Effects of microbeam light on growth and phototropism ofPilobolus crystallinus sporangiophores

A small blue-light beam (50 μm in diam) was used to examine light-growth response and phototropism inPilobolus crystallinus sporangiophores. Continuous irradiation by microbeam of a region 100–150 μm from the apex promoted the growth of a dark-adapted sporangiophore for about 15 min after a lag period of 1–2 min. After the promotion, the growth rate fell below that before the irradiation. Irradiation of the apex of sporangiophore slightly promoted the growth but strongly inhibited the growth after the promotion. A smaller light beam (10 μm in diam) applied continuously at grazing incidence along one side of the sporangiophore caused bending toward the shaded side, implying that the irradiated side grew more rapidly than the shaded side and that the lens effect is involved in the phototropism of young sporangiophores ofP. crystallinus. The involvement of the lens effect was confirmed by the fact that a carotenoid-less mutant was 1.5–2 times more sensitive to unilateral blue light than the wild type, probably because of a smaller intracellular light attenuation during passage through the mutant cell.     

Negative phototropism in the piloboloid mutants of Phycomyces blakesleeanus Bgff.

The sporangiophore (spph) of a piloboloid mutant, genotype pil, of Phycomyces ceases elongation and expands radially in the growth zone shortly after reaching the developmental stage IV b. The pil spph is always negatively phototropic to unilateral visible light when its diameter exceeds 210 m. Photoinduction of spph initiation, light-growth response, threshold of light energy fluence rate for the negative phototropism, avoidance and gravitropism in the pil mutant are all normal. In liquid paraffin, the pil spph shows negative phototropism as does the wild-type spph. Genetic analyses indicate that the negative phototropism of the pil mutant is governed by the phenotypic characteristics of pil but not by specific gene(s) responsible for negative phototropism. These facts imply that the reverse phototropism of the pil mutant results from a loss of the convergent lens effect of the cell because of the increase in cell diameter.Abbreviations spph(s) sporangiophore(s) - wt(s) wild type(s)     

Physiological Studies in the Mucorales: PART I THE PHOTOTROPISM OF SPORANGIOPHORES OF PHYCOMYCES BLAKESLEEANUS

Previous views on the physical basis of phototropism in Phycomycessporangiophores are briefly discussed.

1. It was confirmed thatunilaterally illuminated sporangiophoresimmersed in liquidparaffin show strong negative phototropism.
2. Elongation growthceased and no phototropic response took placeunder anaerobicconditions.
3. By focusing a fine beam of light on to one edgeof the growingzone of a sporangiophore, leaving the other sidein darkness,it was established that greater elongation tookplace in theilluminated zone, the sporangiophore tending tobend out ofthe beam. Rapid reversal of the curvature followedwhen theillumination was transferred to the opposite edge ofthe sporangiophore.

Wassink and Boumann's suggestion that phototropism can be initiatedby a one-quantum-per-cell process is criticized in the lightof this result and other work by Castle.     

A colicin-tolerant mutant of Escherichia coli with reduced levels of cyclic AMP and a strong bias towards lambda lysogeny

Summary A collection of mutants of Phycomyces blakesleeanus with abnormal phototropism is divided into phenotypic groups on the basis of the results of five routine tests of sensory function. The three classes of mutants found permit the formulation of a network of sensory pathways, linking the three types of receptor (light, chemical, gravity) with the three types of response (sporangiophore tropisms, initiation of sporangiophores, induction of -carotene synthesis). A model with a photoreceptor, early and late linear transducers, a logarithmic transducer and a growth-controlling element is proposed for the information channel of the phototropic response. Mutagenesis and the possible functional relationship between carotenes and phototropism are discussed.     

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.     

