Structure of Columella Cells in Primary and Lateral Roots of Ricinus communis (Euphorbiaceae)

Horizontally oriented primary roots of Ricinus communis aremore graviresponsive than similarly oriented lateral roots.The more pronounced graviresponsiveness of primary roots ispositively correlated with their caps having a more extensivecolumella tissue than caps of lateral roots. Individual columellacells of primary roots contain 2.6 times more protoplasm thando columella cells of lateral roots. Similarly, the absolutevolumes of all cellular components in columella cells of primaryroots are larger than those of lateral roots. However, thereare no statistically significant differences in the relativevolumes of any cellular component in columella cells of primaryvs lateral roots. Endoplasmic reticulum is distributed randomlyin columella cells of both types of roots. Columella cells ofprimary and lateral roots contain numerous sedimented amyloplastswhich do not consistently contact any cellular structure. Nucleitend to be located in the middle thirds of the columella cells,and the vacuole is found in largest concentrations in the middleand upper thirds of columella cells of both types of roots.The largest protoplasmic volumes of mitochondria occur in thelower thirds of columella cells, and dictyosomes are found insimilar concentrations throughout the cells. There is no significantdifference in the intracellular distributions of organellesin columella cells of primary vs lateral roots. We believe thatthe differing graviresponsiveness of primary vs lateral rootsof R. communis is probably due to factors other than the structuresof their individual columella cells. Ricinus communis, columella, graviperception, graviresponsiveness, roots, root cap     

Calcium Movement, Graviresponsiveness and the Structure of Columella Cells and Columella Tissues in Roots of Allium cepa L.

Roots of Allium cepa L. cv. Yellow are differentially responsiveto gravity. Long (e.g. 40 mm) roots are strongly graviresponsive,while short (e.g. 4 mm) roots are minimally responsive to gravity.Although columella cells of graviresponsive roots are largerthan those of nongraviresponsive roots, they partition theirvolumes to cellular organelles similarly. The movement of amyloplastsand nuclei in columella cells of horizontally-oriented rootscorrelates positively with the onset of gravicurvature. Furthermore,there is no significant difference in the rates of organellarredistribution when graviresponsive and nongraviresponsive rootsare oriented horizontally. The more pronounced graviresponsivenessof longer roots correlates positively with (1) their caps being9.6 times more voluminous, (2) their columella tissues being42 times more voluminous, (3) their caps having 15 times morecolumella cells, and (4) their columella tissues having relativevolumes 4·4 times larger than those of shorter, nongraviresponsiveroots. Graviresponsive roots that are oriented horizontallyare characterized by a strongly polar movement of ⁴⁵Ca²⁺ acrossthe root tip from the upper to the lower side, while similarlyoriented nongraviresponsive roots exhibit only a minimal polartransport of ⁴⁵Ca²⁺. These results indicate that the differentialgraviresponsiveness of roots of A. cepa is probably not dueto either (1) ultrastructural differences in their columellacells, or (2) differences in the rates of organellar redistributionwhen roots are oriented horizontally. Rather, these resultsindicate that graviresponsiveness may require an extensive columellatissue, which, in turn, may be necessary for polar movementof ⁴⁵Ca²⁺ across the root tip. Allium cepa, onion, root, columella tissue, columella cell, gravitropism, calcium, ultrastructure     

Root Graviresponsiveness and Cellular Differentiation in Wild-type and a Starchless Mutant of Arabidopsis thaliana

