Phototropism mutants of Phycomyces blakesleeanus isolated at low light intensity

Mutants of Phycomyces have played a major role in the analysis of phototropism and other responses. Fifteen new mutants of Phycomyces with abnormal phototropism (genotype mad) have been isolated on the basis of their inability to bend toward dim unilateral blue light (fluence rate 5 × 10⁻⁷ W m⁻²), a protocol different from those employed in previous mutant hunts. One mutant resulted from chemical mutagenesis with ICR-170, and the other 14 were induced with N-methyl-N′-nitro-N-nitro-soguanidine. Seven of the mutants are “night blind”; six have phototropic thresholds intermediate between those of wild type (10⁻⁹ W m⁻²) and madA strains (∼ 10⁻⁴ W m⁻²); and one has a threshold similar to that of night-blind madB and madC mutants. The other eight mutants are “stiff”, with various reductions of tropic responsiveness. Two of them, when compared to previously isolated stiff mutants, show unusually weak responses to light, barriers, and gravity.     

INFLUENCE OF LIGHT OF VERY LOW INTENSITY ON PHOTOTROPIC REACTIONS OF ANIMALS

1. The negative phototropism of certain land isopods was investigated over a large range of intensities, especially low ones. The responses were determined quantitatively by measuring the angle through which an animal turned away from a line perpendicular to the rays of light. 2. In the absence of light the undirected movements set up by obscure stimuli were such as to compensate each other statistically, the average path being a movement in the direction in which the animal was headed. 3. Over a large range of intensities (0.0026 m.c. up) the average turning is maximal, about 55° (Oniscus). This maximal response is due to an anatomical peculiarity, in that the carapace cuts off the light on the eye after the animal has turned 50–60°. This peculiarity probably accounts for specific differences among land isopods. Any light, therefore, which is strong enough to turn an animal through this maximal angle in a radial distance of 10 cm. will give results whose mean will be maximal. 4. Below 0.0026 m.c. the amount of angular deflection becomes less and less, in proportion to the logarithm of the intensity, until at 0.00003 m.c. the movements are the same as in darkness. 5. This proportionality between amount of turning and the logarithm of the intensity indicates the photochemical nature of phototropism on the basis of Hecht''s work with Mya. As a result, Loeb''s theory of phototropism may then be stated in the mathematical form See PDF for Equation in which I ₁ and I ₂ are the two intensities, E ₁ and E ₂, their respective effects, and R, the muscular action set up by the difference in photochemical effect on the two sides.     

System analysis of Phycomyces light-growth response: Single mutants

The white-noise method of system identification has been applied to the transient light-growth response of a set of seven mutants of Phycomyces with abnormal phototropism, affected in genes madA to madG. The Wiener kernels, which represent the input-output relation of the light-growth response, have been evaluated for each of these mutants and the wild-type strain at a log-mean blue-light intensity of 0.1 W m^-2. Additional experiments were done at 3x10^-4 and 10 W m^-2 on the madA strain C21 and wild-type. In the normal intensity range (0.1 W m^-2) the madA mutant behaves similarly to wild-type, but, at high intensity, the madA response is about twice as strong as that of wild-type. Except for C21 (madA), the first-order kernels of all mutants were smaller than the wild-type kernel. The first-order kernels for C111 (madB) and L15 (madC) show a prolonged time course, and C111 has a longer latency. The kernels for C110 (madE), C316 (madF), and C307 (madG) have a shallow and extended negative phase. For C68 (madD), the latency and time course are shorter than in the wild-type. These features are also reflected in the parameters estimated from fits of the anlytical model introduced in the previous paper to the experimental transfer functions (Fourier transforms of the kernels). The kernel for L15 (madC) is described better by a model that lacks one of the two second-order low-pass filters, because its response kinetics are dynamically of lower order.     

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.     

