On the relation between photo- and gravitropically induced spatial memory in maize coleoptiles

The interaction of photo- and gravitropic stimulation was studied by analysing the curvature of maize (Zea mays L.) coleoptiles subjected to rotation on horizontal clinostats. Gravitropic curvature in different directions with respect to the stimulation plane was found to be transient. This instability was caused by an increasing deviation of response direction from the stimulation plane towards the caryopsis. The bending angle as such, however, increased steadily. This reorientation of the gravitropic response towards the caryopsis is thought to be caused by the clinostat-elicited nastic curvature found in maize coleoptiles. In contrast, the response to phototropic stimulation was stable, in both, orientation and curving. Although stimulation by gravity was not capable of inducing a stable tropistic response, it could inhibit the response to opposing phototropic stimulation, if the counterstimulation was given more than 90 min after the onset of gravistimulation. For shorter time intervals the influence of the phototropic stimulus obscured the response to the first, gravitropic stimulation. For time intervals exceeding 90 min, however, the phototropic effects disappeared and the response was identical to that for gravity stimulation alone. This gravity-induced inhibition of the phototropic response was confined to the plane of gravity stimulation, because a phototropic stimulation in the perpendicular direction remained unaffected, irrespective of the time interval between the stimulations. This concerned not only the stable phototropic curving, but also the capacity of the phototropic induction to elicit a stable directional memory as described earlier (P. Nick and F. Schäfer, 1988b, Planta 175, 380–388). This was tested by a second bluelight pulse opposing the first. It is suggested that gravity, too, can induce a directional memory differing from the blue-light elicited memory. The mechanisms mediating gravi- and phototropic directional memories are thought to branch off the respective tropistic signal chains at a stage where photo- and gravitropic transduction are still separate.This work was supported by the Deutsche Forschungsgemeinschaft and a grant of the Studienstiftung des Deutschen Volkes to P. Nick.     

Desensitization by red and blue light of phototropism in maize coleoptiles

The effects of pretreatments with red and blue light (RL, BL) on the fluence-response curve for the phototropism induced by a BL pulse (first positive curvature) were investigated with darkadapted maize (Zea mays L.) coleoptiles. A pulse of RL, giving a fluence sufficient to saturate phytochrome-mediated responses in this material, shifted the bell-shaped phototropic fluence-response curve to higher fluences and increased its peak height. A pulse of high-fluence BL given immediately prior to this RL treatment temporarily suppressed the phototropic fluence-response curve, and shifted the curve to higher fluences than induced by RL alone. The shift by BL progressed rapidly compared to that by RL. The results indicate (1) that first positive curvature is desensitized by both phytochrome and a BL system, (2) that desensitization by BL occurs with respect to both the maximal response and the quantum efficiency, and (3) that the desensitization responses mediated by phytochrome and the BL system can be induced simultaneously but develop following different kinetics. It is suggested that theses desensitization responses contribute to the induction of second positive curvature, a response induced by prolonged irradiation.Abbreviations BL blue light - RL red light CIW-DPB Publication No. 1001     

Relationships between xanthoxin,phototropism, and elongation growth in the sunflower seedling Helianthus annuus L.

For phototropic curvature of a green sunflower seedling, only the hypocotyl has to be illuminated; the tip and cotyledons are not involved in stimulus perception. The etiolated seedling is phototropically insensitive, illumination of only the hypocotyl renders it sensitive. It is concluded that the photoreceptor is located within the responding organ. In curving seedlings, the endogenous indoleacetic acid (IAA) remains evenly distributed. However, the inhibitor, xanthoxin (Xa), accumulates on the illuminated side. The degree of phototropic response is generally related to the concentration of Xa. The amount of phototropic curvature is independent of the rate of elongation growth, the former can be changed without affecting the latter, and vice versa. The data conflict with the Cholodny-Went theory, whereas they support the hypothesis of Blaauw that the phototropic reaction is caused by the local accumulation of a growth-inhibiting substance on the irradiated side.Abbreviations CCC chlormequat, (2-chloroethyl)trimethylammonium chloride - GA₃ gibberellic acid - IAA indole-3-acetic acid - Xa xanthoxin     

