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.     

Abscisic acid and the response of the roots of Zea mays L. seedlings to gravity

Summary Exogeneous application of abscisic acid (ABA) to intact roots of LG 11 maize seedlings inhibits root elongation and induces bending of the root in response to gravity in darkness, even though the roots of these seedlings are not normally positively geotropic in the dark. ABA cannot, however, induce geotropic curvature in dark-exposed decapped roots, thus confirming that the root cap is the site of graviperception in the intact root.Abbreviation ABA abscissic acid     

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.     

Root Growth Inhibitors from Root Cap and Root Meristem of Zea mays L.

A micro-assay based on the growth inhibition of root segmentsof the seminal roots of Zea mays has been used to investigatethe root-growth-inhibiting substances in root caps and meristemsrespectively of the roots of Zea mays. This micro-assay is sensitiveto 50 pg of IAA or less. Paper chromatography of the acid fractionof methanolic extracts shows the presence of one main inhibitorin root caps and a different main inhibitor in root meristems.Neither is IAA, whose presence in meristems is sometimes indicatedby small inhibitions (or stimulations) at the characteristicRf of IAA. A Commelina leaf-epidermis assay shows the presenceof one stomata-closing ABA-like substance in root caps and onein meristems, one corresponding in Rf to the main root-growthinhibitor from the root cap. The implications of these findingsfor the geotropic responses of roots is briefly discussed.     

Physiology of Morphactins: Effect on Gravi- and Photo-response

Plant roots and shoots respond to gravity and light source in a definite way. Thus, there are typical geotropic and phototropic responses for roots and shoots. When seedlings were grown in presence of morphactins, IT 3233 or IT 3456, on a vertical or a horizontal plane, the roots and shoots lost the capacity to respond to gravity or to unilateral light source. This was true for both monocots and dicots. This suggests that basic mechanism (s) of the two tropic responses are the same in the roots and shoots of the two plant groups. The site(s) of action of morphactins is unknown. The reaction (s) controlling geotropism and phototropism may be closely related as morphactins affected both geotropic and phototropic response of the same organ. Indoleacetic acid and gibberellic acid per se did not modify the effect of morphaclins on geotropism. Growth retardation effect of morphactins appears to be controlled by another mechanism.     

Characterizing Pathways by Which Gravitropic Effectors Could Move from the Root Cap to the Root of Primary Roots of Zea mays

Plasmodesmata linking the root cap and root in primary rootsZea mays are restricted to approx. 400 protodermal cells borderingapprox. 110000 µm² of the calyptrogen of the root cap.This area is less than 10% of the cross-sectional area of theroot-tip at the cap junction. Therefore, gravitropic effectorsmoving from the root cap to the root can move symplasticallyonly through a relatively small area in the centre of the root.Decapped roots are non-responsive to gravity. However, decappedroots whose caps are replaced immediately after decapping arestrongly graviresponsive. Thus, gravicurvature occurs only whenthe root cap contacts the root, and symplastic continuity betweenthe cap and root is not required for gravicurvature. Completelyremoving mucilage from the root tip renders the root non-responsiveto gravity. Taken together, these data suggest that gravitropiceffectors move apoplastically through mucilage from the capto the root. Calyptrogen, open meristem, protoderm, root cap, root gravitropism, Zea mays     

Interactions between Hydrotropism and Gravitropism in the Primary Seminal Roots of Triticum eastivum L.

