Experimental control of the activity of the quiescent centre in excised root tips of Zea mays

Summary Quiescent centres have been demonstrated in cultured excised root tips of both Pisum sativum and Zea mays. Upon addition of sucrose to Zea roots which have been deprived of carbohydrate, the cells of the quiescent zone as well as those of the rest of the meristem undergo DNA synthesis. Following the onset of proliferative activity in the meristem, DNA synthesis in the quiescent-centre cells is again arrested. It is suggested that the dividing cells of the meristem are responsible for the maintenance of the quiescent centre. It has also been shown that DNA-synthesising cells do occur within the quiescent centre and that they appear to be localised in specific regions.     

X-Irradiation of Root Meristems

When roots of Zea mays are given heavy doses of X-rays the averagerate of mitosis at first falls in the normally meristematiccells and increases in the quiescent centre. Proliferation inthe quiescent centre gives rise to a new meristem which continuesthe growth of the root and which acquires a quiescent centreof its own. Observations on the micronuclei produced after irradiationsuggest that the quiescent centre is less sensitive to X-raysthan the rest of the meristem. It is probably this lower sensitivitycombined with the initial difference in time at which the radiationdamage to cell viability occurs that gives the quiescent centreits role in the recovery of the meristem.     

Nutrition and the quiescent centre of root meristems

Summary Root apices were fed with tritiated thymidine from the endosperm and from labelled root caps placed on decapped roots. Supply from both sources reaches the meristem, but is attenuated by the demand of the dividing cells. The thymidine reaches the far sides of the quiescent centre from both directions. Starvation is thus unlikely to be the reason for the existence of the quiescent centre although cessation of mitosis in the normally meristematic regions may stimulate the quiescent centre by releasing nutrients.     

The Response of the Primary Root Meristem of Zea mays L. to Various Periods of Cold

Primary roots of maize (Zea mays L.) grown in nutrient solutionat 5?C elongate at about 1% of the rate found at 20?C. The apicalmeristem becomes shorter and shows little proliferative activityat 5?C, but following transfer to 20?C mitoses increase in frequencyand the meristem regrows to its original length. Both the amountby which the meristem shortens and the time for its completeregrowth are related to the period spent at 5?C. The shorteningof the meristem suggests that at the lower temperature the balancewhich normally exists between cell production and differentiationis altered, the latter continuing at a relatively faster ratethan the former. A new, steady-state balance between the twoprocesses is re-established during the recovery period. Themeristem recovers as a result not only of its own mitotic activitybut also through stimulation of cell division in the quiescentcentre. The degree to which the quiescent centre is activated,as judged by its mitotic index and the number of nuclei labelledby feeding with tritiated thymidine, increases as the durationof the preceding cold treatment increases. The close relationshipbetween proliferative activity in the quiescent centre and theminimum length of the meristem following the cold treatmentsuggests that there is communication between these two zoneswhich co-ordinates their respective rates of cell productionand helps to maintain a normal meristem structure. The resultsemphasize the importance of the quiescent centre as a reservoirof cells that can re-establish a meristem rendered non-functionalthrough the impact of unfavourable environmental conditions. Key words: Meristem, quiescent centre, root, temperature, Zea mays     

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.     

Magnetic field affects meristem activity and cell differentiation in Zea mays roots

Abstract

Exposure of Zea mays seedlings to a continuous electromagnetic field (EMF) for 30 h induced a 30% stimulation in the rate of root elongation compared with the controls. It also resulted in a significant increase of cell expansion, in both the acropetal (metaxylem cell lineage) and basipetal (root cap cells) direction. In addition, in EMF-exposed roots a precocious structural disorder was observed both in differentiating metaxylem cells and root cap cells. All these features may be consistent with an advanced differentiation of root cells that are programmed to die. EMF treatment also resulted in a significant reduction in the size of the quiescent centre in the root apical meristem. The extent to which these responses are causally linked is discussed.     

Indoleacetic acid movement in the root cap

Summary When applied on the root cap of Zea mays L., indol-3yl-acetic acid (IAA) may enter the root tip and move basipetally inside the cap. From the cap to the apex (quiescent centre and meristem) the IAA transport is very slow. Polarity of IAA movement, in relation to growth, is discussed.     