Suppression of gravitropic response of primary roots by submergence

Primary roots of six plant species were placed horizontally either in humid air or under water, and their growth and gravitropic responses were examined. In air, all the roots showed a normal gravitropic curvature. Under water without aeration, roots of rice (Oryza sativa L.), oat (Avena sativa L.), azuki bean (Vigna angularis Ohwi et Ohashi), and cress (Lepidium sativum L.) curved downward at almost same rate as in air, whereas the curvature of roots of maize (Zea mays L.) and pea (Pisum sativum L.) was strongly suppressed. Submergence did not cause a decrease in growth rate of these roots. When roots of maize and pea were placed horizontally under water without aeration and then rotated in three dimensions on a clinostat in air, they showed a significant curvature, suggesting that the step suppressed by submergence is not graviperception but the subsequent signal transmission or differential growth process. Constant bubbling of air through the water partly restored the gravitropic curvature of maize roots and completely restored that of pea roots. The curvature of pea roots was also partly restored by the addition of an inhibitor of ethylene biosynthesis, aminooxyacetic acid. In air, ethylene suppressed the gravitropic curvature of roots of maize and pea. Furthermore, the level of ethylene in the intercellular space of the roots was increased by submergence. These results suggest that the accumulation of ethylene in the tissue is at least partly involved in suppression of transmission of the gravity signal or of differential growth in maize and pea roots under conditions of submergence.Abbreviations AOA aminooxyacetic acid - 3-D three-dimensional Dedicated to Professor Andreas Sievers on the occasion of his retirementWe thank Professor H. Suge and Drs. H. Takahashi and H. Kataoka, Tohoku University and Dr. T. Suzuki, Yamagata University, for helpful suggestions. The present study was supported in part by a Grant for Basic Research in Space Station Utilization from the Institute of Space and Astronautical Science, Japan.     

Phytochrome-mediated phototropism in maize seedling shoots

Unilateral irradiation with red light (R) or blue light (BL) elicits positive curvature of the mesocotyl of maize (Zea mays L.) seedlings raised under R for 2 d from sowing and kept in the dark for 1 d prior to curvature induction. The fluenceresponse curve for R-induced mesocotyl curvature, obtained by measuring curvature 100 min after phototropic induction, shows peaks in two fluence ranges, designated first positive range (from the threshold to the trough), and second positive range (above the trough). The fluence-response curve for BL is similar to that for R but shifted two orders of magnitude to higher fluences. Blue light elicits the classical first positive curvature of the coleoptile, whereas this response is not found with R. Positive mesocotyl curvature induced by either R or BL is eliminated by R given from above just before the unilateral irradiation, whereas BL-induced coleoptile curvature is not eliminated. The above results collectively offer evidence that phototropic curvature of the mesocotyl is induced by R-sensitive photosystem(s). Mesocotyl curvature in the second positive range is reduced by vertical far-red light (FR) applied after phototropic induction with R, but is not affected by FR applied before R. Unilateral irradiation with FR following vertical irradiation with a high R fluence leads to negative curvature of the mesocotyl. It is concluded that mesocotyl curvature in the second positive range results from a gradient in the amount of the FR-absorbing form of phytochrome (Pfr) established across the plant axis. Mesocotyl curvature in the first positive range is inhibited by vertical FR given either before or after phototropic induction with R. Since the FR used here is likely to produce more Pfr than the very low fluences of R eliciting the mesocotyl curvature in the first positive range, it is assumed that FR reduces the response in this case by adding Pfr at both sides of the plant axis. By rotating seedlings on a clinostat with its axis horizontal, the kinetics of mesocotyl curvature can be studied in the absence of a counteracting gravitropic response. On the clinostat, the R-induced mesocotyl curvature develops after a lag, through two successive phases having different curvature rates, the late phase is slower than the early phase. Negative curvature of the coleoptile can be induced by either R or BL; the BL-induced negative curvature is found at fluences higher than those giving positive curvature. The clinostat experiments show that the negative coleoptile curvature induced by either R or BL is a gravitropic compensation for positive mesocotyl curvature.Abbreviations BL blue light - FR far-red light - Pfr phytochrome in the far-red-absorbing form - Pr phytochrome in the red-absorbing form - R red light C.I.W.-D.P.B. Publication No. 824     

Induction of beta-glucosidase activity in maize coleoptiles by blue light illumination

The role of beta-glucosidase during the phototropic response in maize (Zea mays) coleoptiles was investigated. Unilateral blue light illumination abruptly up-regulated the activity of beta-glucosidase in the illuminated halves, 10 min after the onset of illumination, peaking after 30 min and decreasing thereafter. The level of 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA), which is released from DIMBOA glucoside (DIMBOA-Glc) by beta-glucosidase, and its degradation compound 6-methoxy-benzoxazolinone (MBOA) were elevated within 30 min in the illuminated halves as compare to the shaded halves, prior to the phototropic curvature. Furthermore, beta-glucosidase inhibitor treatment significantly decreased the phototropic curvature and decreased growth suppression in the illuminated sides. These results suggest that blue light induces the activity of beta-glucosidase in the illuminated halves of coleoptiles causing an increase in DIMBOA biosynthesis and the growth inhibition that leads to a phototropic curvature.     

