Root Cap and Georeaction

CIRCULATING lymphoid cells suffer alterations in a localized disease, chronic viral keratitis; we have established long term lymphoctye cultures from the blood of such patients¹. We have used capillary migration technique to study cellular immunity in the disease. Leucocytes from patients were challenged with corneal antigen and with the virus which had initiated the infection^2,3. Migration inhibition is specific for the antigen to which the animal is sensitive and on contact with antigen, sensitized lymphocytes produce migratory inhibitory factor (MIF).     

Sloughing of Root Cap Cells

Root cap cells of Zen mays sloughed into water have been countedat short intervals and after 24 h. The results have been comparedwith rates of cell production calculated from the use of thestathmokinetic agent, colchicine, on the four active tiers ofthe cap meristem. More cells are produced by the cap meristem and sloughed bythe cap when roots are grown in low densities in water or whenthe water is changed at more frequent intervals. The range atbetween 250 and 50 roots per litre is from 3000 to 7000 cellsper root per day for seedling primary roots grown continuouslyin water at 23 °C for 24 h The results are discussed in relation to previous estimatesof cap cell proliferation and the possibility of periodic sloughingof cells and slime.     

Barbiturate-induced Disorientation of Root and Shoot Geotropism in Germinating Cruciferous Seedlings

Cruciferous seedlings germinated on glass-distilled water andon barbital manifested normal root and shoot geotropisms whereastwisted roots and shoots were evident in seedlings germinatedon amobarbital and secobarbital, the roots often being completelysuspended in the air. The marked difference in germination anddevelopment behaviour of cress and white mustard seedlings germinatedon barbital compared with that on amobarbital and secobarbitalindicate that the Meyer-Overton principle of drug potency andlipid solubility was operative in both species of germinatingseedlings. Lepidium sativum L., Sinapsis alba L., seed germination, geotropism, barbiturates     

The Root Cap: Cell Dynamics, Cell Differentiation and Cap Function

The root cap is a universal feature of angiosperm, gymnosperm, and pteridophyte roots. Besides providing protection against abrasive damage to the root tip, the root cap is also involved in the simultaneous perception of a number of signals – pressure, moisture, gravity, and perhaps others – that modulate growth in the main body of the root. These signals, which originate in the external environment, are transduced by the cap and are then transported from the cap to the root. Root gravitropism is one much studied response to an external signal. In the present paper, consideration is given to the structure of the root cap and, in particular, to how the meristematic initial cells of both the central cap columella and the lateral portion of the cap which surrounds the columella are organized in relation to the production of new cells. The subsequent differentiation and development of these cells is associated with their displacement through the cap and their eventual release, as “border cells”, from the cap periphery. Mutations, particularly in Arabidopsis, are increasingly playing a part in defining not only the pattern of genetic activity within different cells of the cap but also in revealing how the corresponding wild-type proteins relate to the range of functions of the cap. Notable in this respect have been analyses of the early events of root gravitropism. The ability to image auxin and auxin permeases within the cap and elsewhere in the root has also extended our understanding of this growth response. Images of auxin distribution may, in addition, help extend ideas concerning the positional controls of cell division and cell differentiation within the cap. However, firm information relating to these controls is scarce, though there are intriguing suggestions of some kind of physiological link between the border cells surrounding the cap and mitotic activity in the cap meristem. Open questions concern the structure and functional interrelationships between the root and the cap which surmounts it, and also the means by which the cap transduces the environmental signals that are of critical importance for the growth of the individual roots, and collectively for the shaping of the root system.     

