Root Graviresponsiveness and Columella Cell Structure in Carotenoid-deficient Seedlings of Zea mays

Root graviresponsiveness in normal and carotenoid-deficientmutant seedlings of Zea mays was not significantly different.Columella cells in roots of mutant seedlings were characterizedby fewer, smaller, and a reduced relative volume of plastidsas compared to columella cells of normal seedlings. Plastidsin columella cells of mutant seedlings possessed reduced amountsof starch. Although approximately 10 per cent of the columellacells in mutant seedlings lacked starch, their plastids werelocated at the bottom of the cell. These results suggest that(i) carotenoids are not necessary for root gravitropism, (ii)graviresponsiveness is not necessarily proportional to the size,number, or relative volume of plastids in columella cells, and(iii) sedimentation of plastids in columella cells may not resultdirectly from their increased density due to starch content.Plastids in columella cells of normal and mutant seedlings wereassociated with bands of microtubule-like structures, suggestingthat these structures may be involved in ‘positioning’plastids in the cell. Zea mays, graviperception, graviresponsiveness, carotenoids, vp-9 mutant, columella cell, roots     

Root Growth, Secondary Root Formation and Root Gravitropism in Carotenoid-deficient Seedlings of Zea mays L.

The effect of ABA on root growth, secondary-root formation androot gravitropism in seedlings of Zea mays was investigatedby using Fluridone-treated seedlings and a viviparous mutant,both of which lack carotenoids and ABA. Primary roots of seedlingsgrown in the presence of Fluridone grew significantly slowerthan those of control (i.e. untreated) roots. Elongation ofFluridone-treated roots was inhibited significantly by the exogenousapplication of 1 mM ABA. Exogenous application of 1 µMand 1 nM ABA had either no effect or only a slight stimulatoryeffect on root elongation, depending on the method of application.The absence of ABA in Fluridone-treated plants was not an importantfactor in secondary-root formation in seedlings less than 9–10d old. However, ABA may suppress secondary-root formation inolder seedlings, since 11-d-old control seedlings had significantlyfewer secondary roots than Fluridone-treated seedlings. Rootsof Fluridone-treated and control seedlings were graviresponsive.Similar data were obtained for vp-9 mutants of Z. mays, whichare phenotypically identical to Fluridone-treated seedlings.These results indicate that ABA is necessary for neither secondary-rootformation nor for positive gravitropism by primary roots. Zea mays, gravitropism, carotenoid-deficient, Fluridone, root growth, vp-9 mutant     

A Morphometric Analysis of the Redistribution of Organelles in Columella Cells of Horizontally-oriented Roots of Zea mays

In order to determine what structural changes in graviperceptivecells are associated with the onset of root gravicurvature,the redistribution of organelles in columella cells of horizontally-oriented,graviresponding roots of Zea mays has been quantified. Rootgravicurvature began by 15 min after reorientation, and didnot involve significant changes in the (i) volume of individualcolumella cells or amyloplasts, (ii) relative volume of anycellular organelle, (iii) number of amyloplasts per columellacell, or (iv) surface area or cellular location of endoplasmicreticulum. Sedimentation of amyloplasts began within 1 to 2min after reorientation, and was characterized by an intenselystaining area of cytoplasm adjacent to the sedimenting amyloplasts.By 5 min after reorientation, amyloplasts were located in thelower distal corner of columella cells, and, by 15 min afterreorientation, overlaid the entire length of the lower cellwall. No consistent contact between amyloplasts and any cellularstructure was detected at any stage of gravicurvature. Centrally-locatednuclei initially migrated upward in columella cells of horizontally-orientedroots, after which they moved to the proximal ends of the cellsby 15 min after reorientation. No significant pattern of redistributionof vacuoles, mitochondra, dictyosomes, or hyaloplasm was detectedthat correlated with the onset of gravicurvature. These resultsindicate that amyloplasts and nuclei are the only organelieswhose movements correlate positively with the onset of gravicurvatureby primary roots of this cultivar of Zea mays. Zea mays, root gravitropism, ultrastructure, morphometry, graviperception     

