The Development of Cells in the Growing Zones of the Root

1. Data are presented which show the change in volume, dry weight,respiration, and protein content of the cell during developmentfrom the meristematic to the mature fully extended state.
2. Thedata have been obtained by making serial sections of knownthicknessalong the root, making the relevant observations onthe sections,and relating the values obtained to the totalnumber of cellsin the sections. An apparatus is described forobtaining serialsections from a group of roots.
3. It is shown that the weightof the wall, the protein content,and respiration all increaseduring extension, and the increasesduring active growth withrespect to respiration and proteincontent occur in two phases.In the first the increases arelarge per unit increment in volume;in the second they are relativelysmaller. When extension ceasesa third phase in the developmentof the cell sets in in whichrespiration and protein contentdecrease slightly.
4. The datashow that the protein content and the respiration inthe meristematiccell are considerably lower than they are infully vacuolatedcell.

     

Nucleolytic Enzymes in Growing Root Cells

Experimental material consisted of fifteen serial sections 1.0mm. in length, cut from the apex towards the base of bean rootsof standard length. Optimum pH for ribo- and deoxyribonucleaseswere determined. Each enzyme was assayed at optimum pH in correspondinggroups of sections, and the enzyme activity per cell calculated.Results are compared with those obtained using other tissuesand with those of earlier investigations of the enzyme complementof the bean root.     

Proteolytic Enzymes in Growing Root Cells

The experimental material consisted of fifteen serial sections,each 1·o mm. in length, cut from the apex towards thebase of the bean root. Proteolytic enzymes were assayed at variouspH levels on corresponding groups of sections. Values per sectionwere converted to average values per cell by dividing each bythe number of cells occurring in the section. Evidence was obtainedfor the presence of two proteolytic enzymes or groups of enzymeswith different pH optima. Maximum activity of proteolytic enzymesis shown by cells which have ceased to grow and in which thelevel of protein and certain other enzymes is falling. The significanceof these observations in the interpretation of a mechanism ofcell growth is discussed.     

The Development of Root Nodule Xylem Transfer Cells in Trifolium repens

Stereological analysis of micrographs of developing and functioningxylem transfer cells of Trifolium repens indicates that theER and dictyosomes may be involved in wall ingrowth deposition,and the ER in intracellular solute transport.     

Actin in Growing Nerve Cells

AXONAL growth from developing and regenerating nerve cells is a very rapid process. The growth rate is often as high as 50 µm/h¹ and the axon that emerges from a cell less than 0.1 mm in diameter can have a final length of more than a metre. Such a vigorous process, occurring in a non-dividing cell, will probably dominate the biosynthetic activity of the neurone and it is likely that proteins made in large amounts will be major components of the growing fibres. For this reason we have examined the proteins synthesized by growing neurones in the hope that there would be components we could identify and, perhaps, to which we could assign a role in neurite assembly. We have used explanted neurones growing in tissue culture because they can be obtained largely free of other cell types; and we have chosen embryonic sympathetic neurones because of the extremely rapid outgrowth of processes which can be induced by addition of the nerve growth factor (NGF)².     

The Absorption of Potassium by Cells in the Apex of the Root

The purpose of this investigation is the study of the propertiesof the cell with regard to potassium absorption as the cellexpands. Serial sections each 1·5 mm. in length havebeen cut along the course of maize roots over the first 9·0mm. from the tip. The sections have been immersed in 0·01N. solution of potassium chloride in water and in 2 per cent.fructose. Absorption from these solutions has been measuredover a period of 48–72 hours. It is shown that absorptionby the first section which consists almost entirely of meristematiccells is abnormally low, and this is attributed to the absenceof a tonoplast surrounding a central vacuole in the cells ofthis section. The different initial rates with the differentsections indicate that after vacuolation in the root has occurredthe rate per unit area of surface decreases as the cell expands.When the data are reduced to a unit cell basis, however, theyshow that, per cell, absorption increases as expansion occursin the root. This is attributed to an increasing protein content,and the decrease in rate per unit area to a corresponding decreasein protein per unit area. At the same time it is shown thatduring cell growth in the root the cytoplasm differentiatesin such a way that absorption per unit protein increases. After excision, whether growth occurs or not, the protein contentdoes not increase. The final internal concentrations in thedifferent sections vary according to the growth that occurs.When vacuolation has occurred and growth is limited the finalinternal concentration is greater than it is at the beginningof the experiment. When growth occurs, on the other hand, thefinal internal concentration may be lower. This is taken toindicate that absorption depends on an inward secretion intothe vacuole which is independent of surface area and an outwarddiffusion into the medium which increases with increase in surfacearea. It is suggested that the results obtained with sugar supportthis interpretation. When sugar is provided the rate of absorptionis always stimulated and when growth does not occur the finalinternal concentration is enhanced. When growth occurs, however,sugar not only stimulates absorption but also the expansionof the cell, and the latter effect leads to a final concentrationwhich may be lower than that given in the absence of sugar.     