It has been proposed that hydrotropism interacts with gravitropismin seedling roots; that is, roots which are highly gravitropicshow less hydrotropism (Takahashi and Suge, 1991 PhysiologiaPlantarum 82: 24-31; Takahashi and Scott, 1993 Plant, Cell andEnvironment 16: 99-103). Here, we examine varietal differencesin the hydrotropic response and its interaction with gravitropismin wheat roots. Primary seminal roots of wheat (Triticum aestivumL.) were hydrotropically stimulated by different moisture gradientsestablished by placing wet cheesecloth and saturated solutionsof different salts in closed chambers. From equations obtainedby relative humidity (RH) at different distances from the wetcheesecloth, moisture gradients at the root-tip level were estimatedto be 0·03 to 1·84% RH mm^-1, depending upon thesalt introduced into the chamber. The roots showed positivehydrotropism in response to 0·67% RH mm^-1, and the responseapparently increased as the gradient was strengthened. Whenthe primary seminal roots of 12 cultivars were exposed to amoisture gradient of 1·84% RH mm-1, hydrotropic responsesignificantly differed depending upon the cultivar tested. Amongthe cultivars, the roots of Norin 11, Norin 15, Norin 117, andNorin 125 responded hydrotropically more strongly than the others.These roots, with the exception of Norin 11, showed a less vigorousresponse to gravity compared to the remaining cultivars. However,the roots of Norin 20, Norin 38, and Norin 107 were relativelyunresponsive to both a moisture gradient and to gravity. Thus,the primary seminal roots of wheat respond hydrotropically,and the responsiveness differs among cultivars. However, thevarietal difference in hydrotropic response cannot be explainedsolely by converse differences in responsiveness to gravity.Copyright1995, 1999 Academic Press Cultivar, gravitropism, hydrotropism, primary seminal roots, Triticum aestivum L., wheat     

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.     

Defective Secretion of Mucilage is the Cellular Basis for Agravitropism in Primary Roots of Zea mays cv. Ageotropic

Root caps of primary, secondary, and seminal roots of Z. mayscv. Kys secrete large amounts of mucilage and are in close contactwith the root all along the root apex. These roots are stronglygraviresponsive. Secondary and seminal roots of Z. mays cv.Ageotropic are also strongly graviresponsive. Similarly, theircaps secrete mucilage and closely appress the root all alongthe root apex. However, primary roots of Z. mays cv. Ageotropicare non-responsive to gravity. Their caps secrete negligibleamounts of mucilage and contact the root only at the extremeapex of the root along the calyptrogen. These roots become graviresponsivewhen their tips are coated with mucilage or mucilage-like materials.Peripheral cells of root caps of roots of Z. mays cv. Kys containmany dictyosomes associated with vesicles that migrate to andfuse with the plasmalemma. Root-cap cells of secondary and seminal(i.e. graviresponsive) roots of Z. mays cv. Ageotropic are similarto those of primary roots of Z. mays cv. Kys. However, root-capcells of primary (i.e. non-graviresponsive) roots of Z. mayscv. Ageotropic have distended dictyosomal cisternae filled withan electron-dense, granular material. Large vesicles full ofthis material populate the cells and apparently do not fusewith the plasmalemma. Taken together, these results suggestthat non-graviresponsiveness of primary roots of Z. mays cv.Ageotropic results from the lack of apoplastic continuity betweenthe root and the periphery of the root cap. This is a resultof negligible secretion of mucilage by cells along the edgeof the root cap which, in turn, appears to be due to the malfunctioningof dictyosomes in these cells. Corn, dictyosomes, mucilage, root gravitropism, Zea mays cv. Ageotropic, Zea mays cv. Kys     

Quantitative Assessment of Ultrastructural Changes in Primary Roots of Corn (Zea mays L.) after Geotropic Stimulation: I. Root Cap

The root cap is the site of gravity perception. In the study of caps of primary roots of corn (Zea mays L.), we compared the ultrastructure of geotropically responding roots that had received a 661 nm (red) irradiation (60 second) with nonresponding dark control roots kept in the dark, at comparable times following geotropic stimulation for a total of 150 minutes. The outstanding differences in the light-exposed root caps at the ultrastructural level were: (a) significantly more Golgi apparatus (dictyosomes) were found in the top than in the bottom of red-exposed cells; a random distribution is seen in the dark control cells; (b) the nucleus preferred the top in a greater number of the red-exposed cells; (c) the pattern of mitochondria localization was identical in both treatments, a greater preference for the top; however, the number of mitochondria was reduced in the bottom of red-treated cap cells as compared to the control cells. A lowering in number in the bottom of the red-treated cells was noted also in the dictyosomes; and (d) in a small percentage of cells that showed a preferential distribution of endoplasmic reticulum (ER), more red-exposed cells than controls, during the period 30 to 135 minutes after stimulation, had less ER in the top; however, a majority of the cells in both treatments showed no preferred position for ER distribution. Commonalities in ultra-structural behavior also existed between the red- and dark-treated root cap cells: (a) sedimentation of amyloplasts, with no difference in total number between treatments; and (b) a close association between amyloplasts and ER in both groups.

