by causing an E_pl increase of the outer epidermal wall. The driving force comes from the expansion of the cortical cylinder which is under constant compression in the intact section.