Graviresponsiveness and the Development of Columella Tissue in Primary and Lateral Roots of Ricinus communis

Half-tipped primary and lateral roots of Ricinus communis cv Hale bend toward the side of the root on which the intact half-tip remains. Therefore, the minimal graviresponsiveness of lateral roots is not due to the inability of their caps to produce growth effectors (presumably inhibitors). The columella tissues of primary (i.e. graviresponsive) roots are (a) 4.30 times longer, (b) 2.95 times wider, (c) 37.4 times more voluminous, and (d) composed of 17.2 times more cells than those of lateral roots. The onset of positive gravitropism by lateral roots is positively correlated with a (a) 2.99-fold increase in length, (b) 2.63-fold increase in width, and (c) 20.7-fold increase in volume of their columella tissues. We propose that the minimal graviresponsiveness of lateral roots is due to the small size of their columella tissues, which results in their caps being unable to (a) establish a concentration gradient of the effector sufficient to induce gravicurvature and (b) produce as much of the effector as caps of graviresponsive roots.     

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 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     

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.     

Acid-Efflux Patterns of Primary and Secondary Roots of Ricinus communis (Euphorbiaceae)

Graviresponding primary roots of castor bean (Ricinus communis)were characterized by more acid efflux on their upper (i.e.rapidly growing) sides than their lower sides. Acid-efflux patternsof the upper and lower sides of horizontally oriented lateralroots were symmetrical. The onset of graviresponsiveness bysecondary roots correlated positively with the development ofasymmetric patterns of acid efflux similar to those of gravirespondingprimary roots. The addition of 1 mM sodium orthovanadate (aninhibitor of auxin-induced proton efflux) to the growth mediumabolished gravicurvature and the development of acid-effluxasymmetry by primary and secondary roots. These results suggestthat (i) the absence of an auxin-induced asymmetry of acid effluxmay be the factor responsible for uncoupling graviperceptionfrom gravitropism in lateral roots, and (ii) the developmentof an auxin-induced asymmetry of acid efflux may mediate theonset of graviresponsiveness by secondary roots of R. communis. Ricinus communis, castor bean, acid efflux, root gravitropism     

Water and Solute Transport by Exuding Root Systems of Ricinus communis

The influence of range of concentrations of potassium nitrate(C_m)in the medium surrounding the roots of decapitated Ricinuscommunis plants on: (a) the potassium concentration of the exudate(C_s), (b)the rate of exudation (fH₂O), and (c) the flux of potassiuminto the exudate (f_k) was investigated. A change in C_m couldinfluence exudation by altering the osmotic potential of themedium as will as by a change in potassium concentration, andthese two effect were separated in the following way. the effectof varying the potassium concentration was investigated usingmedia made isotonic with mannitol, whilst the osmotic effectwas investigated using mannitol solutions of various concentrationsbut of equal potassium concentration. An analysis of the resultsrevealed the existence of two salt-transfer processes, one independentof the water flux but dependent on the concentration of potassiumin the medium, the other linearly related to the flux of waterthrough the root system. The implications of these findingsin relation to the intact transpiring plant are discussed.     

ATPase and Proton Pumping Activities in Cotyledons and other Phloem-Containing Tissues2Ricinus communis

ATPase activity was investigated in phloem-containing tissuesof Ricinus communis in relation to its proposed role in phloemloading. Cytochemical staining of cotyledons revealed an ATP-hydrolysingactivity on the plasma membrane of the sieve tube/companioncell complex. Microsomal fractions prepared from cotyledonsand main veins contained a Mg²⁺-dependent ATPase activity whichshowed low stimulation by KC1 particularly at pH 6.5. The pHoptimum was at pH 8.5 to 90, although the effect of azide indicatedthe presence of mitochondrial ATPase. At pH 6.5, the cited optimumfor plasma membrane ATPase, the activity showed strong inhibitionby PCMBS, vanadate and DCCD. A high pyrophosphatase activitywas observed at pH 8.5. Acidification of the medium by intactcotyledons was increased by fusicoccin and inhibited by PCMBS,NEM and vanadate. Proton pumping by microsomal vesicles as measuredby quinacrine fluorescence was also inhibited by PCMBS, NEMand vanadate. Sucrose uptake by cotyledon discs showed stronginhibition by PCMBS, NEM and CCCP but was little affected byvanadate. Sucrose uptake varied with the developmental stageof the cotyledons and this correlated with microsomal ATPaseactivity measured at pH 6.5, although the precise cellular originof this activity is not certain. The results are discussed inrelation to the role of ATPase activity and proton pumping inphloem loading. Key words: ATPase, phlocm loading, proton pumping, Ricinus communis, sucrose     