Influence of phototropic response of spore germ tubes on infection process inColletotrichum lagenarium andBipolaris oryzae

The germ tubes ofColletotrichum lagenarium showed negative phototropism to UV-blue (300–520 nm) and far-red (>700 nm) regions with maximum in the near ultraviolet (NUV) region, while monochromatic radiations of 575–700 nm (yellow-red region) induced positive phototropism with maximum in the red region. Green light (520–575 nm) was ineffective. Negative phototropism-inducing wavelength regions inhibited germ tube growth and positive phototropism-inducing wavelength regions promoted it significantly.Bipolaris oryzae did not show any phototropic response and light did not affect the germ tube growth. These results indicate that the lens effect, in combination with the light growth reaction and light growth inhibition, is the mechanism of the phototropism of germ tubes ofC. lagenarium. NUV radiation, which induced negative phototropism ofC. lagenarium, promoted appressorium formation, while red light, which induced positive phototropism, suppressed it significantly. In the case ofB. oryzae, light did not affect the infection structure formation. These results indicate that negative phototropism of germ tubes ofC. lagenarium favors the infection process by facilitating the contact of the tips of germ tubes with the host surface, while positive phototropism has the opposite effect.     

A new allele with abnormal cyclic-AMP phosphodiesterase activity in Phycomyces

Summary A mutant of the fungus Phycomyces blakesleeanus (Burgeff), C21 (madA7) that was isolated for its abnormal phototropism (Bergman et al. 1973) carries a secondary mutation pde-1 which is unlinked to the madA gene. The pde-1 allele causes the loss of about 80% of the cAMP phosphodiesterase activity. This allele is not essential for the photoreactions of the mycelium or the sporangiophore, and the bulk activity of the phosphodiesterase appears to play no role in the phototransduction pathway of Phycomyces.     

Blue-light control of sporangiophore initiation in Phycomyces

Summary Many fungi produce spores or spore-bearing structures under the control of blue light. Sporangiophores of Phycomyces blakesleeanus are produced continuously along racing tube cultures grown in constant darkness or constant light. However, if a dark-grown culture is exposed to light for a short time on one day a narrow, dense band of sporangiophores is observed the next day at that point of the tube occupied by the mycelial tips during the light pulse. A periodic program with short days (e.g., 4 h light; 20 h dark), leads to periodic bands of sporangiophores spaced at intervals corresponding to one period-length (in this case 24 h) of mycelial growth. Sporangiophore initiation is inhibited by a light to dark transition and is stimulated by a dark to light transition. A partial action spectrum of the initiation response, covering the critical 480–540 nm region, strongly suggests that the same photoreceptor pigment is involved as in the phototropic response and light growth response of sporangiophores. Mutants with altered light control of sporangiophore initiation have been found among those selected for altered phototropism. This joint elimination of these two responses to blue light by a single mutation is evidence for a common early transduction system.     

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.     

Growth Retardant-Induced Changes in Phototropic Reaction of Vigna radiata Seedlings

The effect of growth retardants on phototropism has been studied in mung bean (Vigna radiata) seedlings. Ancymidol, tetcyclacis, and paclobutrazol inhibited phototropism while AMO 1618 and CCC were ineffective. The fluence-response relationships for phototropism of etiolated seedlings were similar to those previously described for monocots and other dicots. Ancymidol caused a shift in the maximum phototropic response to higher fluence of light. It is suggested that ancymidol may affect phototropism through an effect on the photoreceptor system.     

Reduced phototropism in pks mutants may be due to altered auxin‐regulated gene expression or reduced lateral auxin transport

Phototropism allows plants to orient their photosynthetic organs towards the light. In Arabidopsis, phototropins 1 and 2 sense directional blue light such that phot1 triggers phototropism in response to low fluence rates, while both phot1 and phot2 mediate this response under higher light conditions. Phototropism results from asymmetric growth in the hypocotyl elongation zone that depends on an auxin gradient across the embryonic stem. How phototropin activation leads to this growth response is still poorly understood. Members of the phytochrome kinase substrate (PKS) family may act early in this pathway, because PKS1, PKS2 and PKS4 are needed for a normal phototropic response and they associate with phot1 in vivo. Here we show that PKS proteins are needed both for phot1‐ and phot2‐mediated phototropism. The phototropic response is conditioned by the developmental asymmetry of dicotyledonous seedlings, such that there is a faster growth reorientation when cotyledons face away from the light compared with seedlings whose cotyledons face the light. The molecular basis for this developmental effect on phototropism is unknown; here we show that PKS proteins play a role at the interface between development and phototropism. Moreover, we present evidence for a role of PKS genes in hypocotyl gravi‐reorientation that is independent of photoreceptors. pks mutants have normal levels of auxin and normal polar auxin transport, however they show altered expression patterns of auxin marker genes. This situation suggests that PKS proteins are involved in auxin signaling and/or lateral auxin redistribution.     