Gravitropism of Primary Roots of Zea mays L. at Different Displacement Angles

Primary roots of Zea mays were oriented at various angles fromthe vertical ranging from 99° to 1° and their subsequentbending analysed from filmed records. The maximum rate of bendingand the time before bending commenced both varied two-fold,but showed no correlation with the initial angle of tip displacement.Roots orientated to small initial angles (< 40°) oftenovershot the vertical and proceeded to oscillate around thisorientation, whereas roots oriented to large initial angles(> 60°) often failed to achieve the vertical. Roots inthis latter group resumed bending after an indeterminate time,or did so immediately after a second displacement of their tip,showing that they were not intrinsically unable to bend. Theapparently spontaneous resumption of bending after a temporaryplagiogravitropic phase is suggested as being due to noise inthe graviperception system in the root cap. The tips of rootsgrowing vertically downwards showed oscillatory bending movementsup to 10° either side of vertical. This angle correspondsto the minimum angle of displacement which induces gravitropicbending. Only when roots were oriented 10-20° from verticaldid they begin unequivocally to show a gravitropism since atsuch angles the deflection of their tips exceeded that due totheir natural oscillation.Copyright 1993, 1999 Academic Press Gravitropism, roots, Zea mays     

RNA Synthesis in the Root Apex of Zea mays

The rate of RNA synthesis in eight regions of the root apexof Zea mays has been determined. Cells of the quiescent centrehave a lower level of RNA synthesis than any other region studied.The level of RNA synthesis appears to be a function of the positionof the cell in the apex. All the newly synthesized RNA detectedin chromatin and cytoplasm can be accounted for by synthesisin the nucleolus and subsequent transport to these compartments.Newly synthesized RNA appears in the three compartments at differentrates depending on the location of the cell.     

Movement of Amyloplasts in the Root Cap Cells of Geotropically Sensitive Roots

The root cap of Lepidium sativum under our culture conditions was found to contain 7 (or occasionally 8) storeys of starch-containing cells. In the youngest one (or two) of these storeys the amyloplasts are small and the cells appear embryonic. In the 6 non-embryonic storeys the amyloplasts are large. Upon inversion of the root, the amyloplasts in the 3 youngest of the 6 non-emhryonic storeys start falling toward the opposite end of the cell at about 72 μ per h (at 21 C), hut they maintain this speed for only 6 to 12 min, after which they virtually come to a stop. As a result, it takes 10 to 12 min before any of the amyloplasts get approximately as close to the ceiling as they were to the floor before the inversion; and this is true only of the 2 youngest of the non-embryonic storeys. When the root is placed horizontal, whether coming from the normal or the inverted position, the amyloplasts reach the lower, longitudinal wall in 15 min or less. The positions of the amyloplasts in the cells of the 3 oldest starch-containing storeys are erratic and show little, if any, dependency on the preceding time of inversion.     

Uronide Deposition Rates in the Primary Root of Zea mays

The spatial distribution of the rate of deposition of uronic acids in the elongation zone of Zea mays L. Crow WF9 × Mo 17 was determined using the continuity equation with experimentally determined values for uronide density and growth velocity. In spatial terms, the uronide deposition rate has a maximum of 0.4 micrograms per millimeter per hour at s = 3.5 mm (i.e., at the location 3.5 mm from the root tip) and decreases to 0.1 mg mm⁻¹ h⁻¹ by s = 10 mm. In terms of a material tissue element, a tissue segment located initially from s = 2.0 to s = 2.1 mm has 0.14 μg of uronic acids and increases in both length and uronic acid content until it is 0.9 mm long and has 0.7 μg of uronide when its center is at s = 10 mm. Simulations of radioactive labeling experiments show that 15 min is the appropriate time scale for pulse determinations of deposition rate profiles in a rapidly growing corn root.     