Tropic Responses of Funaria Spores to Red Light

The tropic responses of Funaria hygrometrica spores to continuous illumination with red light (610 to 690 mμ) have been studied over the intensity range from 10^-5 through 10⁺⁶ erg/cm² second, using both plane polarized light and partial illumination with unpolarized light. From the relative frequency of outgrowth origin in different directions, the following is inferred. (1). The germination direction of chloronemal filaments is directly influenced by red light over this whole intensity range, while that of rhizoids tends to be opposite the chloronema. (2) Three photoreceptor systems direct chloronemal primordia: (a) A low intensity system acting from 10^-5 to 10^-0.5 erg/cm² second. It favors their growth from a cell's brightest part(s). Its photoreceptors are disoriented, excited by the electric vector, and probably are dispersed phytochrome molecules. (b) A medium intensity system which acts largely alone only at 10^0.5 erg/cm² second but is influential from 10⁰ to 10⁵ erg/cm² second. It likewise favors growth from a cell's brightest part(s); its receptor molecules are also excited electrically, but they are tangentially oriented. (c) A high intensity system which acts alone from 10⁵ to 10⁶ erg/cm² second and is influential down to 10¹ erg/cm² second. It favors growth of the chloronemas from a cell's darkest part. Its receptors probably are magnetically excited and tangentially oriented. The polarotropic responses of the chloronemas resemble those directing their origins. One new feature is that under intense (10⁶ erg/cm² second) plane polarized and vertically directed light, many soon grow to form tight helices.     

Phycomyces: Phototropism and light-growth response to pulse stimuli

The relationship between phototropism and the light-growth response of Phycomyces blakesleeanus (Burgeff) sporangiophores was investigated. After dark adaptation, stage-IVb sporangiophores were exposed to short pulses of unilateral light at 450 nm wavelength. The sporangiophores show a complex reaction to pulses of 30 s duration: maximal positive bending at 3·10^-4 and 10^-1 J m^-2, but negative bending at 30 J m^-2. The fluence dependence for the light-growth response also is complex, but in a different way than for phototropism; the first maximal response occurs at 1.8·10^-3 J m^-2 with a lesser maximum at 30 J m^-2. A hypertropic mutant, L85 (madH), lacks the negative phototropism at 30 J m^-2 but gives results otherwise similar to the wild type. The reciprocity rule was tested for several combinations of fluence rates and pulse durations that ranged from 1 ms to 30 s. Near the threshold fluence (3·10^-5 J m^-2), both responses increase for pulse durations below 67 ms and both have an optimum at 2 ms. At a fluence of 2.4·10^-3 J m^-2, both responses decrease for pulse durations below 67 ms. The hypertropic mutant (madH), investigated for low fluence only, gave similar results. In both strains, the time courses for phototropism and light-growth response, after single short pulses of various durations, show no clear correlation. These results imply that phototropism cannot be caused by linear superposition of localized light-growth responses; rather, they point to redistribution of growth substances as the cause of phototropism.     

Negative phototropism of rice root and its influencing factors

Some characteristics of the rice (Oryza sativa L.) root were found in the experiment of unilaterally irradiating the roots which were planted in water: (i) All the seminal roots, adventitious roots and their branched roots bent away from light, and their curvatures ranged from 25° to 60°. The curvature of adventitious root of the higher node was often larger than that of the lower node, and even larger than that of the seminal root, (ii) The negative phototropic bending of the rice root was mainly due to the larger growth increment of root-tip cells of the irradiated side compared with that of the shaded side, (iii) Root cap was the site of light perception. If root cap was shaded while the root was irradiated the root showed no negative phototropism, and the root lost the characteristic of negative phototropism when root cap was divested. Rice root could resume the characteristic of negative phototropism when the new root cap grew up, if the original cells of root cap were well protected while root cap was divested, (iv) The growth increment and curvature of rice root were both influenced by light intensity. Within the range of 0–100 μmol · m² -s⁻¹, the increasing of light intensity resulted in the decreasing of the growth increment and the increasing of the curvature of rice root, (v) The growth increment and the curvature reached the maximum at 30°C with the temperature treatment of 10–40°C. (vi) Blue-violet light could prominently induce the negative phototropism of rice root, while red light had no such effect. (vii) The auxin (IAA) in the solution, as a very prominent influencing factor, inhibited the growth, the negative phototropism and the gravitropism of rice root when the concentration of IAA increased. The response of negative phototropism of rice root disappeared when the concentration of IAA was above 10 mg · L⁻¹     

Molecular genetic analysis of phototropism in Arabidopsis

Plant life is strongly dependent on the environment, and plants regulate their growth and development in response to many different environmental stimuli. One of the regulatory mechanisms involved in these responses is phototropism, which allows plants to change their growth direction in response to the location of the light source. Since the study of phototropism by Darwin, many physiological studies of this phenomenon have been published. Recently, molecular genetic analyses of Arabidopsis have begun to shed light on the molecular mechanisms underlying this response system, including phototropin blue light photoreceptors, phototropin signaling components, auxin transporters, auxin action mechanisms and others. This review highlights some of the recent progress that has been made in further elucidating the phototropic response, with particular emphasis on mutant phenotypes.     

