Effect of xenon on the excited States of phototropic receptor flavin in corn seedlings

The chemically inert, water-soluble heavy atom gas, xenon, at millimolar concentrations specifically quenches the triplet excited state of flavin in solution without quenching the flavin singlet excited state. The preferential quenching of the flavin triplet over the singlet excited state by Xe has been established by showing that the flavin triplet-sensitized photooxidation of NADH is inhibited while the fluorescence intensity and lifetime of flavin are not affected by Xe.     

A single positive phototropic response induced with pulsed light in hypocotyls of Arabidopsis thaliana seedlings

The fluence-response curves were measured for phototropic curvature in response to unilateral 450-nm light in hypocotyls of Arabidopsis thaliana (L.) Heynh. These show the classical first positive (peak curvature of 9–10°), indifferent and second positive phototropic response. Reciprocity is valid only for the first positive response; the fluence requirements for its induction are similar to those for induction of the first positive phototropic response of coleoptiles. Large angles of curvature also may be induced by multiple pulses if the individual pulses are separated by an optimum dark period of about 15 min. The curvature induced by a given fluence, whether applied in continuous irradiation or a sequence of pulses, is a linear function of the duration of continuous irradiation or the duration between first and last pulse, respectively. For a given fluence applied in a sequence of pulses, reciprocity remains valid provided the duration between first and last exposure is kept constant. When the duration between first and last pulse is sufficiently long, the fluence required for high phototropic curvature falls in the first positive fluence range. These results are interpreted to indicate the existence of a kinetic limitation in the transduction sequence, and a relatively short lifetime of an initial physiologically active photoproduct. The apparent existence of more than one positive response may have resulted from these characteristics of the transduction sequence.     

The effects of temperature and light on the carotenoids of seedlings grown from three corn hybrids

Seedlings of three corn hybrids, U. S. 13, Indiana 252, and Indiana 909, were subjected to eight different sets of environmental conditions, and the relative proportions of lutein, β-carotene, violaxanthin, neoxanthin, and zeaxanthin were determined. No significant differences occurred between hybrids, but markedly significant differences were observed between environmental conditions. High light intensity and low temperatures tended to favor the accumulation of zeaxanthin. Changes in this pigment were often approximately equal and opposite to simultaneous changes in β-carotene. This suggested that these two pigments were either interconvertible or formed from a common precursor. Similar interrelationships were observed between lutein and violaxanthin. The results obtained support the interesting possibility that the carotenoids are chemically involved in metabolic functions in green leaves. It is suggested that the techniques developed provide a useful approach for future investigations on the physiological role of the carotenoids in leaves.     

Effect of red light on the phototropic sensitivity of corn coleoptiles

The effect of red light in alteration of the phototropic sensitivity of corn coleoptiles (Zea mays L., cultivar Burpee Barbecue Hybrid) is investigated. Phototropic dosage-response curves for etiolated coleoptiles are compared with those for coleoptiles receiving 1 hour of continuous red light immediately prior to phototropic induction. In the former case, only curvature comparable to the first positive curvature of oat coleoptiles is obtained. There is no evidence for first negative curvature and only minimal second positive curvature. The reciprocity law proved valid for all curvatures obtained. With red light, the sensitivity of the first positive curvature was decreased over ten-fold and there was clear appearance of second positive curvature for which the reciprocity law was not valid. Once again there was no evidence for negative curvature. Time course studies indicated that within 1 hour of the beginning of red light treatment at 25°, reactions leading to the decrease in phototropic sensitivity of the first positive component had gone to completion whether the red light was continuous or consisted of a single 1 second exposure followed by a 1 hour dark period. An action spectrum for the red-induced change in phototropic sensitivity showed a marked peak near 660 mμ with a small broad shoulder between 610 and 630 mμ, characteristic of phytochrome-mediated responses. The effect of red light could be fully reversed by low dosages of far-red light, but longer doses of far red were less effective. Large dosages of far-red light alone induced the same alteration in phototropic sensitivity as did red light.     

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.     

Second positive phototropic response patterns of the oat coleoptile

Summary 1.During second positive irradiation, bending increases steadily with time. Under optimal conditions, the lag between onset of illumination and beginning of parabolic bending behavior is about 3 min. — 2. Shortly after irradiation ceases, bending becomes linear with time. On a clinostat, bending continues for about 2.5 hr. Auxanometric measurements show that the ultimate cessation of bending is not due to failing growth rate. — 3. The second positive response shows a striking dependence on intensity of irradiation. Inactivation occurs when irradiation approaches the intensity of full daylight. — 4. Induction is linear with duration of illumination, both at purely activating intensities and at partially inactivating intensities. — 5. Induction at 2°, while somewhat slower than at 25°, retains linear dependence on exposure duration. This suggests that the reactions immediately following light reception are slowed but not stopped at low temperature. — 6. Growth, which drops to about 0.5 /min at 2°, resumes at about 18 min^-1 as soon as plants are warmed to 25°. Curvature does not seem to begin for about 10 min. Combined with information about lag time for primary auxin action, this suggests that lateral auxin transport, as well as growth, is strongly inhibited at near-freezing temperatures. — 7. The induced transport system is highly stable at 2°. — 8. Under optimal conditions, the lag between onset of irradiation and induction of capacity to produce measurable curvature is only a few seconds. The length of the lag is dependent on the rate of induction. The lag is thought to be due to the requirement that enough induction be accumulated to overcome resistance of the coleoptile. — 9. Induction is dependent on the gradient of light across the coleoptile, whether measured for purely activating or partially inactivating intensities. The light received is probably integrated either across individual cells or across the entire width of the coleoptile.     