NGF Induces the Expression of Group IIA Secretory Phospholipase A2 in PC12 Cells: The Newly Synthesized Enzyme Is Addressed to Growing Neurites

We proposed that group IIA secretory phospholipase A₂ (GIIA) participates in neuritogenesis based on our observations that the enzyme migrates to growth cones and neurite tips when PC12 cells are induced to differentiate by nerve growth factor (NGF) (Ferrini et al., Neurochem Res 35:2168–2174, 2010). The involvement of other secretory PLA₂ isoforms in neuronal development has been suggested by others but through different mechanisms. In the present study, we compared the subcellular distribution of GIIA and group X sPLA₂ (GX) after stimulation of PC12 cells with NGF. We found that GIIA, but not GX, localized at the neuritic tips after treatment with NGF, as demonstrated by immunofluorescence analysis. We also found that NGF stimulated the expression and the activity of GIIA. In addition, NGF induced the expressed myc-tagged GIIA protein to migrate to neurite tips in its active form. We propose that GIIA expression, activity, and subcellular localization is regulated by NGF and that the enzyme may participate in neuritogenesis through intracellular mechanisms, most likely by facilitating the remodelling of glycerophospholipid molecular species by deacylation–reacylation reactions necessary for the incorporation of polyunsaturated fatty acids.     

The Electrical Potential Difference Across the Tonoplast of Root Cells

Changes in electrical potential, measured as a microelectrodewas advanced into epidermal cells and from cell to cell in rootsof Lolium multiflorum and Zea mays, are described. The recordingssuggest that the electrical potential difference between thecytoplasm and vacuole, E_vc is of the order of a few millivolts,the vacuole tending to be the more positive. E_vc appeared tobe approximately the same for epidermal, cortical, endodermal,and pericycle cells.     

The Endoplasmic Reticulum and the Golgi Structures in Maize Root Cells

Maize root tips were fixed in potassium permanganate, embedded in epoxy resin, sectioned to show silver interference color, and studied with the electron microscope. All the cells were seen to contain an endoplasmic reticulum and apparently independent Golgi structures. The endoplasmic reticulum is demonstrated as a membrane-bounded, vesicular structure comparable in many aspects to that of several types of animal cells. With the treatment used here the membranes appear smooth surfaced. The endoplasmic reticulum is continuous with the nuclear envelope and, by contact at least, with structures passing through the cell wall. The nuclear envelope is characterized by discontinuities, as previously reported for animal cells. The reticula of adjacent cells seem to be in contact at or through the plasmodesmata. Because of these contacts the endoplasmic reticulum of a given cell appears to be part of an intercellular system. The Golgi structures appear as stacks of platelet-vesicles which apparently may, under certain conditions, produce small vesicles around their edges. Their form changes markedly with development of the cell.     