Primary roots of a starchless mutant of Arabidopsis thalianaL. are strongly graviresponsive despite lacking amyloplastsin their columella cells. The ultrastructures of calyptrogenand peripheral cells in wild-type as compared to mutant seedlingsare not significantly different. The largest difference in cellulardifferentiation in caps of mutant and wild-type roots is therelative volume of plastids in columella cells. Plastids occupy12.3% of the volume of columella cells in wild-type seedlings,but only 3.69% of columella cells in mutant seedlings. Theseresults indicate that: (1) amyloplasts and starch are not necessaryfor root graviresponsiveness; (2) the increase in relative volumeof plastids that usually accompanies differentiation of columellacells is not necessary for root graviresponsiveness; and (3)the absence of starch and amyloplasts does not affect the structureof calyptrogen (i.e. meristematic) and secretory (i.e. peripheral)cells in root caps. These results are discussed relative toproposed models for root gravitropism. Arabidopsis thaliana, gravitropism (root), plastids, root cap, stereology, ultrastructure     

Graviresponsiveness and columella cell structure in primary and secondary roots of Ricinus communis

In order to determine what structural changes are associated with the onset of graviresponsiveness by plant roots, we have monitored the quantitative ultrastructures of columella (i.e., graviperceptive) cells in primary and secondary roots of Ricinus communis. The relative volumes of cellular components in lateral (i.e., minimally graviresponsive) roots were not significantly different from those of primary roots. The relative volumes of cellular components in secondary roots growing laterally were not significantly different from those of graviresponsive secondary roots. Therefore, the onset of graviresponsiveness by secondary roots of R. communis is not correlated with changes in organellar concentrations in columella cells. These results are discussed relative to a model for the differential graviresponsiveness of plant roots.     

Characterization of root agravitropism induced by genetic, chemical, and developmental constraints

The patterns and rates of organelle redistribution in columella (i.e., putative statocyte) cells of agravitropic agt mutants of Zea mays are not significantly different from those of columella cells in graviresponsive roots. Graviresponsive roots of Z. mays are characterized by a strongly polar movement of 45Ca2+ across the root tip from the upper to the lower side. Horizontally-oriented roots of agt mutants exhibit only a minimal polar transport of 45Ca2+. Exogenously-induced asymmetries of Ca result in curvature of agt roots toward the Ca source. A similar curvature can be induced by a Ca asymmetry in normally nongraviresponsive (i.e., lateral) roots of Phaseolus vulgaris. Similarly, root curvature can be induced by placing the roots perpendicular to an electric field. This electrotropism increased with 1) currents between 8-35 mA, and 2) time between 1-9 hr when the current is constant. Electrotropism is reduced significantly by treating roots with triiodobenzoic acid (TIBA), an inhibitor of auxin transport. These results suggest that 1) if graviperception occurs via the sedimentation of amyloplasts in columella cells, then nongraviresponsive roots apparently sense gravity as do graviresponsive roots, 2) exogenously-induced asymmetries of a gravitropic effector (i.e., Ca) can induce curvature of normally nongraviresponsive roots, 3) the gravity-induced downward movement of exogenously-applied 45Ca2+ across tips of graviresponsive roots does not occur in nongraviresponsive roots, 4) placing roots in an electrical field (i.e., one favoring the movement of ions such as Ca2+) induces root curvature, and 5) electrically-induced curvature is apparently dependent on auxin transport. These results are discussed relative to a model to account for the lack of graviresponsiveness by these roots.     

Dimensions of Root Caps and Columella Tissues of Primary Roots of Ricinus communis Characterized by Differing Degrees of Graviresponsiveness

Primary roots of Ricinus communis having large caps and columellatissues are more graviresponsive than primary roots with smallcaps and columella tissues. The increased graviresponsivenessof roots with larger caps correlates positively with their columellatissues having larger length: width ratios than less graviresponsiveroots having smaller caps. Roots with wider tips typically aremore graviresponsive and have more extensive columellas thanroots with thinner tips. However, the size of the columellatissue correlates positively with graviresponsiveness, irrespectiveof the width of the root tip. These results indicate that differingdimensions of the columella tissue may be the basis for thediffering graviresponses of primary roots of R. communis. Root gravitropism, columella, root cap, primary root, Ricinus communis, castor bean     