Longitudinal Water Movement in the Primary Root of Zea mays

The rates of transfer of tritiated water (THO) along lengthsof excised primary roots of Zea mays have been measured undera variety of conditions. The following values of ‘apparentdiffusion coefficients’ for THO in the root tissue havebeen evaluated: 1.5±0.1x10^-5 cm² sec^-1 in roots boiledfor 3 min before use,0.5±0.03x10^-5 cm² sec^-1 in rootspoisoned with 10^-2 M NaF,0.9±0.07x10^-5 cm² sec^-1 in rootspoisoned with 10^-2 M NaN₃,and 2.1±0.2x10^-5 cm² sec^-1in normal roots. The bathing medium in all cases was 1.0 mMKCl/0.1 mM CaCl₂ with the addition of the inhibitors where appropriate.Thefourfold increase in the rate of THO transfer in normal rootscompared with poisoned ones is attributed to the existence ofa long-distance convective flow in the first case, which isterminated by the addition of inhibitors. Since experimentsshow that this convective flow must occur both acropetally andbasipetally with equal velocity, it is thought to occur in thephloem.By assuming the ‘streaming transcellular strands’model for phloem transport, the rate of movement required togive the observed transfer has been computed as approximately4.5x10^-2 cm sec^-1 (160 cm h^-1).The earlier report of the existenceof a highly impermeable barrier surrounding the xylem vesselshas been further substantiated by the experiments reported here.     

Springback in Root Gravitropism

Conditions under which a gravistimulus of Merit corn roots (Zea mays L.) is withdrawn result in a subsequent loss of gravitropic curvature, an effect which we refer to as `springback.' This loss of curvature begins within 1 to 10 minutes after removal of the gravistimulus. It occurs regardless of the presence or absence of the root cap. It is insensitive to inhibitors of auxin transport (2,3,5-triiodobenzoic acid, naphthylphthalmaic acid) or to added auxin (2,4-dichlorophenoxyacetic acid). Springback is prevented if a clinostat treatment is interjected to neutralize gravistimulation during germination, which suggests that the change in curvature is a response to a `memory' effect carried over from a prior gravistimulation.     

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.     

A Morphometric Analysis of the Ultrastructure of Columella Statocytes in Primary Roots of Zea mays L.

A morphometric analysis of the ultrastructure of columella statocytesin primary roots of Zea mays was performed to determine theprecise location of cellular organelles in graviperceptive cells.Vacuoles occupy the largest volume in the cell (11.4 per centof the protoplasm). The nucleus (9.51 per cent), amyloplasts(7.57 per cent), mitochondria (3.42 per cent), spherosomes (2.13per cent) and dictyosomes (0.55 per cent) occupy progressivelysmaller volumes of the statocytes. All organelles are distributedasymmetrically within the cell. Amyloplasts, spherosomes anddictyosomes are found in greatest numbers (and relative volumes)in the lower (i.e. ‘bottom’) third of the cell.The largest numbers and relative volumes of mitochondria arein the lower and middle thirds of the cell. Nuclei tend to befound in the middle third of the statocytes. Only the hyaloplasmis concentrated in the upper (i.e. ‘top’) thirdof Z. mays statocytes. When the sedimentation of amyloplasts(and the resulting exclusion of other organelles from the lowerthird of the cell) is corrected for, all cellular constituentsremain asymmetrically distributed within the cell. Therefore,the sedimentation of amyloplasts alone is not responsible forthe differential distribution of other cellular organelles inZ. mays statocytes. The quantitative ultrastructure of Z. maysstatocytes is discussed relative to the graviperceptive functionof these cells. Zea mays, corn, maize, root cap, stereology, columella, statocytes, graviperception, ultrastructure     

