Phototropism of Pellia: Evidence for Mediation by Auxin-stimulated Acid Efflux

Flash induced 685 nm fluorescence emission of preilluminated and dark kept Chlamydobotrys stellata has been measured under conditions of CO₂-deprivation. The difference in fluorescence intensity between dark kept and preilluminated cells is taken as a measure for the reduced state of the primary stable electron acceptor of photosystem II, Q, at the given intensity of preillumination. CO₂ removal from growing cultures of this alga for 15 min diminishes photosynthetic electron transport at the oxidizing side of this photosystem. Prolonged CO₂-absence influences also its reducing side. Measurements of flash induced oxygen yields support the conclusion that both sides of photosystem II are affected in the absence of bicarbonate.     

Evidence That Inconsistent Gene Prediction Can Mislead Analysis of Dinoflagellate Genomes

Comparative algal genomics often relies on predicted genes from de novo assembled genomes. However, the artifacts introduced by different gene-prediction approaches, and their impact on comparative genomic analysis remain poorly understood. Here, using available genome data from six dinoflagellate species in the Symbiodiniaceae, we identified methodological biases in the published genes that were predicted using different approaches and putative contaminant sequences in the published genome assemblies. We developed and applied a comprehensive customized workflow to predict genes from these genomes. The observed variation among predicted genes resulting from our workflow agreed with current understanding of phylogenetic relationships among these taxa, whereas the variation among the previously published genes was largely biased by the distinct approaches used in each instance. Importantly, these biases affect the inference of homologous gene families and synteny among genomes, thus impacting biological interpretation of these data. Our results demonstrate that a consistent gene-prediction approach is critical for comparative analysis of dinoflagellate genomes.     

Biochemical and Functional Evidence of p53 Homology Is Inconsistent with Molecular Phylogenetics for Distant Sequences

The tumor suppressor p53 is mutated in ～50% of all human cancer cases worldwide. It is commonly assumed that the phylogenetic history of this important tumor suppressor has been thoroughly studied; however, few detailed studies of the entire extended p53 protein family have been reported, and none comprehensively and simultaneously consider functional, molecular, and phylogenetic data. Herein we examine a diverse collection of reported p53-like protein sequences, including representatives from the arthropods, nematodes, and protists, with the goal of answering several important questions. First, what evidence supports these highly divergent proteins being true homologues to the p53 family? Second, is the inferred overall family phylogeny concordant with known structures and functions? Third, does the extended p53 family possess recognizable conserved sites outside of the within-chordate, highly-conserved DNA-binding domain? Our study shows that the biochemical and functional evidence of p53 homology for nematodes, arthropods, and protists is inconsistent with their implied phylogenetic relationship within the overall family. Although these divergent sequences are always reported as functionally similar to human p53, our results confirm and extend the hypothesis that p63 is a far more appropriate protein for comparison. Within these divergent sequences, we find minimal conservation within the DNA-binding domain, and no conservation elsewhere. Taken together, our findings suggest that these sequences are not bona fide homologues of the extended p53 family and provide baseline criteria for the future identification and characterization of distant p53-family homologues.     

植物根负向光性反应

本文介绍植物根的负向光性反应研究现状和进展。     

水稻根的负负光性（简报）

萌发的水稻种子,用金属丝固定后,排放在透光盛水容器的水面,从单侧方向给种子照光,稻种长出的种子根和次生根均向背光一侧倾斜生长,负向光倾斜角度一般在25°～40°之间,少数可接近45°.感受光的部位和发生倾斜生长的部位均在根尖.根的倾斜生长角度受连续强光促进,而根的生长量则受强光抑制.     

Positive Phototropism in the Thallus of Bryopsis plumosa

Positive phototropism in the thallus of the marine coenocyticgreen alga Bryopsis plumosa was investigated in terms of themode of bending, the photosensitive zone and the effectivenessspectrum. The bending occurred as a consequence of a differencein growth rate between the illuminated and the shaded sidesof the thallus. Elongation on the shaded side was stimulatedwhile that on the illuminated side was inhibited. However, theoverall elongation rate was barely affected. Illumination witha fine beam revealed that a zone from approximately 80 to 120µm below the tip was the most photosensitive. The effectivenessspectrum showed that blue light (<550 nm) was most effective,with light at 467 nm having maximal effectiveness. (Received November 29, 1994; Accepted May 29, 1995)     

Negative Phototropism in the Rhizoid of Bryopsis plumosa

Negative phototropism in the rhizoid of Bryopsis plumosa wasanalyzed. The growth zone of the rhizoid was confined to theapical hemisphere, as is typical of tip growth. Upon unilateralillumination, the rhizoid bent away from the light source witha "bulging" manner. The photoreceptive site for phototropismwas also restricted to the apical hemisphere. The action spectrumfor this negative phototropism was determined from fluence-responsecurves that were obtained after fixing the duration of illuminationat 60 min and varying the fluence rate between 0.1 and 3.0 Wm^-2. The action spectrum had a large peak at 467 nm and smallerpeaks at 378 and 414 nm, resembling the action spectra of certainother blue-light responses. (Received November 29, 1995; Accepted May 29, 1995)     

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.     