Inducing Gravitropic Curvature of Primary Roots of Zea mays cv Ageotropic

Primary roots of the mutant `Ageotropic' cultivar of Zea mays are nonresponsive to gravity. Their root caps secrete little or no mucilage and touch the root only at the extreme apex. A gap separates the cap and root at the periphery of the cap. Applying mucilage from normal roots or substances with a consistency similar to that of mucilage to tips of mutant roots causes these roots to become strongly graviresponsive. Gravicurvature stops when these substances are removed. Caps of some mutants secrete small amounts of mucilage and are graviresponsive. These results indicate that (a) the lack of graviresponsiveness in the mutant results from disrupting the transport pathway between the cap and root, (b) movement of the growth-modifying signal from the cap to the root occurs via an apoplastic pathway, and (c) mucilage is necessary for normal communication between the root cap and root in Zea mays cv Ageotropic.     

Inhibition of gravitropism in primary roots of Zea mays by chloramphenicol

Primary roots of Zea mays seedlings germinated and grown in 0.1 mM chloramphenicol (CMP) were significantly less graviresponsive than primary roots of seedlings germinated and grown in distilled water. Elongation rates of roots treated with CMP were significantly greater than those grown in distilled water. Caps of control and CMP-treated roots possessed extensive columella tissues comprised of cells containing numerous sedimented amyloplasts. These results indicate that the reduced graviresponsiveness of CMP-treated roots is not due to reduced rates of elongation, the absence of the presumed gravireceptors (i.e., amyloplasts in columella cells), or reduced amounts of columella tissue. These results are consistent with CMP altering the production and/or transport of effectors that mediate gravitropism.     

CALCIUM MOVEMENT,GRAVIRESPONSIVENESS, AND THE STRUCTURE OF COLUMELLA CELLS IN PRIMARY ROOTS OF AMYLOMAIZE MUTANTS OF ZEA MAYS

Primary roots of Zea mays cv. Amylomaize were less graviresponsive than primary roots of the wild-type Calumet cultivar. There were no significant differences in: 1) the partitioning of volume to organelles in columella cells, 2) the size or density of amyloplasts, or 3) rates and overall patterns of organelle redistribution in horizontally-oriented roots of the two cultivars. Amyloplasts and nuclei were the only organelles whose movement correlated positively with the onset of root gravicurvature. However, the onset of gravicurvature was not directly proportional to the average sedimentation rate of amyloplasts, since amyloplasts sedimented at equal rates in columella cells of both cultivars despite their differences in root gravicurvature. The more graviresponsive roots of Calumet seedlings were characterized by a more strongly polar movement of ⁴⁵Ca²⁺ from the upper to lower sides of their root tips than the less graviresponsive roots of Amylomaize seedlings. These results suggest that the decreased graviresponsiveness of horizontally-oriented roots of Amylomaize seedlings may be due to a delay in or decreased ability for polar transport of calcium rather than to smaller, more slowly sedimenting amyloplasts as has been suggested for their less graviresponsive coleoptiles.     

A morphometric analysis of the redistribution of organelles in columella cells in primary roots of normal seedlings and agravitropic mutants of Hordeum vulgare

The redistribution of organelles in columella cells of horizontally-oriented roots of Hordeum vulgare was quantified in order to determine what structural changes in graviperceptive (i.e., columella) cells are associated with the onset of the root gravicurvature. The sedimentation of amyloplasts is the only major change in cellular structure that correlates positively with the onset of root gravicurvature, which begins within 15 min after re-orientation. There is no consistent contact between sedimented amyloplasts and any other organelles. Nuclei are restricted to the proximal ends of columella cells in vertically-oriented roots, and remain there throughout gravicurvature after roots are oriented horizontally. Root gravicurvature does not involve significant changes in (1) the volume of columella cells, (2) the relative or absolute volumes of organelles in columella cells, or (3) the distribution of endoplasmic reticulum (ER). The size, number and sedimentation rates of amyloplasts in columella cells of non-graviresponsive roots of mutant seedlings are not significantly different from those of graviresponsive roots of normal seedlings. Similarly, there is no significant difference in (1) cellular volume, (2) distribution or surface area of ER, (3) patterns or rates of organelle redistribution in horizontally-oriented roots, (4) relative or absolute volumes of organelles in columella cells of graviresponsive and non-graviresponsive roots. These results suggest that the lack of graviresponsiveness by roots of mutant seedlings is probably not due to either (1) structural differences in columella cells, or (2) differences in patterns or rates of organelle redistribution as compared to that characteristic of graviresponsive roots. Thus, the basis of non-graviresponsiveness in this mutant is probably different from other agravitropic mutants so far studied.     