Gravitational compensation and the phototropic response of oat coleoptiles

Avena seedlings were germinated and grown while continuously rotated on the horizontal axis of a clinostat. The coleoptiles of these gravity-compensated plants were phototropically more responsive than those of plants rotated on a vertical axis. When the plants were compensated after unilateral irradiation, phototropic curvature of the shoot progressed for the next 6 hours, with the rate of curving decreasing about 3 hours after irradiation. The decrease in rate was less in the plants gravity-compensated before irradiation than in those vertically rotated. In the period 70 to 76 hours after planting, the growth rate of the compensated coleoptiles was significantly less than that of the vertically rotated seedlings. The greater phototropic curvature, the decreased growth rate, and the slower rate of straightening of the curved, compensated shoot can be correlated with several consequences of compensation: an increase in sensitivity to auxin, a lowering of auxin content in the coleoptile tip, and possibly, from an interaction between compensation and phototropic stimulation, an enhanced difference in auxin transport between the illuminated and shaded halves of the unilaterally irradiated shoot.

The phototropic response of the vertically rotated seedling was significantly different from that of the vertical stationary, indicating the importance of vertically rotated controls in clinostat experiments.

     

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.     

High pigment1 mutation negatively regulates phototropic signal transduction in tomato seedlings

Phototropins and phytochromes are the major photosensory receptors in plants and they regulate distinct photomorphogenic responses. The molecular mechanisms underlying functional interactions of phototropins and phytochromes remain largely unclear. We show that the tomato (Lycopersicon esculentum) phytochrome A deficient mutant fri lacks phototropic curvature to low fluence blue light, indicating requirement for phytochrome A for expression of phototropic response. The hp1 mutant that exhibits hypersensitive responses to blue light and red light reverses the impairment of second-positive phototropic response in tomato in phytochrome A-deficient background. Physiological analyses indicate that HP1 functions as a negative regulator of phototropic signal transduction pathway, which is removed via action of phytochrome A. The loss of HP1 gene product in frihp1 double mutant allows the unhindered operation of phototropic signal transduction chain, obviating the need for the phytochrome action. Our results also indicate that the role of phytochrome in regulating phototropism is restricted to low fluence blue light only, and at high fluence blue light, the phytochrome A-deficient fri mutant shows the normal phototropic response.     

S-adenosylmethionine decarboxylase of corn seedlings

S-Adenosylmethionine decarboxylase (EC 4.1.1.50) has been purified 500-fold in 30% yield from the extract of etiolated corn seedlings (cv. Golden Crossbantam Bell). This preparation had a molecular weight of approximately 25,000. The K_m value was 5 micromolar for S-adenosylmethionine. Methylglyoxal bis(guanylhydrazone), hydroxylamine, and sulfhydryl reagents (such as p-hydroxymercuriphenylsulfonate and N-ethylmaleimide) were effective inhibitors of this enzyme. Germination of corn seed was accompanied by a rapid increase in enzyme activity and maximum activity occurred in 5-day-old seedlings.     

Specific inhibition of phototropism in corn seedlings

Geotropism was used as a control for the specificity of potential inhibitors of phototropism by the coleoptiles of corn (Zea mays) seedlings. The compounds tested fall into three categories showing: (a) no inhibition of either phototropism or geotropism (KCl); (b) nonspecific inhibition of both phototropism and geotropism (KCN); and (c) specific inhibition of phototropism (KI, NaN₃, and phenylacetic acid). Simultaneous irradiation of coleoptiles with phototropically inert light in addition to the phototropically active blue light also results in an inhibition of phototropism. Since azide, iodide, and phenylacetic acid are known to interact with flavins while a simultaneous irradiation with a phototropically inert light may depopulate the first triplet state of flavins, these data support the hypothesis that the photoreceptor pigment for phototropism in corn is a flavin.     

RPT2. A signal transducer of the phototropic response in Arabidopsis

The blue light receptor NPH1 (for nonphototropic hypocotyl) has been considered to be the only UV-A/blue light receptor that induces a phototropic response by the hypocotyl and root of Arabidopsis. By analysis of root phototropism (rpt) mutants, we show, however, the involvement of another blue light receptor as well as the existence of two separate signaling pathways working downstream of these receptors in the phototropic response. A newly isolated gene, RPT2, controls one of these pathways. The RPT2 gene is light inducible; encodes a novel protein with putative phosphorylation sites, a nuclear localization signal, a BTB/POZ domain, and a coiled-coil domain; and belongs to a large gene family that includes the recently isolated NPH3 gene. From genetic, physiological, and biochemical evidence, we propose a genetic model of the signaling pathways that induce the phototropic response in Arabidopsis.     