The Effect of 2-thiouracil on the Growth of Cells in the Root

Observations have been made with either intact seedling rootsor segments of roots of Pea (Vicia faba Meteor) and Maize (Zeamais American yellow horsetooth). Thiouracil, uracil and oroticacid were employed—mostly in concentrations ca. I-5 x10^-3 M. With intact roots, two effects of thiouracil must be distinguished:the first, an immediate stimulation on length increase, andthe second (shown with high con-centration), a marked inhibitionwhich in the conditions of the present series of experimentsbecomes well defined after about I2 hours. A change in growth-rateimmediately after a treatment is applied may be taken as aneffect on cell expansion. When a second delayed effect is observed,and particularly when it is different from an initial effect,this may be taken as an effect on division. Accordingly theeffects observed with the intact root suggest that thiouracilmay have a marked inhibiting effect on division and an equallymarked stimulating effect on expansion. With the intact root the final strong inhibition is partiallyrelieved when uracil is supplied simultaneously. When suppliedalone at the same concentration uracil itself has a slightlyinhibiting effect. Uracil, while having no stimulating effecton division, nevertheless mitigates the strong inhibiting effectthat thiouracil has on this process. It is probable that thouracil exerts a stimulating effect asa result of incorporation into RNA. The fact that in the intactroot uracil may reverse the effect suggests as much. The positionwith segments is markedly different from that with intact roots.In the latter case the stimulation is reversed by uracil, withthe isolated segment it is not. With 2.0-4.0 mm pea segmentsa non-reversible stimulation with thiouracil is observed. With1.0-3.0 mm segments, in which thiouracil alone has no effect,the two compounds together give a marked stimulation. In thissystem when thiouracil alone is available, a stimulation inthe basal tissue is compensated by an inhibition in the apical.When uracil is simultaneously applied the inhibition is removedand some of the stimulation due to the thiouracil persists.The inhibition in the youngest tissue may be interpreted inthe same terms as the stimulation that is due to a prolongationof the period of growth. A discussion is given of the interpretation of the experimentalresults. Thiou-racil affects not only the time but also therate of extension, and for the influence on rate no interpretationcan at present be offered.     

低温下抗氧化酶活性与冬油菜根细胞结冰关系的初步研究

以4个抗寒性不同的油菜品种的根为材料,于低温试验箱内连续降温处理(4℃/24 h,0℃/24 h,-3℃/24 h,-6℃/24 h,-9℃/24 h,-15℃/24 h,-21℃/24 h),检测其抗氧化酶(POD、CAT)活性,并结合POD同工酶电泳和冷冻前后幼苗的形态变化特征,探讨油菜根部抗氧化酶活性变化与细胞冰冻状态间的相关性以及油菜根的抗冻机理.结果显示:(1)弱抗寒性油菜品种(‘临油7号’和Vision)在低温冷冻后,叶面出现大量水渍,之后叶片萎缩直至焦枯,根部出现水渍后软化直至干枯死亡;强抗寒性品种(‘陇油6号’和‘陇油8号’)在冷冻后不产生水渍或水渍极少,其根在冻后损伤也较轻微.(2)在连续冷冻条件下,弱抗寒性油菜根部POD和CAT活性在降温初期有较大幅度的变化,之后就保持恒定,说明根部细胞已完全结冰;而强抗寒性油菜根部POD和CAT活性在降温初期变化较小,后期持续大幅增加,表明根部细胞仍保持溶液状态;POD同工酶分析也反映出类似的变化规律.研究表明,低温下冬油菜根部抗氧化酶活性变化和细胞冰冻状态之间存在相关性;细胞结冰通常会给植物造成致死性伤害,强抗寒油菜品种能安全越冬的关键就是能通过某种机制避免细胞内结冰,其结冰温度越低,抗寒性越强.     

The Induction of Glyoxysomal Enzyme Activities in the Aleurone Cells of Germinating Wheat

The aleurone cells of quiescent Triticum vulgare grain wereobserved to contain glyoxysomes, but enzymes known to be locatedin this organelle were not detected. During germination thenumber of glyoxysomes increased, and their associated enzymeactivities appeared, increasing up to the fifth or sixth day.The appearance of ß-oxidation, isocitratase, and malatesynthetase activities were largely dependent upon the presenceof the embryo. Gibberellic acid (GA₂) was effective in replacingthe embryo in this role. It is proposed, therefore, that thedevelopment of glyoxysomal enzyme activities and probably ofthe glyoxysomes themselves, is a gibberellic acid-dependentprocess. The developments of citrate synthetase and malate dehydrogenaseactivities were only partly dependent upon gibberellic acid.Since it is known that these enzymes are located in other compartmentsbesides the glyoxysomes, it is proposed that their gibberellicacid-dependent activities are located in glyoxysomes while theirgibberellic acidindependent activities are located in the cytosoland/or the mitochondria. The developmental courses of the gibberellicacid-independent activities and the results of studies usinginhibitors of protein synthesis support this hypothesis     

The Influence of Growing Temperature on the Growth and Morphology of Cereal Seedling Root Systems