Cytochemical localization of calcium in cap cells of primary roots of Zea mays L

The distribution of calcium (Ca) in caps of vertically- and horizontally-oriented roots of Zea mays was monitored to determine its possible role in root graviresponsiveness. A modification of the antimonate precipitation procedure was used to localize Ca in situ. In vertically-oriented roots, the presumed graviperceptive (i.e., columella) cells were characterized by minimal and symmetric staining of the plasmalemma and mitochondria. No precipitate was present in plasmodesmata or cell walls. Within 5 min after horizontal reorientation, staining was associated with the portion of the cell wall adjacent to the distal end of the cell. This asymmetric staining persisted throughout the onset of gravicurvature. No staining of lateral cell walls of columella cells was observed at any stage of gravicurvature, suggesting that a lateral flow of Ca through the columella tissue of horizontally-oriented roots does not occur. The outermost peripheral cells of roots oriented horizontally and vertically secrete Ca through plasmodesmata-like structures in their cell walls. These results are discussed relative to proposed roles of root-cap Ca in root gravicurvature.     

Cytochemical Localization of Calcium in Cap Cells of Primary Roots of Zea mays L.

The distribution of calcium (Ca) in caps of vertically- andhorizontally-oriented roots of Zea mays was monitored to determineits possible role in root graviresponsiveness. A modificationof the antimonate precipitation procedure was used to localizeCa in situ. In vertically-oriented roots, the presumed graviperceptive(i.e., columella) cells were characterized by minimal and symmetricstaining of the plasmalemma and mitochondria. No precipitatewas present in plasmodesmata or cell walls. Within 5 min afterhorizontal reorientation, staining was associated with the portionof the cell wall adjacent to the distal end of the cell. Thisasymmetric staining persisted throughout the onset of gravicurvature.No staining of lateral cell walls of columella cells was observedat any stage of gravicurvature, suggesting that a lateral flowof Ca through the columella tissue of horizontally-orientedroots does not occur. The outermost peripheral cells of rootsoriented horizontally and vertically secrete Ca through plasmodesmata-likestructures in their cell walls. These results are discussedrelative to proposed roles of root-cap Ca in root gravicurvature. Key words: Antimonate, calcium, columella cell, peripheral cell, root gravitropism, Zea mays L.     

Cellular Differentiation and Tissue Partitioning in Caps of Primary and Lateral Roots of Helianthus annuus

Cellular and tissue volumes in caps of primary and lateral rootsof Helianthus annuus have been measured in order to determinequantitatively how tissues and their functions are partitionedin root caps. Patterns of change in cellular dimensions andvolumes are similar in caps of primary and lateral roots. Significantincreases in cellular dimensions and volume occur during thedifferentiation of columella cells and the innermost peripheralcells. There are no significant changes in cellular dimensionsas either (i) the production and secretion of mucilage begins,or (ii) cells are sloughed from the cap. Tissues are partitionedsimilarly in caps of primary and lateral roots. indeed, rootcaps allocate 7–8 per cent of their volume for regeneration(i.e. calyptrogen tissue), 16–19 per cent of their volumefor graviperception (i.e. columella tissue), and approx. 38per cent of their volume for the production and secretion ofmucilage. These results are discussed relative to patterns ofcellular differentiation and tissue function in root caps. Helianthus annuus, root caps, primary root, lateral root, calyptrogen, columella, peripheral cells, tissue partitioning     