Polarization of organelles occurred in both the geotropically responding and nonresponding roots. The differences in dictyosome and nuclear localization, and dictyosome and mitochondrial number could be correlated with the tropic response in the red-exposed roots and no response in the dark roots, which in turn could be related to the reported hormonal events in the geotropism of roots.

     

Organelle Sedimentation in Gravitropic Roots of Limnobium is Restricted to the Elongation Zone

Roots of the aquatic angiosperm Limnobium spongia (Bosc) Steud.were evaluated by light and electron microscopy to determinethe distribution of organelle sedimentation towards gravity.Roots of Limnobium are strongly gravitropic. The rootcap consistsof only two layers of cells. Although small amyloplasts arepresent in the central cap cells, no sedimentation of any organelle,including amyloplasts, was found. In contrast, both amyloplastsand nuclei sediment consistently and completely in cells ofthe elongation zone. Sedimentation occurs in one cell layerof the cortex just outside the endodermis. Sedimentation ofboth amyloplasts and nuclei begins in cells that are in theirinitial stages of elongation and persists at least to the levelof the root where root hairs emerge. This is the first modernreport of the presence of sedimentation away from, but not in,the rootcap. It shows that sedimentation in the rootcap is notnecessary for gravitropic sensing in at least one angiosperm.If amyloplast sedimentation is responsible for gravitropic sensing,then the site of sensing in Limnobium roots is the elongationzone and not the rootcap. These data do not necessarily conflictwith the hypothesis that sensing occurs in the cap in otherroots, since Limnobium roots are exceptional in rootcap originand structure, as well as in the distribution of organelle sedimentation.Similarly, if nuclear sedimentation is involved in gravitropicsensing, then nuclear mass would function in addition to, notinstead of, that of amyloplasts.Copyright 1994, 1999 AcademicPress Limnobium spongia, gravitropism, roots, sedimentation, cortex     

Residual nutational activity of the sunflower hypocotyl in simulated weightlessness

Does gravity drive circumnutation? One model ascribes hypocotylcircumnutation to a continuing series of geotropic responseseach of which overshoots thereby maintaining a sustained oscillation.However, some features of the observed movements are not easilyreconciled with the model. The critical feature of this modelis the requirement that circumnutation must exhibit an absolutedependence on a g-force. Experiments with Helianthns annum onhorizontal clinostats demonstrated an 80% reduction in the amplitudeof hypocotyl circumnutation (compared with upright plants atone g) although the oscillations continued at simulated "zerog". It is not certain that the clinostat environment adequatelysimulates the weightless environment of space but, if it does,we may expect a space experiment to demonstrate that hypocotylnutation in Helianthus annuus is not fully dependent on gravity. (Received October 13, 1978; )     

Calcium Requirement for the Induction of Hydrotropism and Enhancement of Calcium-Induced Curvature by Water Stress in Primary Roots of Pea, Pisum sativum L.