Correlations Between the Dimensions of Different Zones of Grass Root Apices, and their Implications for Morphogenesis and Differentiation in Roots

The sizes of different zones within root apices of nine speciesof grass were estimated, and statistically significant correlationswere found between certain of them. The volume of the cap isrelated to the volume of the meristem of the root proper. Thecortical and stelar portions of the meristem are also related,and their lengths and volumes correlate with the volume of thequiescent centre. The volume of the quiescent centre also correlateswith the length of the zone in which periclinal divisions arefound in the inner cortex; these divisions generate the rowsof cells across the cortex. Diameter of the procambial cylinder,quiescent centre volume and vascular complexity are related,though from correlations alone it is not possible to say whetherone of these characters directly influences another as has beensuggested by other workers. All the zones within the root apexprobably form a tightly-integrated developmental unit. Root structure seems to be independent of cell size, thoughcell size correlates with nuclear DNA content which is a species-specificfeature. Gramineae, meristems, morphogenesis, root apices     

STUDIES IN THE QUIESCENT ROOT MERISTEM OF THE BULB OF ALLIUM CEPA L.: FINE STRUCTURE AND DNA SYNTHESIS *

Study of the quiescent root meristem of Allium cepa L. bulbs has revealed that its histological organization does not differ significantly from the growing meristem, except for the fact that the cells are all arrested in interphase. Ultrastructure of the quiescent tissue is, however, different in the organization of the nucleolus and in the absence of prominent endoplasmic reticulum, microtubules and golgi complexes. A variety of lomasome-like structures, plasma membrane modifications and vacuoles have been recorded. Most of the cells except for the ones in the root cap and quiescent center are highly vacuolated; vacuolation is maximum in the cortical zone of the meristem. The pattern of ³H thymidine incorporation during early stages of sprouting indicates that asynchrony of the mitotic cycle, which is the characteristic of the growing meristem, is maintained during quiescence by the arrest of nuclei at different subphases of interphase.     

Micronuclei and Radiosensitiviy in the Root Meristem of Vicia faba

Percentages of cells with micronuclei in four regions of theroot meristems of Vicia faba are used as measures of sensitivityto acute X-irradiation. There are two peaks in these percentages,occurring at about four and eight days after 360 rads and twoand six days after 180 rads. Two peaks exist, probably becausethe radiation delays cells that were in G₁ much more than cellsthat were in G₂ in reaching the first post-irradiation mitosisand consequently in displaying micronuclei in the followinginterphase. The relative heights of the two peaks thereforereflect the relative numbers of cells in G₁ and G₂ as well asthe relative sensitivity of the two phases to chromosomal damage. The cells of the quiescent centre are injured least by the radiationas they are mostly held at Gt. The meristem thus obeys the lawof Bergonié and Tribondeau, but differs from that ofZea in that the meristematic cells of the cap initials and steleimmediately adjacent to the quiescent centre resemble the quiescentcentre much more closely than the stele 250 µ away inthe numbers of micronuclei produced. This is consistent withthe differences already known between the two species concerningrates of division in the different regions of the meristem andthe behaviour of the meristem after severe radiation injury.     

Organelle DNA Synthesis in the Quiescent centre of Arabidopsis thaliana (Col.)

The behaviour of cell nuclei and organelle nucleoids (organellenuclei) was studied in the root apical meristem of 3-d-old seedlingsof Arabidopsis thaliana (Col.). Samples were embedded in Technovit7100 resin, cut into thin sections and stained with 4'-6-diamidino-2-phenylindole(DAPI) for observation of DNA. DNA synthesis in cell nucleiand organelle nucleoids was investigated using the incorporationof [³H] thymidine or 5-bromo-2'-deoxyuridine (BrdU). Incorporated[³H] thymidine and BrdU were detected by microautoradiographyor immunofiuorescence microscopy, respectively. Central cellsand cells just above the central cells of the quiescent centre(QC) showed an extremely low activity of DNA synthesis. However,DNA synthesis occurred in at least one organelle nucleoid ofall cells in the QC within 24 h. This suggests the cells inthe QC are quiescent with regard to nuclear DNA synthesis, butnot with regard to the organelle nucleoids. Key words: Arabidopsis thaliana, quiescent centre, root apical meristem, mitochondrial nucleoid (nuclei), plastid nucleoid (nuclei)     

Determination of Stelar Elements in Roots of Pisum sativum L.

Cytochemical studies of esterase activity in 0.5 mm segmentsfrom root tips of Pisum sativum explanted for up to 9 days inbasal culture medium containing 2 per cent sucrose showed retentionof this activity. During this time, all segments from the secondand third 0.5 mm segments of the root tip developed xylem elementsas did the proximal end of the first segment. No xylem elementswere found in the 12–14 cells behind the quiescent centre.It is concluded that the central group of meristem cells aregenerally programmed to form tissues of the stele immediatelyon leaving the quiescent centre, and that the programming forxylem and phloem elements occurs as a second step. Pisum sativum L., garden pea, determination, histochemistry, esterases, stele, root     

Apical meristem organization and lack of establishment of the quiescent center in Cactaceae roots with determinate growth

