Deterministic Patterns of Cellular Growth and Division Within a Meristem

The primary root meristem of maize (Zea mays L.) is composedof longitudinal files of cells arranged in groups of familialdescent (sisters, cousins, etc.). In the proximal portion ofthe meristem, the cells in these groups, or packets, show orderedsequences of division that are transverse with respect to theapico-basal axis of the root. The division sequences fall intoa relatively small number of pathways which can be describedusing deterministic 'bootstrap' L-systems. Although these systemscan operate through the assignment of determinate lifespansto sister cells which thus specify their subsequent interdivisionalperiod, because of their exponential growth kinetics the systemscan also operate with determinate units of cell extension. Thisdeterministic type of system allows simulation not only of thedivision sequences, but also of the lengths of the cells thatare present within the packets which participate in the differentdivision pathways. The types of L-systems used to describe thesepathways also predict the distributions and ranges of cell andpacket lengths found after varying numbers of cell generations.These distributions compare favourably with those actually foundin the maize root meristem. Theoretical aspects of bootstrapL-systems, essential for their application to the one-dimensionalcellular arrays of the meristematic cell-files of the maizeroot apex, are also presented.Copyright 1994, 1999 AcademicPress Cell division, cell elongation, cell polarity, L-system, root meristem, Zea mays     

Histology of Shoot Bud Ontogeny from Seedling Root Segments of Brassica napus L.

Root explants of Brassica napus cultured in vitro form adventitiousshoots. The root buds originated at the base of the newly initiatedlateral root. Cells in association with the differentiatingphloem of the developing lateral roots were the sites for rootbud formation. A nodular mass of cytoplasmic cells developedby day 7 at the base of the lateral root. This group of cellscontinued to divide an enlarge. The cells in the peripheralregion of the nodular cell mass differentiated further intoa meristematic zone. The meristematic cells grew towards theperiphery of the cortex by crushing the outer layer of corticalcells. Further development of the meristematic layer resultedin the formation of shoot primordia with organized shoot apicalmeristems and leaf primordia.Copyright 1993, 1999 Academic Press Brassica napus, canola, cultured root segments, root buds     

Response to Phosphate Deficiency in Bean (Phaseolus vulgarisL.) Roots. Respiratory Metabolism, Sugar Localization and Changes in Ultrastructure of Bean Root Cells

Bean plants (Phaseolus vulgarisL. ‘Zlota Saxa’)were cultured on complete (+P) or phosphate-deficient (-P) nutrientmedium. A large increase in glucose concentration was foundin the meristematic zone of -P roots compared to control roots.The increased glucose concentration in the meristematic zonedid not influence total respiration rate. Glucose or uncoupler(carbonyl cyanide m-chlorophenylhydrazone) failed to increasethe respiration rate in -P root segments, but stimulated respirationin +P roots. The ultrastructure of cortical cells from the meristematicroot zone showed marked differences between +P and -P plants.Large vacuoles, invaginations of the plasmalemma and condensedforms of mitochondria were dominating features in cortical cellsof -P roots. Analysis of extracts after treating roots withdimethylsulfoxide (DMSO) indicated different localization ofsugars in the cell compartments. In roots of -P plants, mostof the reducing sugars were detected in the cytoplasm fractionwhile most sucrose was in the vacuole. Observations of the effectof 10% DMSO on cell ultrastructure indicated partial destructionof the plasmalemma but not the tonoplast. The localization ofreducing sugars in secondary vacuoles or plasmalemma invaginationsin the cells from the meristematic region of -P roots is discussed.Copyright1998 Annals of Botany Company. Bean (Phaseolus vulgarisL.), roots, Pi deficiency, respiration, meristematic zone, ultrastructure, sugar efflux, reducing sugars and sucrose localization.     

