Growth and curvature in seedlings of Pisum sativum and an ageotropic mutant

In an attempt to explain the influence of gravity on the behaviour of ageotropic plant organs, a pea mutant (Pisum sativum ageotropum) and normal pea (Pisum sativum cv. Sabel) were examined. The mutant has a significantly lower germination rate (large seeds: 25%, small seeds: 10%) than normal pea seeds (55%). Removal of testa increased germination dramatically, the values obtained were 63 and 89%, respectively. Immediately after imbibition the mutant from which the testa had been removed, developed more slowly than normal pea seeds; after 28 h the difference in elongation rate between the two types was reversed. When continuously stimulated geotropically in the horizontal position the elongation in the mutant is larger than in the normal pea roots kept in the same position. During a 24 h period starting 48 h after imbibition the mutant root elongated 45.0 mm while the value for the normal pea root was 11.5 mm. The course of the geotropic curvature in roots of the two types has been followed during a period of 24 h. Normal pea roots develop an asymmetry in the extreme root tip region after 30 min of horizontal stimulation. After prolonged stimulation (exceeding 2 h) the asymmetry has disappeared and the curvature distributed over the entire growth region. When roots of normal pea are stimulated continuously at various angles, the optimum angle of geotropic response is 90° with decreasing responses in the order 135° (i.e. the root tip is pointing obliquely upward) and 45°. The presumed ageotropic behaviour of the mutant has only to a certain extent been confirmed in the present study. When stimulated at 135° a slight positive curvature developed; stimulation at 90° and 45° gave a slight negative curvature.     

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     

The Geotropic Curvature in Roots of Norway Spruce (Picea abies) Containing Anthocyanins

Seedlings of Norway spruce (Picea abies L.) have been found to synthesize anthocyanins in the root tips as well as in the hypocotyls upon irradiation with white light when kept at 4°C for 6–8 days. In addition, it has also been found that the elongation and the geotropic curvature of spruce roots are dependent on the light conditions. The course of the geotropic curvature in spruce roots containing anthocyanins has been followed during a period of 5 h, in which the seedlings were geotropically stimulated continuously in the horizontal position. When the stimulation was performed in white light and in darkness at 21°C, significantly larger curvatures were observed in the roots pretreated at 4°C in darkness than in the roots containing anthocyanins. The specific curvature (curvature in degrees per mm elongation), however, was approximately the same in both types of roots stimulated in white light. This was due to a retarded elongation of the roots pretreated with light at 4°C and containing anthocyanins. A smaller difference in elongation rate between roots with and without anthocyanins was observed in the dark than in the light, but even in the dark the anthocyanin-containing roots grew more slowly than roots without anthocyanins. In order to find out if it is the anthocyanin content or the illumination which affects the elongation and geotropic curvature in the roots, a series of similar experiments was performed using cress seedlings grown at 4°C in light or darkness. Roots of cress seedlings cultivated under conditions which would induce anthocyanin formation in spruce roots exhibited the highest geotropic responses both in light and darkness as compared to cress seedlings grown at 4°C in darkness. As in the case of spruce roots an increase in elongation was observed in cress roots illuminated during the geotropic stimulation. These similarities in the behaviour made it relevant to compare the development of the geotropic curvature in cress and spruce roots.     

Transport and Degradation of Auxin in Relation to Geotropism in Roots of Phaseolus vulgaris