Active and Passive Transport of the Major Nutrient Ions across the Root of Ricinus communis

Mesurements have been made of the electrical potential differncebetween the exuding sap of detopped castor-oil plants and anutrient solution bathing the roots. The initial concentrationsof the major nutrient ions in the root medium were (m.equiv./I.):K 0.7, Ca 0.8, Mg 0.4, NO₂ 1.4, So₄ 0.4, and phosphate 0.2,when present, Na was at a level of 0.3 m.equiv./I. and Cl at0.1 m.equiv./I. The concentration of each of these ions in thesap was of the order of 10 times that in the external solution,was —50 mV. Comparison of the measured potential differencewith the Nernst potential calculated for each ion incicatesthat, under the experimental conditions used here, the influxof K, Na, Ca, and Mg is a passive process while the transportof No₃, Cl, SO₄, H₂PO₄ and HPO₄ is in each case an active processagainst the electrochemical potential gradient. There is no evidence that a K—Na pump is working in theseroots although, in the absence of estimates for flux ratios,an outward ‘pump’ is not ruled out for Na, or Caand Mg. K is the only ion which appears to be at equilibriumconcentration in the sap.     

Regulation of Mitosis in Roots by their Caps

WHEN the meristematic cells of root tips are damaged by surgery, X-rays or various chemical treatments, the cells of the quiescent centre (Fig. 1) are stimulated into mitosis. These cells normally have an average rate of mitosis about one-tenth of that of their neighbours more than half of them are not in mitotic cycles and, of those that are, some have a “fast” cycle three or four times as long as those of their neighbours¹. After X-irradiation there is an immediate response in the quiescent centre that suggests that the balance in cell proliferation between different regions of the meristem is not a simple competition effect. One stimulatory action is the removal of the root cap² and I have now found that the cap exerts a complex effect on mitosis in the rest of the meristem.     

Growth and Graviresponsiveness of Primary Roots of Zea mays Seedlings Deficient in Abscisic Acid and Gibberellic Acid

Moore, R. and Dickey, K. 1985. Growth and graviresponsivenessof primary roots of Zea mays seedlings deficient in abscisicacid and gibberellic acid.—J. exp. Bot. 36: 1793–1798. The objective of this research was to determine if gibberellicacid (GA) and/or abscisic acid (ABA) are necessary for graviresponsivenessby primary roots of Zea mays. To accomplish this objective wemeasured the growth and graviresponsiveness of primary rootsof seedlings in which the synthesis of ABA and GA was inhibitedcollectively and individually by genetic and chemical means.Roots of seedlings treated with Fluridone (an inhibitor of ABAbiosynthesis) and Ancymidol (an inhibitor of GA biosynthesis)were characterized by slower growth rates but not significantlydifferent gravicurvatures as compared to untreated controls.Gravicurvatures of primary roots of d-5 mutants (having undetectablelevels of GA) and vp-9 mutants (having undetectable levels ofABA) were not significantly different from those of wild-typeseedlings. Roots of seedlings in which the biosynthesis of ABAand GA was collectively inhibited were characterized by gravicurvaturesnot significantly different from those of controls. These results(1) indicate that drastic reductions in the amount of ABA andGA in Z. mays seedlings do not significantly alter root graviresponsiveness,(2) suggest that neither ABA nor GA is necessary for root gravicurvature,and (3) indicate that root gravicurvature is not necessarilyproportional to root elongation. Key words: Abscisic acid, Ancymidol, Fluridone, gibberellic acid, root gravitropism, Zea mays     

Purification and characterization of Ricinus communis invertase

An invertase from Ricinus communis leaves was purified 4,400-fold. The preparation was homogeneous by criteria of gel electrophoresis, gel permeation, adsorption, and ionic exchange chromatography. One optimum pH at 3.5 was observed with crude invertase; however, purified preparations showed two optima, at pH 3.5 and 5.5. Addition of bovine serum albumin restored one maximum at pH 3.5 and elicited a 30% activation of the invertase. The effect was caused by many other proteins and by heparin, dextran sulfate, and polyvinylpyrrolidone. Fructose, fructose 1,6-diphosphate, maleic, trans-aconitic, malic, and ascorbic acids were simple competitive inhibitors of the purified enzyme. Glucose was a noncompetitive inhibitor. The activation by proteins suppressed these inhibitory effects. The minimum concentration of activator necessary to reach the maximal activation or "point of optimal activation" was always reached at a concentration of 1 X 10(-6) M, independently of the nature of the activator, when 8.6 X 10(-12) mol of enzyme were used. Apparent molecular weight determinations of the enzyme in the presence and absence of activator and molecular weight determinations based on determinations of the point of optimal activation suggested that the purified enzyme is a heptamer (Mr of 77,900, Stokes radius 32 A, frictional ration f/fo 1.1, partial specific volume 0.749 ml/g) and that the activated form is a trimer consisting of two enzyme subunits and one activator molecule. The activation was lost by dilution of the trimer. The enzyme subunit, as isolated by gel filtration in the presence of sodium dodecyl sulfate (Mr 11,000) was inactive but quickly regained activity upon removal of sodium dodecyl sulfate.     