Inhibition of RNA Synthesis and the Relationship between Nucleolar and Mitotic Cycles in Zea mays Root Meristems

The relationship between nucleolar and mitotic cycles has beendetermined after treatment of root apices of Zea mays with ethidiumbromide. In the meristematic regions of the stele the two cyclesare not much displaced in relation to each other except fora delay in the onset of the disorganization phase. A few nucleolipersist into metaphase and a few nuclei undergo an amitoticdivision. In the cap initials the drug greatly delays the onsetof disorganization of the nucleolus, which normally occurs beforeprophase in this region. It also delays the completion of reorganizationso that fully organized nucleoli are no longer available duringthe last half of telophase. In the quiescent centre the onsetof disorganization and the end of reorganization of the nucleoliare also delayed in relation to mitosis. There is no evidencefor a delay in the onset of reorganization in any region ofthe meristem. Some cells form multiple micronucleoli and this aberrant behaviouroccurs more often in the cap initials than elsewhere as doesamitotic division.     

Water Deficit in Pea Root Tips: Effects on the Cell Cycle and on the Production of Dehydrin-like Proteins

Dehydrin-like proteins have been detected in nuclei and cytoplasmof meristematic root tip cells from pea seedlings subjectedto slow dehydration at 90% relative humidity for 48 h or more.Evidence was gained from Western blotting and immunocytochemicalexperiments using an antibody raised against the conserved domainof dehydrin proteins. Flow cytometer analysis has shown thatcycling cells of root tip meristems from dehydrated seedlingsare mostly arrested in G2 phase. Other stress treatments thoughtto involve water depletion (osmotic stress, cold treatment)or to modulate cell response to water deficit (abscisic acid)gave less clear-cut results with all treatments lowering theproportion of cells entering the S phase, but without a definiteand persistent arrest in any preferential phase of the cycle.Possible interrelationships between G2 arrest and dehydrin productionare discussed. Cell cycle; dehydrins; flow cytometry; nuclei; pea; Pisum sativum L.; water stress     

Development of Stem Nodules in a Tropical Forage Legume, Aeschynomene afraspera

Rhizobial infection occurred on the stem of Aeschynomene afrasperaat the site of emergence of adventitious root primordia. Rhizobiainvaded cortical cells at the base of the root primordium. Thefirst infected cell enlarged and collapsed after rhizobia hadmultiplied in large numbers. At this time, a meristematic zonewas initiated some distance beneath the first infected cell.Rhizobial penetration into the deeper cortex was by progressivecollapse of infected cells towards the meristematic zone; rhizobiaentering the cortical cells by invagination of the host cellwall. At the entry point, rhizobia were embedded in digitatecell wall material. These infection structures were restricted,always originating from the cell wall of an adjacent infectedcell. Soon after infection, the cell collapsed progressivelyforming infection strand-like structures which developed upto the meristematic zone. When infection had reached the meristematiczone, invaded host cells ceased to collapse but divided repeatedlyto form the nodule. Key words: Aeschynomene afraspera, stem nodulation     

RML1 and RML2, Arabidopsis genes required for cell proliferation at the root tip.

New cells are produced from the meristematic tissues located at the shoot and root tip throughout the life of higher plants. To investigate the genetic mechanism regulating meristematic activity, we isolated and characterized four single-gene, recessive mutants in Arabidopsis thaliana called root meristemless (rml). Complementation tests identified two RML loci; RML1 maps to chromosome IV and RML2 maps to chromosome III. These mutants produce normal embryonic roots that either did not undergo or experienced limited cell division following germination, resulting in primary roots of less than 2.0 mm in length. Mutants can produce lateral and adventitious roots, which can grow to a length comparable to the embryonic root and arrest, indicating that the growth arrest is unrelated to the embryonic dormancy process. Neither the addition of growth regulators to the media nor the removal of shoots can rescue mutant roots from growth arrest, indicating that the mutant phenotype is not caused by a shortage of known growth regulators or by a transmissible shoot inhibitor. Normal cell division ability in mutant embryo, shoot, and callus cells indicates that the RML gene functions are not part of the general cell division processes; rather, they are involved specifically in activating the cell division cycle in the root apical cells.     