The movement of auxin in Phaseolus vulgaris roots has been examined after injection of IAA^?3H into the basal root/hypocotyl region of intact, dark-grown seedlings. Only a portion of the applied IAA^?3H was transported unchanged to the root tip. The major part of the chromatographed, labelled compounds translocated to the roots was indole-3-acetylaspartic acid (IAAsp) and an unidentified compound running near the front in isopropanol, ammonia, water. The velocity of the auxin transport (7.2 mm per hour) was calculated from scintillation countings of methanol extracts from serial sections of the root. An accumulation of radioactive compounds in the extreme root tip, was observed 5 h after the injection of IAA. The influence of exogenous IAA on the geotropical behaviour of the bean seedling roots was examined. Pretreated roots were stimulated for 5 min in the horizontal position and then rotated parallel to the horizontal axis of the klinostat for 60 or 90 min. The resulting geotropic curvature of IAA-injected and control roots showed significantly different patterns of development. When the stimulation was started 5 h after application of the auxin, the geotropic curvature became larger in roots of the injected plants than in the controls. If, however, the translocation period was extended to 20 h the geotropic curvature was significantly smaller in the roots of the injected plants. The auxin injection did not significally affect the rate of root elongation. The change in geotropical behaviour of the roots is interpreted as a result of the influence of the conversion products of the applied IAA on the geotropical responsiveness.     

Orientation and Geotropic Reaction of Seminal Roots of Wheat

The geotropic orientation of seminal roots of wheat has been studied on seedlings grown in five different positions, stationary and on clinostats. The roots perceive a geoinduc-tion before they have emerged from the grain and perform curvatures inside the grain. These are very sharp and transient, the following root growth is straight in any direction unless the positions are shifted. The roots are insensitive to a static gravi-induction but react to a change in gravitation with a geotropic curvature in positive direction. The roots may not reach or reach, or even pass the plumb-line. The orientation of a root depends upon the direction of its initiation and the geotropic curvature attained before the reaction has ceased. There is no nastic component in the reactions. The ‘plagiotropic’ orientation is explained by the limited positive reaction followed by an ageotropic state. Main root and adventitious roots react in the same way. Reactions to later stimuli give likewise limited curvatures which are weaker but of longer duration. — The effect of temperatures from 10°C to 25°C has been studied and compared to the temperature effect on cell elongation. It is concluded that the whole reaction may be explained by the regular auxin effects on cell elongation. No other hormone should be required and no plagiotropic mechanism is necessary.     

Effect of indoleacetic acid,abscisic acid,root tips and coleoptile tips on growth and curvature of maize roots

The effect of externally applied indoleacetic acid (IAA) and abscisic acid (ABA) on the growth of roots of Zea mays L. was measured. Donor blocks of agar with IAA or ABA were placed laterally on the roots and root curvature was measured. When IAA was applied to vertical roots, a curvature directed toward the donor block was observed. This curvature corresponded to a growth inhibition at the side of the root where the donor was applied. When IAA was applied to horizontal roots from the upper side, normal geotropic downward bending was delayed or totally inhibited. The extent of retardation and the inhibition of curvature were found to depend on the concentration of IAA in the donor block. ABA neither induced curvature in vertical roots nor inhibited geotropic curvature in horizontal roots; thus the growth of roots was not inhibited by ABA. However, when, instead of donor blocks, root tips or coleoptile tips were placed onto vertical roots, a curvature of the roots was observed.Abbreviations ABA abscisic acid - IAA 3-indoleacetic acid     

Gravity-Induced Polar Transport of Calcium across Root Tips of Maize

Calcium movement across primary roots of maize (Zea mays, L.) was determined by application of ⁴⁵Ca²⁺ to one side of the root and collection of radioactivity in an agar receiver block on the opposite side. Ca movement across the root tip was found to be at least 20 times greater than movement across the elongation zone. The rapid movement of Ca across the tip was severely inhibited in roots from which the root cap had been removed. Ca movement across the tip was also strongly retarded in roots pretreated with 2,4-dinitrophenol or potassium cyanide. Orientation of roots horizontally had no effect on Ca movement across the elongation zone but caused a strong asymmetry in the pattern of Ca movement across the tip. In gravistimulated roots, the movement of Ca from top to bottom increased while movement from bottom to top decreased. The data indicate that gravistimulation induces polar movement of Ca toward the lower side of the root cap. An earlier report (Lee, Mulkey, Evans 1983 Science 220: 1375-1376) from this laboratory showed that artificial establishment of calcium gradients at the root tip can cause gravitropic-like curvature. Together, the two studies indicate that Ca plays a key role in linking gravistimulation to the gravitropic growth response in roots.     