The Root Cap: Cell Dynamics, Cell Differentiation and Cap Function

The root cap is a universal feature of angiosperm, gymnosperm, and pteridophyte roots. Besides providing protection against abrasive damage to the root tip, the root cap is also involved in the simultaneous perception of a number of signals – pressure, moisture, gravity, and perhaps others – that modulate growth in the main body of the root. These signals, which originate in the external environment, are transduced by the cap and are then transported from the cap to the root. Root gravitropism is one much studied response to an external signal. In the present paper, consideration is given to the structure of the root cap and, in particular, to how the meristematic initial cells of both the central cap columella and the lateral portion of the cap which surrounds the columella are organized in relation to the production of new cells. The subsequent differentiation and development of these cells is associated with their displacement through the cap and their eventual release, as border cells, from the cap periphery. Mutations, particularly in Arabidopsis, are increasingly playing a part in defining not only the pattern of genetic activity within different cells of the cap but also in revealing how the corresponding wild-type proteins relate to the range of functions of the cap. Notable in this respect have been analyses of the early events of root gravitropism. The ability to image auxin and auxin permeases within the cap and elsewhere in the root has also extended our understanding of this growth response. Images of auxin distribution may, in addition, help extend ideas concerning the positional controls of cell division and cell differentiation within the cap. However, firm information relating to these controls is scarce, though there are intriguing suggestions of some kind of physiological link between the border cells surrounding the cap and mitotic activity in the cap meristem. Open questions concern the structure and functional interrelationships between the root and the cap which surmounts it, and also the means by which the cap transduces the environmental signals that are of critical importance for the growth of the individual roots, and collectively for the shaping of the root system. Current address (Peter W. Barlow): School of Biological Sciences, University of Bristol, Woodland Road, Bristol BS8 1UG, UK     

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.     

Root Cap Structure in Isoetes macrospora Dur

Root meristem cells of Isoetes macrosporausually have one plastidwhich is associated with the prominent nucleus, numerous ribosomesand mitochondria, and small vacuoles. During mitosis each plastidappears to replicate so that each daughter cell contains oneplastid. The cell walls of the meristem cells are traversedby numerous plasmodesmata. Central cells of the root cap lackdistally displaced plastids but have one or more amyloplastsassociated with the nucleus. These cells also contain largeprotein deposits. Peripheral root cap cells are characterizedby being vacuolated, and by possessing a few dictyosomes andprotein deposits. They appear to be sloughed infrequently. Isoetes macrospora Dur, root cap, protein bodies, ultrastructure     

Effects of Cap Analogue or Cap Removal on the Translation of Rat Brain mRNA In Vitro

Abstract: The role of cap structures in the translation of brain mRNA was examined by measuring protein biosynthesis in vitro in wheat germ and reticulocyte systems programmed by mRNA that was either untreated or oxidized by periodate or from which 5'-terminal 7-methylguanosine (m⁷G) was removed by oxidation and β -elimination. In another series of reactions, amino acid incorporation into polypeptides was measured in the absence and in the presence of varying concentrations of the cap analogue 7-methylguanosine 5'-triphosphate (pppm⁷G). The results indicated that any of the above treatments interfered with brain mRNA translation, the degree of inhibition depending on the translation system used, the concentration of mRNA, and the source of initiation factors. Homologous brain initiation factors were superior to reticulocyte factors in providing a partial relief from inhibition of translation caused by these treatments. It was also found that synthesis of the brain-specific protein S-100 was inhibited by β -elimination of mRNA, by pppm⁷G, or by the presence of capped globin mRNA, indicating that the mRNA for this protein was probably capped.     

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     

玉米根冠细胞的脱落及其对根与根际关系的影响

以光学与电子显微镜技术观察了玉米幼苗根冠细胞脱落进程与胞间关系的变化。发现随着外沿细胞的脱离,固有的胞间连丝渐次伸展、孔径扩展,进而断裂为半连丝。半连丝内侧端仍与质膜保持固有的连接;外侧端向壁外伸展,直接与根际介质沟通,从而在根冠共质体与根际质外体之间建立了动态性的单向共质联系。以成列囊泡在半连丝中存在与由断口端溢出、半连丝呈现活跃ＡＴＰ酶活性、外源不透膜示踪物向胞内原生质掺入以及原生质的穿壁等实例证明,半连丝仍具生理活性,可作为根冠共质体与根际介质间物质与信息交流的直接通道。     

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.     