Some species of Cactaceae from the Sonoran Desert are characterized by a determinate growth pattern of the primary root, which is important for rapid lateral-root formation and seedling establishment. An analysis of the determinate root growth can be helpful for understanding the mechanism of meristem maintenance in plants in general. Stenocereus gummosus (Engelm.) Gibson & Horak and Pachycereus pringlei (S. Watson) Britton & Rose are characterized by an open type of root apical meristem. Immunohistochemical analysis of 5-bromo-2-deoxyuridine incorporation into S. gummosus showed that the percentage of cells passing through the S-phase in a 24-h period is the same within the zone where a population of relatively slowly proliferating cells could be established and above this zone in the meristem. This indicated the absence of the quiescent center (QC) in S. gummosus. During the second and the third days of growth, in the distal meristem portion of P. pringlei roots, a compact group of cells that had a cell cycle longer than in the proximal meristem was found, indicating the presence of the QC. However, later in development, the QC could not be detected in this species. These data suggest that during post-germination the absence of the establishment of the QC within the apical meristem and limited proliferative activity of initial cells are the main components of a determinate developmental program and that establishment of the QC is required for maintenance of the meristem and indeterminate root growth in plants.Abbreviations QC quiescent center - RCP root cap-protoderm - BrdU 5-bromo-2-deoxyuridine - FITC fluorescein isothiocyanate - DAPI 4,6-diamidino-2-phenylindole     

The iRoCS Toolbox – 3D analysis of the plant root apical meristem at cellular resolution

To achieve a detailed understanding of processes in biological systems, cellular features must be quantified in the three‐dimensional (3D) context of cells and organs. We described use of the intrinsic root coordinate system (iRoCS) as a reference model for the root apical meristem of plants. iRoCS enables direct and quantitative comparison between the root tips of plant populations at single‐cell resolution. The iRoCS Toolbox automatically fits standardized coordinates to raw 3D image data. It detects nuclei or segments cells, automatically fits the coordinate system, and groups the nuclei/cells into the root's tissue layers. The division status of each nucleus may also be determined. The only manual step required is to mark the quiescent centre. All intermediate outputs may be refined if necessary. The ability to learn the visual appearance of nuclei by example allows the iRoCS Toolbox to be easily adapted to various phenotypes. The iRoCS Toolbox is provided as an open‐source software package, licensed under the GNU General Public License, to make it accessible to a broad community. To demonstrate the power of the technique, we measured subtle changes in cell division patterns caused by modified auxin flux within the Arabidopsis thaliana root apical meristem.     

The Meristem and Quiescent Centre in Cultured Root Apices of the gib-l Mutant of Tomato (Lycopersicon esculentum Mill.)

Cultured root apices of tomato bearing the gib-I mutation, whichreduces the levels of endogenous gibberellins, grew slower andwere thicker than wild-type contols. This was the result ofshorter and broader cells in the menstem of the mutant. Cellsof both cortex and stele were affected, but this did not causeany alteration to the volume fraction occupied by these twotissues in the root meristem. Root caps were longer in the mutantand there were also more layers of rhizodermis. All these effectscould be reproduced in wild-type roots by addition of 0.1µM2S, 3S paclobutrazol (an inhibitor of gibberellin biosynthesis)to the culture medium and could be normalized in mutant rootsby 0.1 µM GA₃. Cell doubling times in the proximal regionof the meristem were similar in mutant and wild-type roots,but were faster in both the quiescent centre (QC) and the capmeristem of the mutant. This latter feature of the mutant rootsis likely to be the cause of their longer caps, while the fasterrate of division in the QC accounts for the additional tiersof cells that were found to build up in the cortical portionof this zone These additional tiers failed to form in mutantroots grown in GA₃, but they could be induced in wild-type rootsby 2S, 3S paclobutrazol. These results suggest that endogenousgibberellins may be partly responsible for the slow rate ofcell growth and proliferation in the QC. Gibberellins, gib-I mutation, Lycopersicon esculentum, meristem, roots, 2S, 3S paclobutrazol, quiescent centre, tomato     

The extent and differences in mitotic activity of the root tip ofVicia faba L.