The root systems of cereal seedlings (wheat, rye, barley, oats),grown in a series of liquid and solid media at 5, 15 and 25?C, were measured at intervals between sowing and emergence(shoot length 4.0 cm). In all cases, the length of seminal axesincreased linearly with time, and the rate of root extensionwas increased significantly by each 10 ?C increment in growingtemperature. In general, extension rates were lower in the solidmedia than in water culture; however, there was a strong interactionbetween medium and growing temperature, indicating that highertemperatures can compensate partly for the detrimental effectsof mechanical impedance. Examination of the influence of growing temperature upon rootaxis diameter was hampered by the observation that, for allfour species, apical root diameter was inversely related toroot length. It was concluded that comparison of the diametersof roots grown at different temperatures was valid only if theroots were of the same length (but different ages). Followingthis principle, it was found that, although cereal roots grownat 5 ?C tended to be slightly thicker than those at 15 ?C or25 ?C, this effect was small and rarely exceeded 10%. Publishedreports of very large differences in root diameter at differentgrowing temperatures were examined and found to be generallyunreliable. Lateral roots did not represent a significant fractionof the root system in any of the treatments studied. Root: shoot relationships at different growing temperatureswere examined by plotting root system length against shoot lengthfor each sample. It was found that, for the same shoot length,the root systems at 5 ?C were much shorter than at 25 ?C, theresponse at 15 ?C being rather more variable. This finding,which indicates that plants grown at low temperatures have alower capacity for water and solute uptake than those grownat laboratory temperatures, has important implications for thedesign of laboratory experiments. The influence of root zonetemperature upon root growth and physiology can be determinedunequivocably only if the roots have been exposed to the experimentaltemperature throughout their growth. Key words: Barley, Oats, Rye, Wheat, Root length, Root diameter, Root, shoot ratio     

Correlation of Pectolytic Enzyme Activity with the Programmed Release of Cells from Root Caps of Pea (Pisum sativum)

In many plant species, the daily release of hundreds to thousands of healthy cells from the root cap into the soil is a normal process, whose function is unknown. We studied the separation of the cells in pea (Pisum sativum) using an aeroponic system in which separated cells were retained on the root until they were washed off for counting. We found that cell separation is a developmentally regulated, temperature-sensitive process that appears to be regulated independently of root growth. No cells were released from very young roots. When plants were grown aeroponically, cell numbers increased with increasing root length to a mean of 3400 cells per root, at which point the release of new cells ceased. The process could be reset and synchronized by washing the root in water to remove shed cells. Cell separation from the root cap was correlated with pectolytic enzyme activity in root cap tissue. Because these cells that separate from the root cap ensheath the root as it grows and thus provide a cellular interface between the root surface and the soil, we propose to call the cells “root border cells.”     

The Growth of the Nucleus in Dividing and Non-dividing Cells of the Pea Root

The growth of the nucleus and the cell in the pea root was followedthrough the mitotic cycle and subsequently in post-mitotic developmentby comparing cells and nuclei from the meristem, at differentstages of interphase, and cells and nuclei from two regionsof the enlarging zone of the root. Measurements of cell andnuclear volumes were made in sections of fixed roots. Measurementsof nuclear volume, DNA content, and dry mass were made on isolatednuclei. Growth in the mitotic cycle was characterized by a doublingof DNA and nuclear dry mass and a five-fold increase of nuclearvolume. Since cell volume doubled, a differential hydrationof cytoplasm and nucleus is inferred. Post-mitotic growth wascharacterized by a four-fold or greater increase in cell volume,with vacuolation and a continued increase of cytoplasmic constituents,but a cessation of nuclear growth except by uptake of water;the only increase in nuclear dry matter appeared to be in cellsbecoming endopolyploid. The concentration of dry matter in thenucleus fell as the nuclei enlarged in the mitotic cycle andin post-mitotic growth. The relationships between the measuredparameters are examined to see whether they might be indicativeof causal relationships.     

根瘤形成过程中豆科植物根毛和根外层传递细胞的诱发

对7种豆科植物接种根瘤菌后根部的形态和内部结构进行了研究.结果表明:根瘤菌可诱发根瘤形成部位根段的根毛增生、形变和根外层传递细胞的发育.根外层传递细胞发生在根毛伸长形变时期,一直可持续到根瘤形成,传递细胞壁内突发育过程是先由根表皮细胞外切向壁一侧细胞质膜向细胞质内陷形成囊状壁傍体,次生细胞壁物质在初生壁上沉积并逐渐充满囊状体,最终形成传递细胞典型的壁内突结构.根瘤形成过程中根外层传递细胞的诱发与培养方式(水培、固培)没有直接关系.在不接菌的对照苗的根段内未发现壁内突结构,研究证明豆科植物根外层传递细胞的形成是由根瘤菌诱导所致.