The Root Cap Cuticle: A Cell Wall Structure for Seedling Establishment and Lateral Root Formation

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Electrotropism of Maize Roots : Role of the Root Cap and Relationship to Gravitropism

We examined the kinetics of electrotropic curvature in solutions of low electrolyte concentration using primary roots of maize (Zea mays L., variety Merit). When submerged in oxygenated solution across which an electric field was applied, the roots curved rapidly and strongly toward the positive electrode (anode). The strength of the electrotropic response increased and the latent period decreased with increasing field strength. At a field strength of 7.5 volts per centimeter the latent period was 6.6 minutes and curvature reached 60 degrees in about 1 hour. For electric fields greater than 10 volts per centimeter the latent period was less than 1 minute. There was no response to electric fields less than 2.8 volts per centimeter. Both electrotropism and growth were inhibited when indoleacetic acid (10 micromolar) was included in the medium. The auxin transport inhibitor pyrenoylbenzoic acid strongly inhibited electrotropism without inhibiting growth. Electrotropism was enhanced by treatments that interfere with gravitropism, e.g. decapping the roots or pretreating them with ethyleneglycol-bis-[β-ethylether]-N,N,N′,N′-tetraacetic acid. Similarly, roots of agravitropic pea (Pisum sativum, variety Ageotropum) seedlings were more responsive to electrotropic stimulation than roots of normal (variety Alaska) seedlings. The data indicate that the early steps of gravitropism and electrotropism occur by independent mechanisms. However, the motor mechanisms of the two responses may have features in common since auxin and auxin transport inhibitors reduced both gravitropism and electrotropism.     

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     

A Local Auxin Gradient Regulates Root Cap Self-Renewal and Size Homeostasis

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Root Cap Mucilage and Extracellular Calcium as Modulators of Cellular Growth in Postmitotic Growth Zones of the Maize Root Apex*

Abstract: The control of maize root growth by root cap mucilage and extracellular calcium (Ca) was examined. Special attention was paid to the influence of these factors on cellular aspects of root growth, such as cell shape and organization of the microtubular (MT) cytoskeleton. Externally supplied Ca impaired the transition of early post-mitotic cells from a more-or-less apolar mode of expansion to a strictly anisotropic mode of elongation accompanied by their more rapid growth. However, this inhibitory effect of Ca was not associated with any re-arrangement of the cortical MTs, their transverse arrays, with respect to the root axis, being maintained under these conditions. Root mucilage, collected from donor root caps and placed around root tips, exerted a similar effect on cell shapes as did externally supplied Ca. In contrast, roots grown in a medium of low Ca content, or from which the root cap mucilage was continually removed, had more elongated cell shapes in their post-mitotic growth regions when compared to the control roots. These findings are consistent with a notion that Ca is present in the root cap mucilage in physiologically relevant amounts and can mediate growth responses in both the PIG region and the apical part of the elongation zone. Integrating several known effects of Ca ions on growth at the root apex, a hypothesis is proposed that a Ca-mediated and MT-independent control of cell growth in the PIG region might be involved in morphogenetic root movements (e.g. gravitropism), and that root growth responses could be initiated by an asymmetric distribution of extracellular calcium, or root cap slime, around the growing root tip.     

Effect of Removal of a Part of the Root System on the Subsequent Growth of the Root and Shoot

Removal of up to 50 per cent. of the roots of barley and ryehas no effect on the growth-rate of the root which is the sameas in the intact plant. In contrast the growth-rate of the shootdecreases as more roots are removed. When more than 50 per cent.of the roots are removed, root growth declines but not so rapidlyas that of the shoot. Similar results are obtained by the removalof lateral roots of tomato but root growth begins to declinewhen 40 per cent. of the lateral roots are removed. The uptake of potassium by barley plants with proportions ofthe root system excised is closely proportional to the dry-matterincrease when the nutrient supply is not limiting. In conditionsof low nutrition the potassium uptake is less than the dry-matterincrease.