Mitotic activity does not stop for different meristematic cells of the root apex at the same distance from the initials. The differences are connected with the functional heterogeneity of the apical meristem of the root. The arrangement of vascular bundles,i.e. the alternation of independent xylem and phloem groups, is of major importance. In broad bean roots, the protophloem sieve elements stop dividing first. The centre of the stelei. e. late metaxylem elements stop dividing next. Division in the stele gradually ceases centrifugally, while it ceases centripetally in the peripheral part of the root. The cylindrical region with prolonged cell division includes internal layers of the cortex including endodermis, pericycle and adjoining cells of the stele. Proximally apical meristem is reduced to isolated strands of cells adjacent to the protoxylem poles. Pericycle cells stop dividing last at a distance of approx. 9–10 mm from the initials. The number of the division cycles is limited and is specific for individual cell types. Epidermal and cortical cells divide in broad bean roots transversely approximately seven times, cells of late metaxylem approximately five times. Root apical meristem is an asynchronous cell population with a different duration of the mitotic cycle. We determined local variations in the duration of the mitotic cycle in the apical meristem of broad bean root by means of colchicine-induced polyploidy. The cells of the quiescent centre had the longest mitotic cycle after colchicine treatment. The region of the proper root adjacent to the quiescent centre was mixoploid (2n and 4n). Isolated cells with a long cycle occurred also in the cortex and in the central cylinder. Cells with a division cycle of 18h were found in the root cap, in the epidermis, in the cortex and in the central cylinder. Relatively numerous cells with the shortest division cycle, approx. 12 h, occurred farther of the quiescent centre in the epidermis, in the cortex, in the pericycle, and in adjacent layers of the stele through-out the entire meristematic region. The results derived from the analysis of the apical meristem are discussed in connection with the ontogenesis of different types of cells taking part in the primary structure of the root.     

Proliferating and Quiescent Cells in the Apical Meristem of Elongating Lateral Roots of Vicia faba L.

Whole root systems of Vicla faba were continuously exposed to³H-TdR for periods of up to 72h, following which LI was determinedin the cap initials, in the quiescent centre or in that partof the meristem in which a quiescent centre will develop, andin the stele and in the cortex-epidermis at intervals alongthe apical 800 µm basal to the junction between the capinitials and the rest of the meristem, in newly-emerged (NE),0.2 and 4.0 cm long lateral roots, after each exposure period.Cell doubling time (Td), mean cycling time (Tc) and the sizeof the growth fraction (GF) were then calculated for each partof the meristem investigated in each batch of roots, from thecurves recording increase in labelling index (LI) with increasein duration of the period of exposure to ³H-TdR and from therate of increase in LI over the initial l-12h labelling period.Since it is extremely difficult to eliminate all sources oferror in calculating GF from the values obtained for LI in continuouslabelling experiments, it is emphasized that the values of GFreported in the present paper may not be totally accurate. Thisis also true of the results obtained for Tc as Tc was derivedfrom the product of the corresponding values for Td and GF. Cell doubling time and mean cycling time were both longer inthe cells forming the quiescent centre in the 0.2 and 40 cmlong roots than in any other part of the apical meristem examined.The size of the GF was found to decrease basally along the steleand the cortex-epidermis from the most apical to the most basalsegment examined in the NE, while Td increased in duration.Similar changes took place along the stele of the 0.2 cm longlaterals, but not in the cortex-epidermis of these roots or,to any great extent, in any of the tissues examined in the 4.0cm secondary roots. No consistent trend was apparent in theduration of Tc basally along either tissue examined in theseroots. It was concluded from these results, and from supportingdata in the literature, that, as the laterals elongated fromNE to 4.0 cm, the apical meristem increased in length.     

Effect of abscisic acid on the cell cycle in the growing maize root

The mechanism by which the rate of cell proliferation is regulated in different regions of the root apical meristem is unknown. The cell populations comprising the root cap and meristem cycle at different rates, proliferation being particularly slow in the quiescent centre. In an attempt to detect the control points in the cell cycle of the root apical meristem of Zea mays L. (cv. LG 11), quiescent-centre cells were stimulated to synthesise DNA and to enter mitosis either by decapping or by immersing intact roots in an aqueous 3,3-dimethyl-glutaric acid buffer solution. From microdensitometric and flow-cytometric data, we conclude that, upon immersion, the G₂ phase of the cell cycle of intact roots was shortened. However, when 50 M abscisic acid (ABA) was added to the immersion buffer, parameters of the cell cycle were restored to those characteristic of intact roots held in a moist atmosphere. On the other hand, decapping of primary roots preferentially shortened the G₁ phase of the cell cycle in the quiescent centre. When supplied to decapped roots, ABA reversed this effect. Therefore, in our model, applied ABA retarded the completion of the cell cycle and acted upon the exit from either the G₁ or the G₂ phase. Immersion of roots in buffer alone seems to trigger cells to more rapid cycling and may do so by depleting the root of some ABA-like factor.Abbreviations ABA cis-abscisic acid - DGA 3,3-dimethyl-glutaric acid - DAPI 4,6-diamidino-2-phenylindole - LI labelling index We thank Pierre Zaech of the Ludwig Institute, Epalinges, Switzerland, for expert assistance in flow cytometry and Dr. Jean-Marcel Ribaut of our Institute for providing data on exodiffusion and metabolism of ABA.