The source and lateral transport of growth inhibitors in geotropically stimulated roots of Zea mays and Pisum sativum

Summary The positive geotropic responses of the primary roots of Zea mays and Pisum sativum seedlings depend upon at least one growth inhibiting factor which arises in the root cap and which moves basipetally through the apex into the extending zone. The root apex (as distinct from the cap) and the regions more basal to the extending zone are not sources of growth regulators directly involved in the geotropic response. A difference in the concentration or effectiveness of the inhibitory factor(s) arising in the cap must be established between the upper and lower halves of a horizontal root. Positive geotropic curvature in a horizontal root is attributable, at least in part, to a downward lateral transport of inhibitor(s) from the upper to the lower half of the organ.     

Movement of Cell Organelles and the Geotropic Curvature in Roots of Norway Spruce (Picea abies)

The geotropic development in roots of Norway spruce [(Picea abies (L.)] H. Karst, has been followed by light and electron microscopy and compared with the movement of cell organelles (statoliths) in the root cap cells. The geotropic curvature develops in two phases: (a) an initial curvature in the root cap region, which results in an asymmetry in the extreme root tip and which appears after about 3 h stimulation in the horizontal position; and (b) the geotropic curvature in the basal parts of the root tip, which after 8 h is distributed over the entire elongation zone. A graphic extrapolation, based on measurements of the root curvatures after various stimulation periods, indicates a presentation time in the range of 8 to 10 min. The root anatomy and ultrastructure have been examined in detail in order to obtain information as to which organelles may act as gravity receptors. The root cap consists of a central core (columella) distinct from the peripheral part. The core contains three to four rows of parenchymatic cells each consisting of 15 to 18 storeys of statocyte cells with possibly mobile cell organelles. Amyloplasts and nuclei have been found to be mobile in the root cap cells, and the movement of both types of organelles has been followed after inversion of the seedlings and stimulation in the horizontal position for various periods of time at 4°C and 21°C. Three-dimensional reconstructions of spruce root cap cells based on serial sectioning and electron microscopy have been performed. These demonstrate that the endoplasmic reticulum (ER)-system and the vacuoles occupy a considerable part of the statocyte cell. For this reason the space available for free movement of single statolith particles is highly restricted.     

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.     

Auxin transport in roots

Summary Light promotes the net acropetal movement of ¹⁴C through 6-mm subapical segments of dark-grown roots of Zea mays supplied at their basal ends with 1 M IAA-1-¹⁴C in agar blocks. This promotion occurs only when the segments are irradiated during the transport period, and both red and blue light appear to be as effective as white light at the radiant flux densities used in this investigation. The promotion is not found if the segments are pretreated with light and then returned to darkness before the trasport of IAA-1-¹⁴C is determined. The very slight basipetal movement of ¹⁴C through the segments supplied with an apical source of IAA-1-¹⁴C is unaffected by light.Only one radioactive substance is found in the apical receiver blocks. This substance has an R_f virtually identical to those of the stock solution of IAA incorporated into the donor block and of unlabelled IAA. The movement of radioactivity into the receiver blocks through, the illuminated segments therefore appears to reflect the movement of IAA. Light thus increases the acropetal movement of IAA through the Zea root segment.The primary roots of Zea mays var. Giant Horse Tooth seedlings grown in total darkness do not exhibit a positive geotropic response. When the seed is orientated with the embryo uppermost the radicle grows out horizontally. On exposure to light, however, the roots bend down. This reaction appears about 3–9 hours after the onset of illumination, and white, red and blue light appear to be equally effective at the flux densities employed in this study. Green light in the spectral band between 510–530 nm did not appear to induce this positive geotropic responsiveness.     