Hypoxic Induction of Anoxia Tolerance in Root Tips of Zea mays

When root tips of fully aerobic, intact maize (Zea mays L.) seedlings are made anaerobic, viability normally is only 24 hours or less at 25°C. We find that viability can be extended to at least 96 hours if seedlings are given a hypoxic pretreatment for 18 hours by sparging the solution with 4% O₂ in nitrogen (v/v) before anoxia. Fully aerobic root tips (sparged with 40% O₂) had very low alcohol dehydrogenase (ADH) activity (per gram root fresh weight), and the level remained low under anoxia. In hypoxically pretreated roots, however, high levels of ADH activity were induced, and activity rose further during the initial 24 hours of anoxia, and then remained high at about 20 times that of controls in 40% O₂. ADH activity in roots in solution sparged with air (21% O₂) was about three times that in 40% O₂. Improved viability of hypoxically pretreated root tips was associated with maintenance of a high energy metabolism (ATP concentration, total adenylates, and adenylate energy charge). Roots that were not pretreated lost 94% of the total adenylates and ATP at 24 hours of anoxia. The relation between induced ADH activity, energy metabolism, and improved anoxia-tolerance in acclimated maize root tips is discussed.     

Free Radicals in the Root System and Stem of Zea mays

The concentration of free radicals in different parts of theroot system and the stem of 130, 158 and 165 d old maize plantswere measured. The highest concentrations of free radicals werefound in the tissue of the primary root and in the adventitiousroots of the 1st and 2nd nodes. In the stem tissues, the highestconcentration of free radicals was found in the tissues of the14th to 15th internode. The concentration of free radicals inthe tissues of the nodal roots decreased with increasing nodenumber, whereas in the tissues of the overground stem it increasedwith internode number. This pattern may be correlated with themetabolic activity of tissues.     

Chimeras and the Origin of Lateral Root Primordia in Zea mays

The difficulty of determining the contribution made by the pericycleand endodermis of mother roots to lateral primordia in the Gramineaehas been solved by inducing polyploid chimeras at initiation. The endodermis forms a layer covering the primordium, but thislayer does not form the epidermis of the lateral. It does formthe root cap of the young primordium, but this is replaced ata variable stage of development by the quiescent centre donatinga new set of cap initials of pericyclic origin. Reasons forthe previous diversity of interpretations are presented. chimera, lateral root primordia, Zea mays     

Inhibition of Acid-Enhanced Elongation of Zea mays Root Segments by Galactose

The effect of sugars and metabolic inhibitors on the elongation of Zea mays root segments was analyzed by a rhizometer which records the elongation of each of 32 root segments at the same time. Galactose suppressed the acid-enhanced rapid elongation after a lag period of 1.5 hours, but it did not inhibit the slow elongation at pH 7. Mannose was less inhibitory than galactose. Arabinose, xylose, glucose, sucrose, mannitol, and sorbitol caused no inhibition. When galactose was removed after a 1-hour treatment, the elongation was partially recovered. Cycloheximide and 2-deoxyglucose suppressed acid-enhanced elongation when these were applied at the same time as acid treatments, whereas cordycepin (3′-deoxyadenosine) inhibited elongation only if it was applied prior to acid treatment. Over the 9-hour period of elongation studied, the inhibition by galactose was comparable to that of cycloheximide. Since galactose has been reported to suppress the sugar metabolism necessary for the cell wall synthesis, the later phase of acid-enhanced elongation of root segments may at least partially depend on the synthesis or metabolism of cell wall components. The inhibition of root growth by galactose may be partially ascribed to a direct effect on the elongation process in roots, an effect that is enhanced by the acidification of the cell walls.     