Hypocotyl growth orientation in blue light is determined by phytochrome A inhibition of gravitropism and phototropin promotion of phototropism

How developing seedlings integrate gravitropic and phototropic stimuli to determine their direction of growth is poorly understood. In this study we tested whether blue light influences hypocotyl gravitropism in Arabidopsis. Phototropin1 (phot1) triggers phototropism under low fluence rates of blue light but, at least in the dark, has no effect on gravitropism. By analyzing the growth orientation of phototropism-deficient seedlings in response to gravitropic and phototropic stimulations we show that blue light not only triggers phototropism but also represses hypocotyl gravitropism. At low fluence rates of blue light phot1 mutants were agravitropic. In contrast, phyAphot1 double mutants grew exclusively according to gravity demonstrating that phytochrome A (phyA) is necessary to inhibit gravitropism. Analyses of phot1cry1cry2 triple mutants indicate that cryptochromes play a minor role in this response. Thus the optimal growth orientation of hypocotyls is determined by the action of phyA-suppressing gravitropism and the phototropin-triggering phototropism. It has long been known that phytochromes promote phototropism but the mechanism involved is still unknown. Our data show that by inhibiting gravitropism phyA acts as a positive regulator of phototropism.     

High-and low-intensity photosystems in Phycomyces phototropism: Effects of mutations in genes madA,madB, and madC

Sporangiophores of Phycomyces blakesleeanus Burgeff that have been grown in darkness and are then suddenly exposed to unilateral light show a two-step bending response rather than a smooth, monotonic response found in light-adapted specimens (Galland and Lipson, 1987, Proc. Natl. Acad. Sci. USA 84, 104–108). The stepwise bending is controlled by two photosystems optimized for the low-and high-intensity ranges. These two photosystems have now been studied in phototropism mutants with defects in genes madA, madB, and madC. All three mutations raise the threshold of the low-intensity (low-fluence) photosystem by about 10⁶-fold and that of the high-intensity (high-fluence) system by about 10³-fold. Estimates for the light-adaptation time constants of the low-and high-intensity photosystems show that the mutants are affected in adaptation. In the mutants, the light-adaptation kinetics are only slightly affected in the low-intensity photosystem but, for the high-intensity photosystem, the kinetics are considerably slower than in the wild type.Abbreviations WT wild type     

Genetic Analysis of Phototropism of Neurospora crassa Perithecial Beaks Using White Collar and Albino Mutants

Positive phototropism of perithecial beaks in the fungus Neurospora crassa has been demonstrated. The effect was shown to be mediated by blue light. When mutants (white collar-1 and white collar-2) which are blocked in the light induction of enzymes in the carotenoid biosynthetic pathway were used as the protoperithecial parent in crosses, the resulting perithecial beaks did not show a phototropic response. However, when wild type, albino-1, albino-2, or albino-3 strains were used as the protoperithecial parent, phototropism occurred.

The results show that both photoinduced carotenogenesis and phototropism in N. crassa are controlled by the white collar-1 and white collar-2 loci. Thus, the sensory transduction pathways for the two photoresponses must have some steps in common. The results further support the proposal that the white collar strains are regulatory mutants blocked in the light induction process, whereas the albino-1, albino-2, and albino-3 strains can carry out light induction but have the albino phenotype because they are each defective for a different enzyme in the carotenoid biosynthetic pathway.

     

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.     