Phototropism of maize coleoptiles Influences of light gradients

The lateral fluence-rate gradients in unilaterally irradiated maize (Zea mays L.) coleoptiles were calculated on the basis of the proportions of P _fr (far-red-absorbing form of phytochrome) measured spectroscopically in transverse slices of the coleoptiles (top 1 cm). The results showed the occurrence of significant gradients that are wavelength-dependent. The gradient at 449 nm was steeper than those measured at 516, 534 and 551 nm, which were steeper than that measured at 665 nm. The ratios between the sides proximal and distal to the light source were, for example, 1:0.12 (449 nm), 1:0.23 (534 nm), and 1:0.28 (665 nm). Fluence-response curves for coleoptile phototropism (first positive curvature produced by less than 100 s unilateral irradiation) were measured at 449, 516, 534 and 551 nm. Comparison of the threshold fluences indicated that the responsiveness to 551 nm is about 10^4.8 less than that to 449 nm. Increasing wavelengths led to a decrease in maximal curvature, which correlated with the decrease of the fluence-rate ratios between the proximal and distal sides. Phototropic fluence-response curves were also measured using bilateral irradiation (449 nm). In one set of experiments, the fluence ratio was kept constant (either 1:1/2, 1:1/4 or 1:1/16) and the total fluence was varied, and in the other set the fluence applied to one side was kept constant and the fluence ratio was varied. A simple model based on the assumption that only one photoreaction occurs, and that the response is a function of the difference between the proximal and distal sides in the local photoreceptor action was tested. A fluence-response curve for this local photoreceptor action was calculated based on the fluence-rate ratio and the phototropic fluence-response curve measured for 449 nm. This curve was used, in conjunction with the measured fluence-rate ratios, as a basis for calculating phototropic fluence-response curves for other wavelengths and those for 449 nm obtained with bilateral irradiation. The calculated fluence-response curves showed excellent agreement with the experimental data. It is concluded that the threshold for maize coleoptile phototropism reflects the apparent photoconversion cross-section of the blue-light receptor whereas the maximal curvature depends on the steepness of the light gradient across the coleoptile.Abbreviations and symbols I(x) fluence rate at the depth x - P _fr phytochrome (far-red absorbing) - P _r phytochrome (red absorbing) - P _tot total phytochrome (P _r+P _fr) - photoconversion cross-section     

Characterization of Adaptation in Phototropism of Arabidopsis thaliana

Phototropic curvature has been measured for etiolated Arabidopsis thaliana seedlings with and without a preirradiation. A bilateral preirradiation with 450-nm light at a fluence greater than about 0.1 micromole per square meter causes a rapid desensitization to a subsequent 450-nanometer unilateral irradiation at 0.5 micromole per square meter. Following a refractory period, the capacity to respond phototropically recovers to the predesensitization level, and the response is then enhanced. The length of the refractory period is between 10 and 20 minutes. Both the time needed for recovery and the extent of enhancement increase with increasing fluence of the bilateral preirradiation. Based on the relative spectral sensitivities of desensitization and enhancement, these responses can be separated. Desensitization is induced by blue light but not by red light. Enhancement, however, is induced by both blue and red light. Thus, enhancement can be induced without desensitization but not vice versa. Both desensitization and enhancement affect only the magnitude of the response and do not affect the fluence threshold.     

Phototropism, Adaptation, and the Light-Growth Response of Phycomyces

Phototropic bending can be initiated without the transient changes in growth speed that characterize a light-growth response. The conditions required are a change from a symmetric to an asymmetric illumination pattern while the cell receives a constant radiant flux. Phototropism is thus basically a steady state process. It cannot be founded on differential light-growth responses as in Blaauw's theory. A possible model system for the unequal partition of growth during steady bending is discussed. The fact that light-growth responses show adaptation while phototropic bending does not follows from the different natures of the two responses.     