Positive hydrotropic curvature in the roots of the agravitropicpea (Pisum sativum L.) mutant, ageotropum, occurred when theroot cap was exposed to a gradient of water potential by anasymmetric application of agar containing sorbitol [Takano etal. (1995) Planta 197: 410]. As previously reported [Takahashiand Suge (1991) Physiol. Plant. 82: 24], in this study the hydrotropicresponse due to unilateral application of sorbitol to the rootcap was totally inhibited by pretreatment with ethyleneglycol-bis-(ß-amino-ethylether)N,N,N',N'-tetraacetic acid (EGTA). However, hydrotropic responseof the EGTA-treated roots was recovered only when EGTA was replacedby a 10 mM calcium (CaCl₂) solution prior to hydrostimulation.A calcium channel blocker, lanthanum (LaCl₃), also inhibitedhydrotropic curvature of ageotropum roots, whereas the hydrotropicresponse was affected by neither nifedipine nor vera-pamil.Application of calcium ionophore, A23187 [GenBank] , resulted in a significantpromotion of hydrotropic curvature. Furthermore, ageotropumroots curved away from a calcium source when an agar block containing10 mM calcium was asymmetrically applied to the root cap. Thiscalcium-induced curvature was found to be accelerated by waterstress and significantly inhibited by LaCl₃. While the calcium-inducedcurvature commenced within 1 h after application, hydrotropiccurvature became visible 3 to 4 h after an exposure to a gradientof water potential. These results indicate that apoplastic calciumand its influx through the plasmamembrane are involved in theinduction of hydrotropism in roots. A gradient of water potentialin the root cap may cause a physiological change that is mediatedby calcium, which ultimately leads to the curvature in the elongationregion associated with the hydrotropic response. (Received October 21, 1996; Accepted January 10, 1997)     

The Physiology of Plant Nutation: I. NUTATION AND GEOTROPIC RESPONSE

The nutation of plant organs has been considered to be eitherthe result of a geotropic feedback loop or produced by an endogenousoscillator. Observations have been made of the angular displacementof Phaseolus seedlings during response to a gravitational stimulus.Nutational oscillations have been observed during the correctivemovement of the stimulated plants in many cases. Geotropic movementcan, however, occur in plants not exhibiting nutation. Theseobservations are considered to support the hypothesis that themovements of nutation in Phaseolus have an endogenous originunconnected with geotropic reactions. A theoretical model isproposed to account for the observed nutational behaviour ofbean seedlings.     

Effects of Light on the Georeaction and Growth Inhibitor Content of Roots

The positive geotropic response of the apical segments prepared from the primary roots of Zea mays depends upon at least one growth inhibitor, produced by the root cap, moving basipetally into the extending zone of the root in which it accumulates in the lower part. Anjou maize reacts in both darkness and light while Kelvedon maize is, for the first few hours, geotropic only in light. The production (or activity) of the growth-inhibiting substance — tested by using vertical half-decapitated root segments — is quite similar to the georeaction. This finding provides strong evidence that, in the case of Kelvedon maize roots, the inhibitory substance may depend on light. Observations related to the root segment of Anjou and Kelvedon maizes of which the tips are exchanged, are in agreement with the above results.     

The Effects of Red, Far-red, and Blue Light on the Geotropic Response of Coleoptiles of Zea mays

The effects of red, far-red, and blue light on the geotropicresponse of excised coleoptiles of Zea mays have been investigated.Seedlings were grown in darkness for 5 or 6 days, exposed tovarious light treatments, and then returned to darkness fordetermination of the geotropic response. The rate of response of the coleoptiles is decreased after theyhave been exposed to red light (620–700 mµ, 560ergs cm^–2sec^–1 for the 24 hrs, but not for the 4hrs, preceding stimulation by gravity. Furthermore, their rateof response is greatly reduced if they are exposed to red lightfor 10 min and then returned to darkness for 20 hrs before geotropicstimulation. At 25° C an interval of 6 to 8 hrs elapses between a 10-minexposure to red light and the first detectable decrease in thegeotropic response of the coleoptile. This interval can be lengthenedby exposing the seedlings to low temperatures (0° to 2°C) after the light treatment but cannot be greatly shortenedby increasing the duration of exposure to red light. Using a standard procedure of exposing 5-day-old etiolated seedlingsto light for various times, replacing them in darkness for 20hrs and then determining the response of the coleoptiles to4 hrs geotropic stimulation, it has been found that: (a) Exposureto red light for 15 sec significantly decreases the geotropiccurvature of the coleoptiles and that further reduction occurson increasing the length of the light treatment to 2 and 5 min.(b) Far-red light has no effect on the geotropic response ofthe coleoptiles but it can completely reverse the effect ofred light. After repeated alternate exposure to red and far-redlight the geotropic response of the coleoptile is determinedby the nature of the last exposure, (c) Complete reversal ofthe effect of red light by far-red radiation only occurs whenexposure to far-red follows immediately after exposure to red.The reversing effect of far-red radiation is reduced if a periodof darkness intervenes between the red and far-red light treatments,and is lost after a dark interval of approximately 2 hrs. The effect of red light on the rate of geotropic response ofthe coleoptiles is independent of their age and length at thetime of excision. Blue light acts in a similar way to red light, but the seedlingsare less sensitive to blue than to red light. Coleoptiles grown throughout in a mixture of continuous, weak,red, and far-red light have a lower rate of geotropic responsethan etiolated coleoptiles.     