Characterization of ATP-Dependent Fructose 6-Phosphate 1-Phosphotransferase Isozymes from Leaf and Endosperm Tissues of Ricinus communis

Plastid and cytosolic isozymes of ATP:fructose 6-phosphate 1-phosphotransferase (PFK_p and PFK_c, respectively) have been isolated from leaves and developing endosperm tissues of the castor oil plant (Ricinus communis L). Endosperm PFK_p has been purified to apparent homogeneity. Polyclonal antibodies raised against one of the four polypeptides associated with potato tuber PFK (molecular mass, 46 kilodaltons) immunoprecipitated developing endosperm and leaf PFK_p, but not PFK_c isozymes. Western blots, sodium dodecyl sulfate polyacrylamide gel electrophoresis, and analytical gel filtration show that PFK_p from developing endosperm is a 220 kilodalton homotetramer composed of 57 kilodalton subunits. Kinetic studies of leaf PFK_p and PFK_c isozymes reveal both similarities and differences to the characteristics of the respective endosperm isozymes studied previously (WJ Garland, DT Dennis [1980] Arch Biochem Biophys 204: 302-317). The immunological and kinetic data suggest that leaf and developing endosperm PFK_p are different but structurally related proteins.     

Root cooling strongly affects diel leaf growth dynamics,water and carbohydrate relations in Ricinus communis

In laboratory and greenhouse experiments with potted plants, shoots and roots are exposed to temperature regimes throughout a 24 h (diel) cycle that can differ strongly from the regime under which these plants have evolved. In the field, roots are often exposed to lower temperatures than shoots. When the root‐zone temperature in Ricinus communis was decreased below a threshold value, leaf growth occurred preferentially at night and was strongly inhibited during the day. Overall, leaf expansion, shoot biomass growth, root elongation and ramification decreased rapidly, carbon fluxes from shoot to root were diminished and carbohydrate contents of both root and shoot increased. Further, transpiration rate was not affected, yet hydrostatic tensions in shoot xylem increased. When root temperature was increased again, xylem tension reduced, leaf growth recovered rapidly, carbon fluxes from shoot to root increased, and carbohydrate pools were depleted. We hypothesize that the decreased uptake of water in cool roots diminishes the growth potential of the entire plant – especially diurnally, when the growing leaf loses water via transpiration. As a consequence, leaf growth and metabolite concentrations can vary enormously, depending on root‐zone temperature and its heterogeneity inside pots.     

The Uptake and Flow of C, N and lons between Roots and Shoots in Ricinus communis L.

Seedlings of Ricinus communis L. cultivated in quartz sand weresupplied with a nutrient solution containing either 1 mol m^–3NO^–₃ or 1 mol m^–3 NH⁺₄ as the nitrogen source. Duringthe period between 41 and 51 d after sowing, the flows of N,C and inorganic ions between root and shoot were modelled andexpressed on a fresh weight basis. Plant growth was clearlyinhibited in the presence of NH⁺₄. In the xylem sap the majornitrogenous solutes were nitrate (74%) or glutamine (78%) innitrate or ammonium-fed plants, respectively. The pattern ofamino acids was not markedly influenced by nitrogen nutrition;glutamine was the dominant compound in both cases. NH⁺₄ wasnot transported in significant amounts in both treatments. Inthe phloem, nitrogen was transported almost exclusively in organicform, glutamine being the dominant nitrogenous solute, but theN-source affected the amino acids transported. Uptake of nitrogenand carbon per unit fresh weight was only slightly decreasedby ammonium. The partitioning of nitrogen was independent ofthe form of N-nutrition, although the flow of nitrogen and carbonin the phloem was enhanced in ammonium-fed plants. Cation uptakerates were halved in the presence of ammonium and lower quantitiesof K⁺, Na⁺ and Ca²⁺ but not of Mg²⁺ were transported to theshoot. As NH⁺₄ was balanced by a 30-fold increase in chloride in thesolution, chloride uptake was increased 6-fold under ammoniumnutrition. We concluded that ammonium was predominantly assimilated inthe root. Nitrate reduction and assimilation occurred in bothshoot and root. The assimilation of ammonium in roots of ammonium-fedplants was associated with a higher respiration rate. Key words: Ricinus communis, nitrogen nutrition (nitrate/ammonium), phloem, xylem, transport, partitioning, nitrogen, carbon, potassium, sodium, magnesium, calcium, chloride     

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     

Structure and stability of Ricinus communis haemagglutinin.