A Novel Effect in Phycomyces Phototropism : Positive Bending and Compensation Spectrum in Far UV

A novel effect—positive phototropic bending under far UV irradiation (between 260 and 305 nanometers) at low intensities—is reported. Natural compensation points (intensities which cause no bending under unilateral irradiation) have been determined for different wavelengths. The curve connecting these points, the compensation spectrum, divides the intensity-wavelength plane into areas of negative and positive tropism. It is further shown that a highly asymmetrical pattern of light stimulus within the sporangiophore underlies the symmetrical growth response at each compensation point. This suggests that some unknown additional factor is involved in perceiving a UV stimulus at the level of the photoreceptor. It is also demonstrated here that positive tropism in the UV range is due to a lens effect. We conclude that the hypothesis of optical attenuation of the stimulus (considered until now as the most plausible explanation of negative tropism in the UV spectral range) must be dismissed. The results presented here represent the first application of our quantitative theoretical consideration of spatial factors in phototropism heretofore neglected by others.     

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     

Negative phototropic response of rhizoid cells in the fern Adiantum capillus-veneris

In general, phototropic responses in land plants are induced by blue light and mediated by blue light receptor phototropins. In many cryptogam plants including the fern Adiantum capillus-veneris, however, red as well as blue light effectively induces a positive phototropic response in protonemal cells. In A. capillus-veneris, the red light effect on the tropistic response is mediated by phytochrome 3 (phy3), a chimeric photoreceptor of phytochrome and full-length phototropin. Here, we report red and blue light-induced negative phototropism in A. capillus-veneris rhizoid cells. Mutants deficient for phy3 lacked red light-induced negative phototropism, indicating that under red light, phy3 mediates negative phototropism in rhizoid cells, contrasting with its role in regulating positive phototropism in protonemal cells. Mutants for phy3 were also partially deficient in rhizoid blue light-induced negative phototropism, suggesting that phy3, in conjunction with phototropins, redundantly mediates the blue light response.     

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.     

Phytochrome modulation of blue-light-induced phototropism

Red light enhances hypocotyl phototropism toward unilateral blue light through a phytochrome‐mediated response. This study demonstrates how the phytochromes modulate blue‐light‐induced phototropism in the absence of a red light pre‐treatment. It was found that phytochromes A, B, and D have conditionally overlapping functions in the promotion of blue‐light‐induced phototropism. Under very low blue light intensities (0.01 µmol m^?2 s^?1) phyA activity is necessary for the progression of a normal phototropic response, whereas above 1.0 µmol m^?1 s^?2 phyB and phyD have functional redundancy with phyA to promote phototropism. PhyA also contributes to attenuation of phototropism under high fluence rates of unilateral blue light, which was previously shown to be dependent on the phototropins and cryptochromes. From these results, it appears that phytochromes are required to develop a robust phototropic response under low fluence rates, whereas under high irradiances where phototropism may be less important, phyA suppresses phototropism.     

Tropisms in Phycomyces: sine law for gravitropism, exponential law for photogravitropic equilibrium

Sporangiophores of Phycomyces blakesleeanus that are gravitropically stimulated by inclining them relative to the earth's gravitational vector obey the sine law for inclination angles between 0 degrees and 150 degrees. The quantitative relation between gravitropism and phototropism was analyzed for sporangiophores that were kept in balance between opposing gravitational and phototropic stimuli. The gravitropism of inclined sporangiophores was compensated with unilateral light impinging at right angles relative to the axis of the sporangiophore. The fluence rate of unilateral blue light (466 nm) that was required to counteract the negative gravitropism increased exponentially with the sine of the inclination angle of the sporangiophore. The establishment of photogravitropic equilibrium during continuous unilateral irradiation is thus determined by two different laws: the well-known sine law for gravitropism and a novel exponential law of phototropism described in this work. Furthermore, the specific form of the exponential relationship depends on the presence of statoliths (vacuolar protein crystals) and on wavelength.     