Action Spectrum in the Ultraviolet Region for Phototropism of Bryopsis Rhizoids

The phototropic response of the rhizoid of the marine coenocyticgreen alga Bryopsis plumosa to ultraviolet light (250–350nm) was investigated. The rhizoid exhibited negative bendingthat was due to bulging upon absorption of light in the UV region,as well as in the visible region, of the spectrum. The negativebending might not be a result of the inhibition of growth onthe irradiated side of the apical hemisphere by UV irradiationbecause growth inhibition was observed after bending had reacheda maximum within one to two hours. The action spectrum obtainedfrom fluence rate-response curves had a pronounced peak at 260nm and a small peak at 310 nm. The quantum effectiveness at260 nm was about five times that in the visible region. Phenylaceticacid (PAA), a potent inhibitor of flavin photoreactions, inhibitedthe phototropic response to both UV light and blue light withoutany obvious effect on tip growth. The inhibition of the phototropicresponse to blue light by PAA was partially overcome by rinsingthe alga with riboflavin-containing medium, which result suggeststhe involvement of flavins in the phototropism of Bryopsis rhizoids. (Received February 6, 1995; Accepted June 19, 1995)     

Phototropism and gravitropism in lateral roots of Arabidopsis

Gravitropism and, to a lesser extent, phototropism have been characterized in primary roots, but little is known about structural/functional aspects of these tropisms in lateral roots. Therefore, in this study, we report on tropistic responses in lateral roots of Arabidopsis thaliana. Lateral roots initially are plagiogravitropic, but when they reach a length of approximately 10 mm, these roots grow downward and exhibit positive orthogravitropism. Light and electron microscopic studies demonstrate a correlation between positive gravitropism and development of columella cells with large, sedimented amyloplasts in wild-type plants. Lateral roots display negative phototropism in response to white and blue light and positive phototropism in response to red light. As is the case with primary roots, the photoresponse is weak relative to the graviresponse, but phototropism is readily apparent in starchless mutant plants, which are impaired in gravitropism. To our knowledge, this is the first report of phototropism of lateral roots in any plant species.     

高等植物的向光性信号转导

对高等植物向光性信号转导的最新研究进展进行了综述,并对向光素的性质、结构和可能的作用方式及其下游可能的信号传递途径作了分析讨论.     

Diurnal Phototropism in Solar Tracking Leaves of Lavatera cretica

On a clear day, leaf laminas of Lavatera cretica tracked the solar position throughout the day. The laminar azimuth did not diverge from the solar azimuth by more than 12° from sunrise to sunset. Tracking of the solar elevation started 1 to 2 hours after sunrise and ceased 1 to 2 hours before sunset. On an overcast day, the laminas reoriented horizontally. After sunset, following a clear day, the laminas performed a nocturnal reorientation, with three well defined phases. During the initial phase the laminas relaxed their strained sunset-facing orientation to one perpendicular to their petioles. This equilibrium configuration was maintained throughout the following phase, which was apparently concerned with time-measuring. During the final phase, the laminas reoriented, before sunrise, to a position facing the direction of the anticipated sunrise. This directional information is phototropic and was retained for 3 to 4 diurnal cycles, probably in the pulvinus itself, which is the site of the response. Laminas of plants transferred from sunlight either to darkness, or to a simulated natural photoperiod under overhead illumination, were facing the originally anticipated direction of sunrise at the time of each of the three to four subsequent sunrises (after which they reverted to the dark orientation in darkness, or to the horizontal one with overhead illumination). Cotyledonary laminas required directional information for the nocturnal reorientation during 3 or 4 cycles of simulated sunrise to sunset transitions.     

向光素调节植物向光性及其与光敏色素/隐花色素的相互关系

蓝光受体向光素(PHOT1/PHOT2)调节蓝光诱导的植物运动反应, 包括植物向光性、叶绿体运动、气孔运动和叶片伸展等。其中, 向光素介导的植物向光性能够促使植物弯向光源, 确保其以最佳取向捕获光源, 优化光合作用。光敏色素和隐花色素作为光受体也参与植物的向光性调节。该文综述了向光素介导的拟南芥(Arabidopsis thaliana)下胚轴向光弯曲信号转导及其与光敏色素、隐花色素协同作用的分子机制, 以期为改造植物光捕获能力及提高光利用效率提供理论基础。     

Growth Distribution during Phototropism of Arabidopsis thaliana Seedlings

The elongation rates of two opposite sides of hypocotyls of Arabidopsis thaliana seedlings were measured during phototropism by using an infrared imaging system. In first positive phototropism, second positive phototropism, and red light-enhanced first positive phototropism, curvature toward the light source was the result of an increase in the rate of elongation of the shaded side and a decrease in the rate of elongation of the lighted side of the seedlings. The phase of straightening that followed maximum curvature resulted from a decrease in the elongation rate of the shaded side and an increase in the elongation rate of the lighted side. These data for the three types of blue light-induced phototropism tested in this study and for the phase of straightening are all clearly consistent with the growth rate changes predicted by the Cholodny-Went theory.