X-Ray Micro-Analysis of Cells and Cell Compartments of Atripiex spongiosa: II. ROOTS

Atripiex spongiosa was grown in hydroponic culture with additionof 0, 50, 200, 400 and 600 mM NaCl. Frozen, hydrated cells ofthe roots were analysed by X-ray micro-analysis. Comparisonswere made of meristematic cells at the root apex and vacuolatedcells 5.0 mm from the apex. High selectivitiy for K relativeto Na was found for the cytoplasm of meristematic cells andthere was little effect of increasing salinity on the ratiosNa/K, Cl/K, Na/P and CI/P. In the cell vacuoles of the cortex,selective uptake of K relative to Na also occurred, but to alesser extent than in the meristematic cells. Gradients werefound of decreasing ratio of Na/K from the epidermis to thestele. Measurements of chemical content of the roots and shoots ofthese plants showed that the ratio of Na/K was higher in theshoot than in the cortical cell vacuoles and higher again thanin the stele or meristematic cytoplasm. It is suggested thattransport of ions to the shoot of Atripiex spongiosa involvesselective exclusion of Na from the xylem parenchyma into thexylem, and that this may be general to other halophytes. Key words: Micro-analysis, X-ray, Cells, Atripiex spongiosa     

Radiomimetic Effect of Cisplatin on Cucumber Root Development: the Relationship between Cell Division and Cell Growth

Cisplatin [DDP, cis-dichlorodiammine platinum (II)], a strongcytostatic and antineoplastic agent, was tested on seedlingsof cucumber Cucumis sativus L. for its general effect on rootdevelopment and its particular effects on root cell divisionand cell growth. DDP was characterized as a radiomimetic compoundsince both DDP (1·3 x 10^-5 M) and -irradiation (2·5-10kGy) drastically and irreversibly stopped development of embryoniclateral root primordia (LRPs) in the radicle by inhibiting bothmitotic activity and cell growth. In 20% of the LRPs of DDP-treatedroots, cells did not divide at all. Dividing cells completedno more than two cell cycles. These effects were specific becausewhen DDP was available to the roots only at the onset of celldivision, cell proliferation and cell growth were similar tothat produced by constant incubation. Neither DDP nor -irradiationaffected non-meristematic cell elongation. It was concludedthat cell growth of meristematic cells is closely related tocell division. However, non-meristematic cell growth is independentof DNA damage. This suggests DDP as a tool to reveal these autonomousprocesses in plants development and to detect tissue compartmentsin mature plant embryos which contain potentially non-meristematiccells.Copyright 1993, 1999 Academic Press Cucumis sativus, cucumber, cisplatin, cell growth, cell elongation, cell division, lateral root, root development     

Histology of Somatic Embryogenesis from Leaf Explants of the Oil Palm Elaeis guineensis

Histological analysis was carried out during the sequence ofevents which lead to the obtaining of somatic embryos of oilpalm. Calluses from the division of perivascular cells formedat the veins of young leaf explants. Subsequent proliferationof histologically similar nodules was by means of a cambium-likezone. Under certain conditions these calluses consisted almostentirely of meristematic cells. They then differentiated rapidly:the cambium-like zone fragmented, leading to protuberance inwhich the cells divide rapidly; epidermal structures were formed,with a network of procambial strands, and synthesis of storagelipids accompanied the formation of these embryo-like structureswhich developed into clumps of true somatic embryos, each witha shoot apex and a root apex. Other structures frequently observedduring in vitro culture are also described and show that alternatepathways do exist. The structure and evolution of somatic embryosare compared to those of zygotic embryos. Storage lipids emergeas an early tracer of the satisfactory development of tissuetowards somatic embryogenesis. Oil palm, Elaeis guineensis, histology, somatic embryogenesis, callogenesis, storage lipids     

Initiation, Development and Structure of the Primary Haustorim in Striga gesnerioides (Scrophulariaceae)

A light-microscopic study is reported on the initiation, establishmentand structure of the primary haustorium of Striga gesnerioideson the host, cowpea (Vigna unguiculata). The radicular apexof the germinated parasite seed dissolves its way through thehost root cortex to the stele. Thus, it is converted into aprimary haustorium. Some of the haustorial front-line cellsin contact with the host endodermis penetrate into the steleand make contact with the xylem vessels. Differentiation ofthese haustorial cells into xylem vessels occurs and extendsbackwards through the median axial region of the haustorialtract in the host cortex to connect with the conductive xylemof the radicle outside the host root. Subsequently the parasite'splumule develops into a leafy shoot. On penetrating the steleof the host, the haustorium stimulates cell division in thehost pericycle whose triggered proliferation together with expansionof the parasite haustorial tissues result in the formation ofa large, tuberous primary haustorium. At various points of thehost-parasite interface, differentiation of xylem elements occurs,presumably maximizing nutrient transfer from host to parasite.In spite of this, many proliferated host cells at the interfaceremain apparently meristematic showing densely-stained cytoplasmand prominent nuclei.     