Interaction of gravi- and phototropic stimulation in the response of maize (Zea mays L.) coleoptiles

The influence of gravitropic stimulation upon blue-light-induced first positive phototropism for stimulations in the same (light source and center of gravity opposite to each other) and in opposing directions was investigated in maize cole-optiles by measuring fluence-response patterns. As a result of gravitropic counterstimulation, phototropic bending was transient with maximum curvature occurring 100 min after stimulation. On a horizontal clinostat, however, the seedlings curved for 20 h. Gravistimulation in the opposite direction acted additively upon blue-light curvature. Gravistimulation in the same direction as phototropic stimulation produced a complex behaviour deviating from simple additivity. This pattern can be explained by a gravitropically mediated sensitization of the phototropic reaction, an optimal dependence of differential growth on the sum of photo-and gravistimulation, and blue-light-induced inhibition of gravitropic curvature at high fluences. These findings indicate that several steps of photo-and gravitransduction are separate. Preirradiation with red light desensitized the system independently of applied gravity-treatment, indicating that the site of red-light interaction is common to both transduction chains.Abbreviations BL blue light - G⁺ stimulation by light and gravity in the same direction (i.e. light source and center of gravity opposite to each other) - G^- stimulation by light and gravity in opposing directions     

Studies on the growth and development in Spirogyra

The phototropic response of Spirogyra sp. filaments and its relation with the different phases of their diurnal movements were studied. The filaments rapidly responded to unilateral irradiation; curvature towards the light source began in their tip region but shifted down to more basal regions. However, this typical and steady phototropic curvature could be observed only in the GnSt phase of the diurnal movement. In the other phases competitive states between the phototropic movement and other kinds of movement were evident. Thus, from the results of our previous and present studies it is proposed that the diurnal movement of Spirogyra filaments is composed of various individual movements, including a phototropic one; among these movements there exists a certain balance determined by the culture conditions and the time of day, and the phototropic movement is relatively inferior to the other movements.Abbreviations Gn negative gravitropism - Gp positive gravitropism - Sh shrinking (spiralling) growth - St straight form of growth V=Tanaka et al. (1983)     

Inhibition of polar calcium movement and gravitropism in roots treated with auxin-transport inhibitors

Primary roots of maize (Zea mays L.) and pea (Pisum sativum L.) exhibit strong positive gravitropism. In both species, gravistimulation induces polar movement of calcium across the root tip from the upper side to the lower side. Roots of onion (Allium cepa L.) are not responsive to gravity and gravistimulation induces little or no polar movement of calcium across the root tip. Treatment of maize or pea roots with inhibitors of auxin transport (morphactin, naphthylphthalamic acid, 2,3,5-triiodobenzoic acid) prevents both gravitropism and gravity-induced polar movement of calcium across the root tip. The results indicate that calcium movement and auxin movement are closely linked in roots and that gravity-induced redistribution of calcium across the root cap may play an important role in the development of gravitropic curvature.Abbreviations 9-HFCA 9-hydroxyfluorenecarboxylic acid - NPA naphthylphthalamic acid - TIBA 2,3,5-triiodobenzoic acid - IAA indole-3-acetic acid     

ATP production after pH shift in membrane vesicles from maize coleoptiles

Microsomal membrane vesicles and purified plasma membranevesicles obtained from coleoptiles of maize (Zea mays L.) weresubjected to pH shifts from pH 7.8 to 4.7. In the presence of ATPaseinhibitors such as vanadate, net accumulation of radiolabelled butyricand indole-3-acetic acid (IAA) remained higher than in controls. When 2min after the pH shift, at 4°C, the microsomal vesicles weredenatured the amount of ATP could be determined using theluciferin/luciferase assay. Significantly increased ATP production overcontrol values – no pH-shift or ionophore treatment – wasfound. Therefore, such vesicles might produce ATP for in vitrotransport processes such as auxin efflux.     

The role of the apex in the phototropic curvature of Avena coleoptiles: positive curvature under conditions of continuous illumination

The differential growth causing second positive phototropic curvature in intact, black-capped and decapitated Avena coleoptiles has been measured. In all cases the curvature is brought about by a cessation in growth of the illuminated side. The fact that shading the apex does not significantly alter the initial steps of differential growth means that the subapical zones can perceive and respond to unilateral illumination. Decapitation significantly reduces coleoptile growth, especially in the most apical zone. However, the fact that differential growth is still evident in the other zones of decapitated coleoptiles within 30 min of unilateral illumination requires one to conclude that the apex cannot be controlling the differential growth in those basal zones.     