Gravity perception in decapped roots of Zea mays

Time-lapse photography and light microscopy were used to determine whether or not sedimentation of the newly developed amyloplasts in the apex of Zea mays L. roots occurred at the time when geotropic responsiveness reappears following removal of the cap. All decapped roots exhibiting a geotropic response had some amyloplast sedimentation in the apical cortical cells. Exposing decapped roots to a centrifugal acceleration of 25 g for 4 h showed that amyloplasts of a similar size and development were not displaced within the cytoplasm when this treatment began 12 h after decapping, whereas displacement did occur when the treatment began 24 h after decapping. This finding indicates the occurrence of a change in the physical characteristics of the cytoplasm between 12 h and 24 h after removing of the cap, which allows amyloplast movement and thus restores gravity perception.     

Microsurgical removal of epidermal and cortical cells: evidence that the gravitropic signal moves through the outer cell layers in primary roots of maize

There is general agreement that during root gravitropism some sort of growth-modifying signal moves from the cap to the elongation zone and that this signal ultimately induces the curvature that leads to reorientation of the root. However, there is disagreement regarding both the nature of the signal and the pathway of its movement from the root cap to the elongation zone. We examined the pathway of movement by testing gravitropism in primary roots of maize (Zea mays L.) from which narrow (0.5 mm) rings of epidermal and cortical tissue were surgically removed from various positions within the elongation zone. When roots were girdled in the apical part of the elongation zone gravitropic curvature occurred apical to the girdle but not basal to the girdle. Filling the girdle with agar allowed curvature basal to the girdle to occur. Shallow girdles, in which only two or three cell layers (epidermis plus one or two cortical cell layers) were removed, prevented or greatly delayed gravitropic curvature basal to the girdle. The results indicate that the gravitropic signal moves basipetally through the outermost cell layers, perhaps through the epidermis itself.     

Induction of Curvature in Maize Roots by Calcium or by Thigmostimulation : Role of the Postmitotic Isodiametric Growth Zone

We examined the response of primary roots of maize (Zea mays L. cv Merit) to unilateral application of calcium with particular attention to the site of application, the dependence on growth rate, and possible contributions of thigmotropic stimulation during application. Unilateral application of agar to the root cap induced negative curvature whether or not the agar contained calcium. This apparent thigmotropic response was enhanced by including calcium in the agar. Curvature away from objects applied unilaterally to the extreme root tip occurred both in intact and detipped roots. When agar containing calcium chloride was applied to one side of the postmitotic isodiametric growth zone (a region between the apical meristem and the elongation zone), the root curved toward the side of application. This response could not be induced by plain agar. We conclude that curvature away from calcium applied to the root tip results from a thigmotropic response to stimulation during application. In contrast, curvature toward calcium applied to the postmitotic isodiametric growth zone results from direct calcium-induced inhibition of growth.     

The role of the epidermis and cortex in gravitropic curvature of maize roots

In order to determine the role of the epidermis and cortex in gravitropic curvature of seedling roots of maize (Zea mays L. cv. Merit), the cortex on the two opposite flanks was removed from the meristem through the growing zone; gravitropic curvature was measured with the roots oriented horizontally with the cut flanks either on the upper and lower side, or on the lateral sides as a wound control. Curvature was slower in both these treatments (53° in 5 h) than in intact roots (82°), but there was no difference between the two orientations in extent and rate of curvature, nor in the latent time, showing that epidermis and cortex were not the site of action of the growth-regulating signal. The amount of cortex removed made no difference in the extent of curvature. Curvature was eliminated when the endodermis was damaged, raising the possibility that the endodermis or the stele-cortex interface controls gravitropic curvature in roots. The elongation rate of roots from which just the epidermis had been peeled was reduced by 0.01 mM auxin (indole-3-acetic acid) from 0.42 to 0.27 mm h^-1, contradicting the hypothesis that only the epidermis responds to changes in auxin activity during gravistimulation. These observations indicate that gravitropic curvature in maize roots is not driven by differential cortical cell enlargement, and that movement of growth regulator(s) from the tip to the elongating zone is unlikely to occur in the cortex.Abbreviations df degrees of freedom - IAA indole-3-acetic acid     

Analysis of growth patterns during gravitropic curvature in roots ofZea mays by use of a computer-based video digitizer