Relation of Phytochrome-enhanced Geotropic Sensitivity to Ethylene Production

Brief exposure of etiolated pea (Pisum sativum cv. Alaska) seedlings to red light enhances subsequent development of geotropic curvature of the stem. Both this response and inhibition of ethylene production by red light become maximal 8 hours after illumination. Very low concentrations of applied ethylene inhibit development of geotropic curvature, whereas hypobaric treatment enhances geotropic sensitivity by removing endogenous ethylene. Increased geotropic sensitivity after illumination is accompanied by increased lateral migration of ³H-indoleacetic acid in response to gravity, and ethylene inhibits this lateral migration. It is suggested, therefore, that red light-enhanced geotropic sensitivity is caused by increased lateral auxin transport resulting from a reduction in ethylene production after illumination.     

Cap Formation during the Elongation of Lateral Roots of Vicia faba L.

Cell proliferation was examined in the cap initials of newly-emerged0·2 and 4·0 cm long lateral roots of Vicia fabafrom an investigation of the passage of labelled cells throughmitosis following a 1-h pulse with tritiated thymidine. Celldoubling time was found to increase in this group of initialcells as the secondary roots elongated, this increase beinga result of a gradual lengthening in the duration of the mitoticcycle of both the fast and slow cycling meristematic cells andof a decrease in the size of both the growth fraction and thoseproliferating cells with a short cycle time. The rate of cellproduction by the cap initials was maximal in the newly emergedsecondary roots and showed a gradual decline with subsequentlateral elongation. This change in the rate of cell proliferationin the cap initials has been shown to be related to the initiationof the quiescent centre following lateral root emergence fromthe tissues of the primary. The number of cells making up the cap of 0·2–4·0cm long secondary roots is known to be constant and thus therate at which cells are sloughed off of the cap into the soilmust be the same as the rate of cap cell formation in theseroots, all of the cap cells being replaced every 6–9 days.The rate of cap cell formation in 0·2–4·0cm long secondary roots was used to calculate that one 11-dayold Vicia plant will have contributed between 56 000 and 85000 cap cells to the rhizosphere from its root system sincegermination.     

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.     

The Influence of Light on Geotropism in Cress Roots

Light affects the growth and orientation of roots of cress seedlings(Lepidium sativum L. cv. Curled). The effects are manifest eitheras increased rates of geotropic curvature or, if the roots arehorizontal, as distorted and crinkled forms of growth. Blue,red, and far-red irradiation can bring about these effects,but with differences of detail: at equal fluence rates duringthe period of geostimulus, blue is more effective than red atincreasing the rate of geocurvature; however, with irradiationprior to a geostimulus, only the stimulatory effects of redirradiation persist for 2–4 h of darkness. Short periods(5 min) of radiation, if given at the time of geostimulus, enhancegeocurvature, again with blue most, and far-red least, effective,but there are no clear indications of red/far-red reversibility.The possibility of there being more than one photosystem responsiblefor the effects of white light on the geotropic responsivenessof roots is discussed.