A maize CONSTANS-like gene, conz1, exhibits distinct diurnal expression patterns in varied photoperiods

Maize (Zea mays ssp. mays L.) was domesticated from teosinte (Z. mays L. ssp. parviglumis Iltis & Doebley), a plant requiring short day photoperiods to flower. While photoperiod sensitive landraces of maize exist, post-domestication breeding included efforts to grow maize in a broad range of latitudes. Thus, modern maize is often characterized as day-neutral because time to flower is relatively unaffected by photoperiod. We report the first identification of maize constans of Zea mays1 (conz1), a gene with extensive sequence homology to photoperiod genes CONSTANS (CO) in Arabidopsis (Arabidopsis thaliana (L.) Heynh.) and Heading date1 (Hd1) in rice (Oryza sativa L.). conz1 maps to a syntenous chromosomal location relative to Hd1. Additionally, conz1 and two maize homologs of another photoperiod gene exhibit diurnal expression patterns notably similar to their Arabidopsis and rice homologs. The expression pattern of conz1 in long days is distinct from that observed in short days, suggesting that maize is able to discern variations in photoperiod and respond with differential expression of conz1. We offer models to reconcile the differential expression of conz1 with respect to the photoperiod insensitivity exhibited by temperate inbreds. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Sequence data from this article can be found in the GenBank () data library under the following accession numbers: conz1 mRNA: EU098139, EU098140; gigz1A: BK006299; gigz1B: BK006298.     

Phototropism involves a lateral gradient of growth inhibitors, not of auxin. A review

During phototropic curvature, indolyl-3-acetic acid (IAA) remains evenly distributed in the hypocotyl of sunflower (Helianthus annuus L.) and in the oat (Avena sativa L.) coleoptile. At the irradiated side, growth inhibiting substances accumulate. In sunflower, basipetal movement of a growth factor is not involved, since the top of the seedling can be covered or removed without affecting the photo-tropic response; this response, moreover, is independent of the rate of elongation growth. The chemical nature of the growth-inhibiting substances is only partly known. In the hypocotyl they occur in the neutral fraction: in sunflower cis-xanthoxin is one of them, in radish (Raphanus sativus L.) cis- and trans-raphanusanins, and possibly raphanusamide, are involved. The inhibitor(s) in the oat coleoptile are acidic. During curvature, their amount remains rather constant but the distribution changes with an accumulation at the irradiated side. It is concluded that phototropic curvature is brought about by an accumulation, at the irradiated side, of growth-inhibiting substances that unilaterally reduce cell elongation even though the IAA distribution is uniform.     

Immunological estimation of growth regulator distribution in phototropically reacting sunflower seedlings

The distribution of immunoassayable xanthoxin (XA), abscisic acid (ABA) and indole-3-acetic acid (IAA) in all parts of sunflower ( Helianthus annuus L.) seedlings was determined. During the course of phototropic curvature, including the lag phase (5 min), the distribution of these growth regulators was analyzed in the illuminated and shaded side of the hypocotyl, as well as in the peripheral and central tissues. All three growth regulators showed no detectable asymmetries between the illuminated and shaded hypocotyl halves during the lag phase and early phototropic curvature. Also, no indication for an exchange of XA, ABA or IAA between the peripheral and central tissues was observed. Partial removal of the peripheral cell layers revealed that changes in the growth properties of this tissue, preferentially at the illuminated side of the hypocotyl, are required for the phototropic reaction. Complete removal of the peripheral cell layers abolishes the phototropic response. In dark-incubated, green sunflower seedlings, the loss of sensitivity to phototropic stimulation is correlated with decreasing levels of IAA immunoreactivity, whereas no changes in the levels of ABA- and XA immunoreactivity were recorded. The findings are discussed with respect to the involvement of ABA, XA and IAA in phototropic reactions of green dicotyledonous shoots.     

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.     

Allelopathic effects of sunflower extracts on mustard seed germination and seedling growth

The allelopathic potential of two cultivars of sunflower (Helianthus annuus L.) Lech and Ogrodowy was studied. Leaf aqueous extracts of sunflower at 2.5, 5, and 10 % (m/v) concentrations were applied to determine their effect on mustard (Sinapis alba) seed germination and seedlings growth under laboratory conditions. Increasing concentration of aqueous extracts of sunflower inhibited seed germination, up to almost complete failure of germination, observed in the presence of 10 % extract from sunflower cv. Ogrodowy. The decrease in germinability was well correlated with increased membrane deterioration, assayed as electrical conductivity and enhanced lipid peroxidation, detected as increased malondialdehyde content.     