The molecular properties of the haemagglutinin of Ricinus communis (RCA I or RCA 120) were evaluated by analytical ultracentrifugation, light-scattering, c.d. and fluorescence. The native molecule had a fairly expanded structure (f/f0 = 1.43) and dissociated into two subunits of equal size in 6 M-guanidinium chloride. This native structure was stable in alkali (up to pH 11) and resistant to thermal denaturation at neutrality. A pH-triggered change in the haemagglutinin conformation was observed and characterized by analytical ultracentrifugation, c.d. and fluorescence between pH 7 and 4.5, the range in which its affinity for galactosides decreased [Yamasaki, Absar & Funatsu (1985) Biochim, Biophys. Acta 828, 155-161]. These results are discussed in relation to those reported in the literature for other lectins and more especially ricin, for which a pH-dependent conformation transition has been observed in the same range of low pH.     

Characteristics of Nectar Secretion by the Extrafloral Nectaries of Ricinus communis

The characteristics of nectar secretion by excised extrafloralnectaries of Ricinus have been examined. Secreted nectar wasfound to contain three sugars: sucrose, glucose and fructose,with glucose and fructose occurring in a 1: 1 ratio. All threesugars supported secretion when used in the culture medium andthe yield of nectar sugar was found to be concentration-dependent.Other sugar sources failed to support secretion. Experimentsusing ¹⁴C-sugars and ¹⁴CO₂ fed to intact plants allowed themovement of sugars through the nectary to be examined. Sucrosesynthesis occurs when excised glands are fed glucose and thisoccurs very early in the transport through the nectary. Themain sugar transported was sucrose, with little hydrolysis occurringuntil the final step of secretion. There was no evidence thatsucrose hydrolysis occurs either by invertase in the nectaror by a microbial flora. Inhibitors of respiration were foundto inhibit secretion as did anaerobiosis. Temperature also hada marked effect, with a temperature coefficient of 1.8. However,secretion of sucrose was not affected by anaerobic conditions,low temperatures or inhibitors of respiration as markedly asthat of glucose and fructose. Electron microscopy revealed the presence of a thickened andheavily stained wall at the inner border of the secretory epidermallayer. This wall contained numerous plasmodesmata at a frequencyof 14 per µm² and may represent an apoplastic barrier.Light microscope cytochemistry revealed that acid phosphataseis primarily located in the nectiferous tissue, while ATPaseis concentrated in the epidermis. The possibility that the nectarycontains two pathways for sucrose secretion, both apoplasticand symplastic, is discussed. Key words: Invertase, nectary, plasmodesmata, Ricinus communis, sucrose     

Effects of Iodination on Cytoagglutination by and Toxicity of Ricinus communis Lectins

Iodinations of two Ricinus communis lectins, ricin D and hemagglutinin (CBH), with potassium iodide at pH 7.0 and 0°C led to inactivation of the cytoagglutinating activity on sarcoma 180 ascites tumor cells as well as the toxicity to HeLa cells of ricin D, whereas the cytoagglutinating activity of CBH was affected slightly. In the presence of lactose, which binds to ricin D, one tyrosyl residue in the B-chain of ricin D was protected from iodination and 40% of the cytoagglutinating activity was retained. This protection against iodination was not observed in the presence of glucose, which does not bind to ricin D. This suggested that the protected tyrosyl residue in the B-chain of ricin D may be situated at or near the saccharide binding site and directly involved in the binding to the saccharide moieties of the cellular receptors.

Adsorption of the iodinated ricin D to Sepharose 4B indicated that one of the two saccharide binding sites in ricin D is still intact and participates in the binding to saccharide: ricin D was altered from divalent to monovalent by the iodination.

We found from binding experiments with ¹²⁵I-labeled iodinated ricin D to HeLa cells, that the low toxicity of the iodinated ricin D may be attributed mainly to its decreased internalization into the cells and that the divalent binding of ricin D to the cellular receptors is important for this internalization.