Cellular differentiation in root caps of Zea mays that do not secrete mucilage

We quantified the structural changes accompanying cellular differentiation in root caps of Zea mays cv. Ageotropic to determine the developmental basis for the nongraviresponsiveness of their primary roots. Cells of the calyptrogen and columella of primary roots of the ageotropic mutant have structures indistinguishable from those of caps of primary roots of Z. mays cv. Kys the graviresponsive, wild-type parent of Z. mays cv. Ageotropic. However, the relative volumes of dictyosomes, dictyosome-derived vesicles and starch in the outermost peripheral cells of wild-type roots were significantly lower than were those in peripheral cells of mutant roots. This corresponds to a dramatic accumulation of starch and mucilage-filled vesicles in peripheral cells of mutant roots. Cellular differentiation in root caps of graviresponsive seminal roots of the Ageotropic mutant resembled that of primary and seminal roots of the wild-type cultivar, and differed significantly from that of primary roots of the mutant. We conclude that the mutation that blocks secretion of mucilage from peripheral cells of Ageotropic roots: (1) expresses itself late in cellular differentiation in root caps; (2) is expressed only in primary (but not seminal) roots of the Ageotropic mutant; and (3) is consistent with malfunctioning dictyosomes and dictyosome-derived vesicles being the cellular basis for agravitropism of primary roots of this mutant.     

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     

A Morphometric Analysis of the Redistribution of Organelles in Columella Cells in Primary Roots of Normal Seedlings and Agravitropic Mutants of Hordeum vulgare

Moore, R. 1985. A morphometric analysis of the redistributionof organellcs in columella cells in primary roots of normalseedlings and agravitropic mutants of Hordeum vulgare.—J.exp. Bot. 36:1275–1286. The redistribution of organeUes m columella cells of horizontally-orientedroots of Hordeum vulgare was quantified in order to determinewhat structural changes in graviperceptive (i.e, columella)cells are associated with the onset of root gravicurvature.The sedimentation of amyloplasts is the only major change incellular structure that correlates positively with the onsetof root gravicurvature, which begins within 15 min after re-orientation.There is no consistent contact between sedimented amyloplastsand any other organelles. Nuclei are restricted to the proximalends of columella cells in vertically-oriented roots, and remainthere throughout gravicurvature after roots are oriented horizontally.Root gravicurvature does not involve significant changes in(1) the volume of columella cells, (2) the relative or absolutevolumes of organelles in columella cells, or (3) the distributionof endoplasmic reticulum (ER). The size, number and sedimentationrates of amyloplasts in columella cells of non-graviresponsiveroots of mutant seedlings are not significantly different fromthose of graviresponsive roots of normal seedlings. Similarly,there is no significant difference in (1) cellular volume, (2)distribution or surface area of ER, (3) patterns or rates oforganelle redistribution in horizontally-oriented roots, or(4) relative or absolute volumes of organelles in columellacells of graviresponsive and non-graviresponsive roots. Theseresults suggest that the lack of gravi-responsiveness by rootsof mutant seedlings is probably not due to either (1) structuraldifferences in columella cells, or (2) differences in patternsor rates of organelle redistribution as compared to that characteristicof graviresponsive roots. Thus, the basis of non-graviresponsivenessin this mutant is probably different from other agravitropicmutants so far studied. Key words: Agravitropic mutant, barley, columella cell, gravitropism (root), Hordeum vulgare, ultrastructure     

Root Gravitropism in a Cultivar of Zea mays Whose Columella Cells Contain Starch-deficient Amyloplasts