The role of cotyledons in phototropism of de-etiolated seedlings

Simulated phototropic curvatures caused by differential masking of the cotyledons of de-etiolated seedlings exposed to white light are unconnected with true phototropism. In Cucumis sativus L. and Helianthus annuus L. such curvatures result from a red-light-induced inhibition coming from the exposed cotyledon. True phototropic bending in these species under long-term exposure to fairly high irradiances (as in nature) is a response to blue light. It occurs even when cotyledons are completely covered. These results show that the cotyledons do not perceive the phototropic stimulus and need not be illuminated for phototropism to occur.     

A novel mutant with modified tropic responses in<Emphasis Type="Italic"> Pisum sativum</Emphasis> L.

A single-gene recessive mutant which displays increased phototropic and gravitropic responses has been isolated in Pisum sativum L. cv. Torsdag and is provisionally named mtr-1, for its modified tropic response. Mutant plants attain a greater degree of bending during both phototropic and gravitropic induction due to an extension of the curvature phase. In addition to their increase in tropic curvature, mutant plants have longer and narrower leaves as mature plants, attenuated blue-light-induced ion flux responses, and lower levels of PsPK5 mRNA (a PHOT1 orthologue). Possible causes of these effects are discussed.     

Daily changes in nitrate influx,efflux and metabolism in maize and pearl millet

Maize (Zea mays L.) and pearl millet (Pennisetum americanum (L.) Leeke) seedlings were exposed to [¹⁵N]nitrate for 1-h periods at eight times during a 24-h period (16–8 h light-dark for maize; 14–10 h for millet). Influx of [¹⁵N]nitrate as well as its reduction and translocation were determined during each period. The efflux of previously absorbed [¹⁴N]nitrate to the uptake solution was also estimated. No marked diurnal changes in [¹⁴N]nitrate efflux or [¹⁵N]nitrate influx were evident in maize. In contrast, [¹⁴N]nitrate efflux from millet increased and eventually exceeded [¹⁵N]nitrate influx during the late dark and early light periods, resulting in net nitrate efflux from the roots. The dissimilarity of their diurnal patterns indicates that influx and efflux are independently regulated. In both species, [¹⁵N]nitrate reduction and ¹⁵N translocation to shoots were curtailed more by darkness than was [¹⁵N]nitrate influx. In the light, maize reduced 15% and millet 24% of the incoming [¹⁵N]nitrate. In darkness, reduction dropped to 11 and 17%, respectively. Since the accumulation of reduced-¹⁵N in shoots declined abruptly in darkness, whereas that in roots was little affected, it is suggested that in darkness [¹⁵N]nitrate reduction occurred primarily in roots. The decrease in nitrate uptake and reduction in darkness was not related to efflux, which remained constant in maize and did not respond immediately to darkness in pearl millet.Paper No. 6722 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh     

Interaction of auxin, light, and mechanical stress in orienting microtubules in relation to tropic curvature in the epidermis of maize coleoptiles

Summary Changes in the orientation of cortical microtubules (longitudinal vs. transverse with respect to the long cell axis) at the outer epidermal wall of maize coleoptile segments were induced by auxin, red or blue light, and mechanical stresses (cell extension or compression produced by bending). Immunofluorescent techniques were used for the quantitative determination of frequency distributions of microtubule orientation. Detailed kinetic studies showed that microtubule reorientations are temporally correlated with the simultaneously measured changes in growth rate elicited by auxin, red light, or blue light. Growth inhibition induced by depletion of endogenous auxin produces a longitudinal microtubule pattern that can be changed into a transverse pattern in a dose-dependent manner by applying exogenous auxin. A mid-point pattern with equal frequencies of longitudinal and transverse microtubules was adjusted at 2 mol/1 auxin. Bending stress applied under these conditions adjusts permanent, maximally longitudinal and transverse microtubule orientations at the compressed and extended segment sides, respectively, quantitatively mimicking the responses to differential flank growth during phototropic and gravitropic curvature. During tropic curvature the changes in microtubule pattern reflect the distribution of growth rather than the distribution of auxin. The microtubule pattern responds to auxin-dependent growth changes and mechanical stress in a synergistic manner, confirming the functional equivalence of these factors in affecting microtubule orientation. Similar results were obtained when segment growth was altered by blue or red light instead of auxin in the presence or absence of mechanical stress. It is concluded from these results that growth changes, elicited by auxin, light, etc., and mechanical stress affect microtubule orientation through a common signal perception and transduction chain.Abbreviations IAA indole-3-acetic acid (auxin) - MT cortical microtubule