Red light enhancement of the phototropic response of etiolated pea stems

In the subapical third internode of 7-day-old etiolated pea seedlings, the magnitude of phototropic curvature in response to continuous unilateral blue illumination is increased when seedlings are pre-exposed to brief red light. The effect of red light on blue light-induced phototropism becomes manifest maximally 4 or more hours after red illumination, and closely parallels the promotive action of red light on the elongation of the subapical cells. Ethylene inhibits phototropic curvature by an inhibitory action on cell elongation without affecting the lateral transport of auxin. Pretreatment of seedlings with gibberellic acid causes increased phototropic curvature, but experiments using ¹⁴C-gibberellic acid indicate that gibberellic acid itself is not laterally transported under phototropic stimuli. Neither red light nor gibberellic acid treatment has any promotive effect on blue light-induced lateral transport of ³H-indoleacetic acid. Under conditions where phototropic curvature is increased by red light treatment, low concentrations of indoleacetic acid applied in lanolin paste to the apical cut end of the seedling cause an increased elongation response in subapical tissue. This could explain increased phototropic curvature caused by red light treatment.     

A new gene (madI) involved in the phototropic response of Phycomyces

Summary Only eight genes are known to be involved in the phototropic response of Phycomyces (madA-H). Mutants affected in these genes have played a major role in the analysis of photosensory transduction processes in this system. A set of new mutants isolated by Alvarez et al. (1989) that are unable to bend towards dim unilateral blue light were studied by complementation and recombination. Two of these mutants have mutations in madE, one has a mutation in madF and one is a double madE madF mutant. The three remaining mutants tested did not complement each other and showed positive complementation with strains carrying mutations in the genes madA, madB, and madC, indicating that they carried mutations in a new gene designated madI. Recombination analysis showed that madI is unlinked to madA, madB and madC.     

Lysosomes of root tip cells in corn seedlings

Summary Nine acid hydrolases are present in lysosomes which are found in the mitochondrial fraction of a cell-free extract prepared from root tips of corn seedlings.Light and heavy lysosomes can be distinguished. The latter are sedimentable in a sucrose-medium, the former only in sorbitol-medium. The fraction of heavy lysosomes is in turn composed of at least three populations of lysosomes differing in density and enzyme content.Light lysosomes are membrane-bound particles with diameters from 0.3 to 1.5 . Electron micrographs of frozen-etched tissue and isolated particles provide evidence that light lysosomes are identical with small vacuoles. This type of lysosome is characterized by presence of transaminases in addition to that of hydrolases. Heavy lysosomes are small spheres (diameters from 0.1–0.3 ) with membranes resembling those of vacuoles and of the endoplasmic reticulum. These lysosomes are characterized by high specific activities of two oxydoreductases known to occur also in the membranes of the reticulum.The different types of particles are thought to represent stages of the development of the lysosomal apparatus; according to this hypothesis the large vacuole of parenchymatous cells represents the end product of this process.     

Nitrate translocation by detopped corn seedlings

Five-day-old seedlings of corn (Zealpha mays L.) grown without nitrate were decapitated and exposed to 0.5 mm KNO(3) or 0.5 mm KCl in aerated solutions at 30 C. Uptake of nitrate, chloride, and potassium was determined by replacing solutions hourly and measuring their depletion. Translocation of these ions and of organic nitrogen was determined by hourly analysis of the vascular exudate. Nitrate reduction was estimated by the difference between nitrate uptake and nitrate recovered in the tissue and exudate. Nitrate uptake exhibited its usual pattern of apparent induction resulting in the development of an accelerated uptake phase. Chloride uptake remained fairly constant throughout the experimental period. Translocation of nitrate increased progressively for at least 7 hours whereas chloride translocation reached a maximum about the 3d hour and then declined to a lower rate than nitrate translocation. Nitrate uptake and translocation were restricted by anaerobiosis, by 20 and 40 C relative to 30 C, and by 0.05 mm 6-methylpurine, an RNA-synthesis inhibitor. Accumulation, reduction and translocation of nitrate had different sensitivities to all these factors. The effect of 0.05 mm 6-methylpurine was more detrimental to nitrate translocation and nitrate reduction than to nitrate uptake.Ambient nitrate, relative to chloride, enhanced the exudation volume and the translocation of organic nitrogen within 4 hours from initiation of the experiments. Translocation of nitrate and organic nitrogen decreased shortly after removal of external nitrate. The higher rates of organic nitrogen translocation which occurred during nitrate uptake indicates either (a) rapid translocation of amino acids synthesized from the entering nitrate, or (b) an accelerated rate of protein turnover and a resulting enhancement in translocation of endogenous amino acids.