Starch occupies 4.2 per cent of the volume of plastids in calyptrogencells in primary roots of Zea mays L. cv. vp-7 wild type. Plastidsin calyptrogen cells are distributed randomly around large,centrally located nuclei. The differentiation of calyptrogencells into columella cells is characterized by cellular enlargementand the sedimentation of plastids to the bottom of the cells.Although sedimented plastids in columella cells do not containsignificantly more starch than those in calyptrogen cells, primaryroots are graviresponsive. The onset of root gravicurvatureis not associated with a significant change in the distributionof plastids in columella cells. These results indicate thatin this cultivar of Z. mays (1) the sedimentation of plastidsin columella cells is not based upon their increased densityresulting from increased starch content alone, (2) starch-ladenamyloplasts need not be present in columella cells for rootsto be graviresponsive, and (3) the onset of root gravicurvaturedoes not require a major redistribution of plastids in columellacells. Columella cell, gravitropism (root), plastids, root cap, Zea mays     

A morphometric analysis of cellular differentiation in caps of primary and lateral roots of Helianthus annuus

In order to determine if patterns of cell differentiation are similar in primary and lateral roots, I performed a morphometric analysis of the ultrastructure of calyptrogen, columella, and peripheral cells in primary and lateral roots of Helianthus annuus. Each cell type is characterized by a unique ultrastructure, and the ultrastructural changes characteristic of cellular differentiation in root caps are organelle specific. No major structural differences exist in the structures of the composite cell types, or in patterns of cell differentiation in caps of primary vs. lateral roots.     

The Possible Involvement of Root-cap Mucilage in Gravitropism and Calcium Movement Across Root Tips of Allium cepa L

Roots of Allium cepa L. grown in aerated water elongate rapidly,but are not graviresponsive. These roots (1) possess extensivecolumella tissues comprised of cells containing numerous sedimentedamyloplasts, (2) lack mucilage on their tips, and (3) are characterizedby a weakly polar movement of calcium (Ca) across their tips.Placing roots in humid air correlates positively with the (1)onset of gravicurvature, (2) appearance of mucilage on tipsof the roots, and (3) onset of the ability to transport Ca polarlyto the lower side of the root tip. Gravicurvature of roots previouslysubmerged in aerated water is more rapid when roots are orientedvertically for 1–2 h in humid air prior to being orientedhorizontally. The more rapid gravicurvature of these roots correlatespositively with the accumulation of mucilage at the tips ofroots during the time the roots are oriented vertically. Therefore,the onset of gravicurvature and the ability of roots to transportCa to the lower sides of their tips correlate positively withthe presence of mucilage at their tips. These results suggestthat mucilage may be important for the transport of Ca acrossroot caps. Allium cepa, root gravitropism, root mucilage, calcium, onion     

Influence of microgravity on root-cap regeneration and the structure of columella cells in Zea mays

We launched imbibed seeds and seedlings of Zea mays into outer space aboard the space shuttle Columbia to determine the influence of microgravity on 1) root-cap regeneration, and 2) the distribution of amyloplasts and endoplasmic reticulum (ER) in the putative statocytes (i.e., columella cells) of roots. Decapped roots grown on Earth completely regenerated their caps within 4.8 days after decapping, while those grown in microgravity did not regenerate caps. In Earth-grown seedlings, the ER was localized primarily along the periphery of columella cells, and amyloplasts sedimented in response to gravity to the lower sides of the cells. Seeds germinated on Earth and subsequently launched into outer space had a distribution of ER in columella cells similar to that of Earth-grown controls, but amyloplasts were distributed throughout the cells. Seeds germinated in outer space were characterized by the presence of spherical and ellipsoidal masses of ER and randomly distributed amyloplasts in their columella cells. These results indicate that 1) gravity is necessary for regeneration of the root cap, 2) columella cells can maintain their characteristic distribution of ER in microgravity only if they are exposed previously to gravity, and 3) gravity is necessary to distribute the ER in columella cells of this cultivar of Z. mays.