The Ultrastructure of the Regenerating Root Cap of Zea mays L

Removal of the cap from the primary root of Zea mays activatescell division in the quiescent centre. It is the descendentsof these cells that eventually regenerate a new cap—aprocess that is complete in about 4 days at 23 °C. The ultrastructureof the cells of the regenerating cap was examined at daily intervals.During the first day after decapping the dictyosomes in theexposed outer layer of cells change from a relatively quiescentstate to one where they are secreting a polysaccharide slimewhich accumulates between the plasmalemma and the outer cellwall. Amyloplasts grow in size and appear to divide, and theendoplasmic reticulum proliferates. Many different cytoplasmicfeatures that are normally characteristic of cells in distinctlocations within the undisturbed cap occur, at first, all togetherwithin the few cells that are the source of the new regeneratingtissue. Regeneration of a normal structure in the new cap isachieved by progressive changes in the structures of the cellorganelles, apparently in response to the position that thecells containing them occupy within the growing cellular ensembleat the root apex. Zea mays, regeneration, root cap, ultrastructure     

Localization of Acid Hydrolase Activity in Zea mays L. Root Tips

Naphthol AS-BI phosphatase, esterase, aryl sulphatase, glucuronidase,and ß-glycerophosphatase have been studied in frozensections of maize root tips. In general these enzymes showedhighest activities at the root surface and at particulate sitesin the cytoplasm although the indigogenic method for esteraseshowed no particulate activity and the naphthol AS-D acetatereaction gave no pronounced surface activity. With electronmicroscopy highest activity for ß-glycerophosphatasewas observed in the cell walls and associated with the vacuoles.The significance of these observations are discussed in relationto the function of surface hydrolytic activity and to the presenceof lysosome-like bodies in higher plant cells.     

A Correlation between Structure and Function in the Root of Zea mays

The exudation rates of fluid and potassium ions from isolatedmaize roots were determined before and after excision of certainlengths of root tip. The results of this study suggest thatexcised maize roots possess the ability to absorb potassium(and presumably chloride) ions and concomitant amounts of waterover a considerable distance (10 cm) from the tip. Moreover,the observed power of absorption of ions and water into thetranslocatory pathway decreases in passing from the tip towardsthe base of the root. Both light and electron microscope techniques were used to examinethe anatomy of primary roots similar to those used in the physiologicalexperiments. The principal observation was that the xylem vesselsnear the root tip contain membrane-bounded cytoplasm with organelles.The number of mature xylem vessels, i.e. without cytoplasm,progressively increased in transverse sections cut from 1 to10 cm from the root tip; above 10 cm from the root tip all ofthe xylem vessels were found to be completely mature. It isevident that prima facie a connexion exists between this singleaspect of root anatomy and fluid exudation from excised roots. The uptake of tritiated water by roots and its transport intoexudates was examined. These data were analysed on the assumptionthat the exchange of external labelled water with the exudatewas achieved by the fluid exudation itself; this analysis indicatedthat an operational volume, similar to that of the total xylemvolume within the root, must become labelled during the formationof the exudate.     

Calcium-Regulated in Vivo Protein Phosphorylation in Zea mays L. Root Tips

Calcium dependent protein phosphorylation was studied in corn (Zea mays L.) root tips. Prior to in vivo protein phosphorylation experiments, the effect of calcium, ethyleneglycol-bis-(β-aminoethyl ether)-N-N′-tetraacetic acid (EGTA) and calcium ionophore (A-23187) on phosphorus uptake was studied. Calcium increased phosphorus uptake, whereas EGTA and A-23187 decreased it. Consequently, phosphorus concentration in the media was adjusted so as to attain similar uptake in different treatments. Phosphoproteins were analyzed by two-dimensional gel electrophoresis. Distinct changes in phosphorylation were observed following altered calcium levels. Calcium depletion in root tips with EGTA and A-23187 decreased protein phosphorylation. However, replenishment of calcium following EGTA and ionophore pretreatment enhanced phosphorylation of proteins. Preloading of the root tips with ³²P in the presence of EGTA and A-23187 followed by a ten minute calcium treatment, resulted in increased phosphorylation indicating the involvement of calcium, calcium and calmodulin-dependent protein kinases. Calmodulin antagonist W-7 was effective in inhibiting calcium-promoted phosphorylation. These studies suggest a physiological role for calcium-dependent phosphorylation in calcium-mediated processes in plants.