Interaction between gravitropism and phototropism in sporangiophores of Phycomyces blakesleeanus.

The interaction between gravitropism and phototropism was analyzed for sporangiophores of Phycomyces blakesleeanus. Fluence rate-response curves for phototropism were generated under three different conditions: (a) for stationary sporangiophores, which reached photogravitropic equilibrium; (b) for sporangiophores, which were clinostated head-over during phototropic stimulation; and (c) for sporangiophores, which were subjected to centrifugal accelerations of 2.3g to 8.4g. For blue light (454 nm), clinostating caused an increase of the slope of the fluence rate-response curves and an increase of the maximal bending angles at saturating fluence rates. The absolute threshold remained, however, practically unaffected. In contrast to the results obtained with blue light, no increase of the slope of the fluence rate-response curves was obtained with near-ultraviolet light at 369 nm. Bilateral irradiation with near-ultraviolet or blue light enhanced gravitropism, whereas symmetric gravitropic stimulation caused a partial suppression of phototropism. Gravitropism and phototropism appear to be tightly linked by a tonic feedback loop that allows the respective transduction chains a mutual influence over each other. The use of tropism mutants allowed conclusions to be drawn about the tonic feedback loop with the gravitropic and phototropic transduction chains. The results from clinostating mutants that lack octahedral crystals (implicated as statoliths) showed that these crystals are not involved in the tonic feedback loop. At elevated centrifugal accelerations, the fluence-rate-response curves for photogravitropic equilibrium were displaced to higher fluence rates and the slope decreased. The results indicate that light transduction possesses a logarithmic transducer, whereas gravi-transduction uses a linear one.

The Phycomyces lens: measurement of the sporangiophore intensity profile using a fiber optic microprobe.

A fiber optic microprobe, 5.5 µm in diameter, was used as a detector to measure the light intensity profile at the distal cell surface of Phycomyces blakesleeanus (Burgeff) sporangiophores that were irradiated unilaterally by a collimated xenon source. The light intensity at a fixed location of the cell surface showed large random variations over time which were probably the result of optical effects of particles being carried past the probe by cytoplasmic streaming. The intensity profile, formed around the distal periphery of the cell by the lens action of the sporangiophore, was determined from intensity measurements made while the probe was held fixed and the incident beam direction was varied in angle of azimuth. The resulting profile consisted of two steeply rising sides enclosing a central plateau or shallow well which ranged in fluence rate from 1.6 to 2.2 times that of the incident beam. These experimental findings differ from theoretical modeling where much greater contrast between the sides and central portion of the lens profile was predicted. These results also indicate that the mechanism of phototropic sensory perception in Phycomyces may filter out cytoplasmic light flicker and may not require strong contrasting regions within the lens profile to detect light direction.

The gravireceptor of Phycomyces. Its development following gravity exposure.

The gravitropism of a mature stage IV Phycomyces sporangiophore has a shorter and more uniform latency if the sporangiophore is exposed horizontally to gravity during its earlier development (stage II and stage III). This early exposure to an altered gravitational orientation causes the sporangiophore to develop a gravireceptor as it matures to stage IV and resumes elongation. A technique has been developed to observe the spatial relationship between the vacuole and the protoplasm of a living sporangiophore and to show the reorganization caused by this exposure to altered gravity. Possible gravireceptor mechanisms are discussed.

;hycomyces: Interference Between the Light Growth Response nd the Avoidance Response.

Phycomyces sporangiophores showl a growth response to a light stimus and an avoidance bending response to a physical barrier. A bluie-light stimnlluts Imninisteredl in conjunction with a barrier interferes with the avoidance bending sponse. This interference begins ifter a latency of abotit 3 minultes atndt continues a period of 4 to 5 minutes.

Intracellular rotation and the phototropic response of Phycomyces.

Experimental evidence indicates that during phototropism, Phycomyces sporangiophores use their own net rotation to convert an apparently spatial stimulus to a temporal one. Conversion to a continuous temporal stimulus insures that phototropism never adapts as long as the spatial asymmetry in illumination is maintained. If this temporal stimulus is circumvented by rotating the cell backwards so that there is no net rotation of some of the receptors relative to the light, the response can be reduced by two-thirds. The system thus adapts to the incident light, resulting in a reduced response. For the illumination of a transparent cell, this compensating rotation speed is 10 degrees/min counterclockwise and probably corresponds to the photoreceptor rotation in the most effective part of the growing zone. We infer that this region is in the upper portion of the growing zone and that the receptor system rotates integrally with that region of the cell.

Phototropism and local adaptation in Phycomyces sporangiophores.

Phototropic responses to unilateral ultraviolet stimuli were studied to determine whether the response of one side of the cell is affected by the previous exposure of the opposite side to ultraviolet. It has been found that the direction of bending is not parallel to the stimulus direction, but is along a straight line rotated 17 degrees clockwise from the stimulus direction. This deviation indicates that the photoreceptors may be in a state of continual clockwise rotation. If before the stimulus the cell is exposed briefly to ultraviolet and rotated through 90 degrees , the response is not along the 17 degrees line, but is deviated a greater or lesser amount, depending on whether the 90 degrees rotation is clockwise or counterclockwise. This difference is evidence that the first ultraviolet exposure leaves a persistent patch of light-adapted receptors and the shaded part of the cell remains dark adapted. The phototropic stimulus straddles the edge between light- and dark-adapted regions, and the differing responses of the two regions affects the direction of phototropic bending. A phototropic mechanism is proposed which combines the features of local adaptation and photoreceptor rotation.

Phycomyces sporangiophores: fungal stretch receptors.

The application of mechanical force to the sporangiophore of Phycomyces elicits a transient growth response. The immediate stimulus may be the deformation of the sporangiophore under the applied force. Compression of the cell causes an interval of faster growth, and extension causes an interval of slower growth.