Roles of Chemosensory Pathways in Transient Changes in Swimming Speed of Rhodobacter sphaeroides Induced by Changes in Photosynthetic Electron Transport

The response of free-swimming Rhodobacter sphaeroides to increases and decreases in the intensity of light of different wavelengths was analyzed. There was a transient (1 to 2 s) increase in swimming speed in response to an increase in light intensity, and there was a similar transient stop when the light intensity decreased. Measurement of changes in membrane potential and the use of electron transport inhibitors showed that the transient increase in swimming speed, following an increase in light intensity, and the stop following its decrease were the result of changes in photosynthetic electron transport. R. sphaeroides has two operons coding for multiple homologs of the enteric chemosensory genes. Mutants in the first chemosensory operon showed wild-type photoresponses. Mutants with the cheA gene of the second operon (cheA_II) deleted, either with or without the first operon present, showed inverted photoresponses, with free-swimming cells stopping on an increase in light intensity and increasing swimming speed on a decrease. These mutants also lacked adaptation. Transposon mutants with mutations in cheA_II, which also reduced expression of downstream genes, however, showed no photoresponses. These results show that (i) free-swimming cells respond to both an increase and a decrease in light intensity (tethered cells only show the stopping on a step down in light intensity), (ii) the signal comes from photosynthetic electron transfer, and (iii) the signal is primarily channelled through the second chemosensory pathway. The different responses shown by the cheA_II deletion and insertion mutants suggest that CheW_II is required for photoresponses, and a third sensory pathway can substitute for CheA_II as long as CheW_II is present. The inverted response suggests that transducers are involved in photoresponses as well as chemotactic responses.     

Light-dependent gravitropism and negative phototropism of inflorescence stems in a dominant Aux/IAA mutant of Arabidopsis thaliana,axr2

Gravitropism and phototropism of the primary inflorescence stems were examined in a dominant Aux/IAA mutant of Arabidopsis, axr2/iaa7, which did not display either tropism in hypocotyls. axr2-1 stems completely lacked gravitropism in the dark but slowly regained it in light condition. Though wild-type stems showed positive phototropism, axr2 stems displayed negative phototropism with essentially the same light fluence-response curve as the wild type (WT). Application of 1-naphthaleneacetic acid-containing lanolin to the stem tips enhanced the positive phototropism of WT, and reduced the negative phototropism of axr2. Decapitation of stems caused a small negative phototropism in WT, but did not affect the negative phototropism of axr2. p-glycoprotein 1 (pgp1) pgp19 double mutants showed no phototropism, while decapitated double mutants exhibited negative phototropism. Expression of auxin-responsive IAA14/SLR, IAA19/MSG2 and SAUR50 genes was reduced in axr2 and pgp1 pgp19 stems relative to that of WT. These suggest that the phototropic response of stem is proportional to the auxin supply from the shoot apex, and that negative phototropism may be a basal response to unilateral blue-light irradiation when the levels of auxin or auxin signaling are reduced to the minimal level in the primary stems. In contrast, all of these treatments reduced or did not affect gravitropism in wild-type or axr2 stems. Tropic responses of the transgenic lines that expressed axr2-1 protein by the endodermis-specific promoter suggest that AXR2-dependent auxin response in the endodermis plays a more crucial role in gravitropism than in phototropism in stems but no significant roles in either tropism in hypocotyls.     

A Common Fluence Threshold for First Positive and Second Positive Phototropism in Arabidopsis thaliana

The relationship between the amount of light and the amount of response for any photobiological process can be based on the number of incident quanta per unit time (fluence rate-response) or on the number of incident quanta during a given period of irradiation (fluence-response). Fluence-response and fluence rate-response relationships have been measured for second positive phototropism by seedlings of Arabidopsis thaliana. The fluence-response relationships exhibit a single limiting threshold at about 0.01 micromole per square meter when measured at fluence rates from 2.4 × 10⁻⁵ to 6.5 × 10⁻³ micromoles per square meter per second. The threshold values in the fluence rateresponse curves decrease with increasing time of irradiation, but show a common fluence threshold at about 0.01 micromole per square meter. These thresholds are the same as the threshold of about 0.01 micromole per square meter measured for first positive phototropism. Based on these data, it is suggested that second positive curvature has a threshold in time of about 10 minutes. Moreover, if the times of irradiation exceed the time threshold, there is a single limiting fluence threshold at about 0.01 micromole per square meter. Thus, the limiting fluence threshold for second positive phototropism is the same as the fluence threshold for first positive phototropism. Based on these data, we suggest that this common fluence threshold for first positive and second positive phototropism is set by a single photoreceptor pigment system.     