A computer-based video digitizer system is described which allows automated tracking of markers placed on a plant surface. The system uses customized software to calculate relative growth rates at selected positions along the plant surface and to determine rates of gravitropic curvature based on the changing pattern of distribution of the surface markers. The system was used to study the time course of gravitropic curvature and changes in relative growth rate along the upper and lower surface of horizontally-oriented roots of maize (Zea mays L.). The growing region of the root was found to extend from about 1 mm behind the tip to approximately 6 mm behind the tip. In vertically-oriented roots the relative growth rate was maximal at about 2.5 mm behind the tip and declined smoothly on either side of the maximum. Curvature was initiated approximately 30 min after horizontal orientation with maximal (50°) curvature being attained in 3 h. Analysis of surface extension patterns during the response indicated that curvature results from a reduction in growth rate along both the upper and lower surfaces with stronger reduction along the lower surface.     

The importance of anatomical structural study of roots for the understanding of physiological processes

Abstract

Besides being species‐specific, the inner structure of the root is influenced by the place and time of origin during the growth period. From the root tip up to the base of a particular root, the zones of cell division, cell elongation, formation of root hairs and root branching can be distinguished. The root tip that is covered by a root cap and mucilage is protected against evaporation and water contact. From the end of the lateral parts of the root cap, the cells become exposed to the surrounding environment. The cells can elongate by water uptake or can shrink by water loss. All processes of geotropic growth take place there. In this study, some differences are illustrated using Zea mays plants. Radicle and roots emerging from several nodes of the shoot as well as lateral roots are compared. The distances from the tip and from the base of the root are also very important for characterization of particular root functions. Distinctive features such as root diameter, size of the stele and of the cortex, ratio of cortex to stele, number and width of the xylem vessels, size of cells, special thickenings and stage of lignification as well as symptoms of maturation are observed.     

Effects of Morphactin on Indol-3yl-acetic Acid Transport, Growth, and Geotropic Response in Cereal Coleoptiles

The effects of the morphactin 2-ehloro-9-hydroxyfluorene-9-carboxylicacid methyl ester [CFM] on growth, geotropic curvature and transportand metabolism of indol-3yl-acetic acid [IAA-5-³H] in the coleoptilesof Zea mays and A vena saliva have been investigated. A strongcorrelation has been found to exist between the inhibition ofthe geotropic response and the inhibition of auxin transport.CFM supplied at concentrations sufficient to abolish auxin transporthas been shown to promote the elongation of Zea, but not ofAvena, coleoptile segments. CFM does not change the patternof metabolism of IAA in Zea coleoptile segments. In these segmentsIAA is metabolized when its concentration is high, but the radioactivitytransported basipetally, or laterally in geotropically stimulatedcoleoptiles, is virtually confined to the IAA molecule. Radioactivityexported into the basal receiver blocks is wholly confined toIAA. It is concluded that CFM inhibits the geotropic responsein coleoptiles by suppression of the longitudinal and lateralauxin transport mechanisms. The growth-promoting propertiesof this substance cannot be linked with its effects on eitherauxin metabolism or transport.     

Spectral dependence of the light-induced geotropic response in Zea roots

The induction by light of geotropic responsiveness in the primary roots of Zea mays L. (cv. Golden Cross Bantam 70) was found to be governed by the all-or-none law. The response was induced by light energies above a threshold value, but the maximal curvature of geo-stimulated roots was constand irrespective of the light energy above that threshold. The action spectrum for this light effect showed a large peak at 650, a small peak at 410, and a shoulder at 663 nm. The effect of red light was not reversed by far-red light. Thus, the geotropic response in Zea roots may not be controlled by phytochrome.     

Kinetics of the light-induced georeactivity of maize roots

Apical root segments of Zea mays L. cv. Orla 264 undergo some geotropic curvature in complete darkness but the curvature increases considerably if prior to geostimulation the segments are given a light pretreatment. If the light treatment is follwed by a dark treatment before the root is geostimulated the light-induced response is not changed by dark periods up to 2 h but declines with longer ones, and disappears completely after 5 h of darkness.