Control of hypocotyl gravitropism by photochrome in a dicotyledonous seedling (Sesamum indicum L.)

Abstract The present study was prompted by the question as to whether the strong effect of red and far-red light treatments on blue-light-mediated phototropism in the sesame (Sesamum indicum L.) hypocotyl (Woitzik & Mohr, 1988) should be attributed in part to changes initialed by light in the gravitropic counter-response. Light treatments, operating through phytochrome, do indeed strongly affect the gravitropic response. However, the direction of the light effect is the same in gravitropism, as in phototropism. Thus, the gravitropic counter-response leads to an underestimate, rather than an overestimate, of the importance of phytochrome action on phototropic responsiveness. The effect of red and far-red light, operating via phytochrome, on the gravitropic response of the sesame hypocotyl could be studied in the present paper without any interference due to phototropism or light control of longitudinal growth. It was found that the effects of red and far-red pretreatments (given prior to the onset of the stimulus) as well as the action of simultaneously applied red or far-red light (simultaneous to the phototropic or gravitropic stimulus) are very similar in both phototropism and gravitropism. In particular, the seedling is capable of superimposing information about the actual light conditions during bending on the ‘memory’ it has about the light conditions prior to the onset of phototropism or gravitropic stimulation, This striking similarity between the phototropic and gravitropic responses possibly indicates that phytochrome affects the signal-response-chain at a relatively late stage, after the phototropic and the gravitropic signal-response chains have merged. From a teleonomic point of view the action of red and far-red light on phototropic, as well as gravitropic, responsiveness can be conceived as part of a shade escape strategy.     

What do we know about the non-uniform perception of a phototropic stimulus in Phycomyces?

For a comprehensive study of phototropism in sporangiophores of the fungus Phycomyces, quantitative treatment of spatial aspects is necessary. The first step in quantifying spatial factors of phototropic signal processing is the elucidation of the non-uniform light profile, predominantly caused by a lens effect in the cylindrical body of the sporangiophore. Herein we compare recently presented theoretical and experimental studies of light profiles. Errors and ambiguities arising from instrumental limitations and arbitrary assumptions are revealed. On the other hand, by combining theoretical and experimental results we have been able to select out the reliable information, which can now be applied in phototropic studies.Dedicated to Professor Hans Mohr on the occasion of his 60th birthday     

Root phototropism: from dogma to the mechanism of blue light perception

In roots, the “hidden half” of all land plants, gravity is an important signal that determines the direction of growth in the soil. Hence, positive gravitropism has been studied in detail. However, since the 19th century, the response of roots toward unilateral light has also been analyzed. Based on studies on white mustard (Sinapis alba) seedlings, botanists have concluded that all roots are negatively phototropic. This “Sinapis-dogma” was refuted in a seminal study on root phototropism published a century ago, where it was shown that less then half of the 166 plant species investigated behave like S. alba, whereas 53% displayed no phototropic response at all. Here we summarize the history of research on root phototropism, discuss this phenomenon with reference to unpublished data on garden cress (Lepidium sativum) seedlings, and describe the effects of blue light on the negative bending response in Thale cress (Arabidopsis thaliana). The ecological significance of root phototropism is discussed and the relationships between gravi- and phototropism are outlined, with respect to the starch-statolith-theory of gravity perception. Finally, we present an integrative model of gravi- and blue light perception in the root tip of Arabidopsis seedlings. This hypothesis is based on our current view of the starch-statolith-concept and light sensing via the cytoplasmic red/blue light photoreceptor phytochrome A and the plasma membrane-associated blue light receptor phototropin-1. Open questions and possible research agendas for the future are summarized.