Phytochromes A and B mediate red-light-induced positive phototropism in roots

The interaction of tropisms is important in determining the final growth form of the plant body. In roots, gravitropism is the predominant tropistic response, but phototropism also plays a role in the oriented growth of roots in flowering plants. In blue or white light, roots exhibit negative phototropism that is mediated by the phototropin family of photoreceptors. In contrast, red light induces a positive phototropism in Arabidopsis roots. Because this red-light-induced response is weak relative to both gravitropism and negative phototropism, we used a novel device to study phototropism without the complications of a counteracting gravitational stimulus. This device is based on a computer-controlled system using real-time image analysis of root growth and a feedback-regulated rotatable stage. Our data show that this system is useful to study root phototropism in response to red light, because in wild-type roots, the maximal curvature detected with this apparatus is 30 degrees to 40 degrees, compared with 5 degrees to 10 degrees without the feedback system. In positive root phototropism, sensing of red light occurs in the root itself and is not dependent on shoot-derived signals resulting from light perception. Phytochrome (Phy)A and phyB were severely impaired in red-light-induced phototropism, whereas the phyD and phyE mutants were normal in this response. Thus, PHYA and PHYB play a key role in mediating red-light-dependent positive phototropism in roots. Although phytochrome has been shown to mediate phototropism in some lower plant groups, this is one of the few reports indicating a phytochrome-dependent phototropism in flowering plants.     

Cochlear Responses to Condensation and Rarefaction Clicks

Auditory nerve responses to condensation and rarefaction clicks (CC and RC) have been recorded over a wide intensity range with gross electrodes. At low intensities the RC responses are nearly identical to CC responses. At high intensities RC and CC response waveforms are similar, but the latency of the N₁ peak in the RC response is 0.2 msec. shorter than that for the corresponding CC response. At intermediate intensities the RC and CC response waveforms are quite different. These results can be interpreted in terms of a model in which there are two excitatory mechanisms for the neural response, which are operative in different intensity ranges. The cochlear microphonic potential and a “slow” potential are suggested as possible excitatory mechanisms.     

Effect of Light on Uredospore Germination and Germ Tube Growth of Soybean Rust (Phakopsora pachyrhizi Syd.)

The effect of light on uredospore germination and germ tube growth of Phakopsora pachyrhizi was studied. Frequency of uredospore germination was only partially reduced by high light intensity (> 1,9 * 10⁴ mW * m^?2). In uredospores unilaterally irradiated with polychromatic light germ tubes always emerged from the shadowed side. Already developed germ tubes showed a negative phototropic response. Both effects were inducible by low light intensities. Negative phototropism of germ tubes was a blue light effect. Light of 441 nm was more effective than that of 422 nm or 372 nm. Red light (> 600 nm) was ineffective, green light (513 nm) induced medium responses. In half-side illumination studies longitudinal halves of germ tube tips and spores were irradiated under a microscope. The tips of the germ tubes bent into the illuminating beam. In half-side illumination studies germ tubes always emerged from the illuminated spore halves. Under unilateral illumination liquid paraffin reversed this light “polarization” of spores and the negative phototropism of germ tubes. These results suggest that during unilateral illumination spores and germ tube tips act as a lens focussing the light on the wall farthest away from the light source., There, in uredospores emergence of germ tubes is stimulated and in germ tubes growth is inhibited. As a consequence, under unilateral illumination germ tubes emerge at the shadowed side of the spores and grow away from the light.     

Sinusoidal and delta function responses of visual cells of the Limulus eye

Dynamic responses of visual cells of the Limulus eye to stimuli of sinusoids and narrow pulses of light superimposed on a nonzero mean level have been obtained. Amplitudes and phase angles of averaged sinusoidal generator potential are plotted with respect to frequency of intensity modulation for different mean levels of light adaptation. At frequencies above 10 CPS, generator potential amplitudes decrease sharply and phase lag angle increases. At frequencies below 1 CPS, amplitude decreases. A maximum of amplitude in the region of 1 to 2 CPS is apparent with increased mean intensity. The generator potential responses are compared with those of differential equation models. Variation of gain with mean intensity for incremental stimuli is consistent with logarithmic sensitivity of the photoreceptor. Frequency response of the photoreceptor derived from narrow pulses of light predicts the frequency response obtained with sinusoidal stimuli, and the photoreceptor is linear for small signals in the light-adapted state.