Entry of a basic protein, lysozyme, through the region of elongation in roots of Iasione montana

Ultrastructural effects of a basic protein, lysozyme, on rootcells of Iasione montana have been related to cellular mechanismsof root growth inhibition. Lysozyme is found to disrupt cellwalls and to disintegrate cellular membranes of elongating cells,but not membranes of mature and meristematic cells. Lysozymeseems to penetrate roots only in the region of elongation anddoes not affect meristematic cells; meristematic cells of Iasionemontana possess very thick peripheral cell walls covered byan electronopaque layer. As the cells elongate, primary cellwalls become thinner, the microfibrillar network becomes looserand the dense layer breaks off. Elongating cells are more vulnerableto penetration by lysozyme; however, as the cells mature, thereis a barrier to lysozyme formed that consists of secondary cellwalls; these appear to be a tightly arranged network of macromolecules. ¹ Present address: Department of Biology, Massachusetts Instituteof Technology, Cambridge, Massachusetts 02139, U.S.A. (Received November 29, 1974; )     

Distribution of copper and iron in heterotrophic phase of development and growth of vicia faba L.

Distribution of Cu and Fe from the cotyledons to the root and the epicotyl of Vicia faba L. was studied during the heterotrophic phase of development and growth in dark. During this phase there was no significant difference in the distribution of the quantity of Cu in the root and in the epicotyl. When the plants had changed to the autotrophic phase, the distribution of Fe was significantly higher in the epicotyl than in the root. In roots both Cu and Fe were localized in the zone of root differentiation. The quantities of these elements in meristematic and elongation zones were about 15% and 25%, respectively. In regions of cell division and cell elongation of the root, the level of Cu was almost the same. On the contrary, the content of Fe in the zone of cell elongation was atout 3 times higher than in the meristematic zone.     

Structural Changes During the in vitro Germination of Vanilla planifolia (Orchidaceae)

A study is reported of histogenesis and organogenesis duringthe processes leading up to seedling formation in cultures ofVanilla planifolia. Prior to germination, all cells of the embryoincreased in size rupturing the seed coat and initiating theprotocorm stage. The cells of the protocorm were heavily ladenwith starch grains. Although all of the cells of the matureembryo were heavily laden with protein bodies, these were confinedto the terminal cell descendents on emergence of the embryofrom the seed coat, and they disappeared during differentiationof the meristem, indicating that some reserves were mobilizedand utilized during germination. The terminal locus of embryonal axis did not differentiate intoa cotyledon and epicotyl as in other angiosperm embryos butformed a thick meristematic layer. Bipolar differentiation withinthe meristem produced the shoot, and after a few leaves hadbeen formed, the first root differentiated endogenously fromthe base of the meristem. Subsequent roots, however, appearedto originate more superficially. The chain of events appearsto be quite unique to Vanilla amongst the angiosperms. Vanilla planifolia, protocorm, starch grains, protein bodies     

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     

Lateral Root Development in Attached and Excised Roots of Zea mays L. Cultivated in the Presence or Absence of Indol-3-yl Acetic Acid

The initiation of lateral root primordia and their subsequentemergence as secondary roots have been examined in attachedand excised roots of Zea mays grown in the presence or absenceof indol-3-yl acetic acid (IAA). Exposure to IAA enhanced anlageinception in both batches of roots. In the attached roots, theIAA-induced stimulation of primordium initiation was followedby a similar increase in lateral emergence. IAA treatment, however,had no effect on the number of laterals produced, per centimetreof root, in the excised primaries. Thus, exposure to IAA didnot directly enhance lateral emergence in the attached rootsnor did it stimulate such emergence in the excised ones. Nocorrelation was found between proliferative activity in themeristem at the apex of the primary or the rate of root elongationon the one hand, and either the number of primordia initiated,or the number of laterals produced, per centimetre of primary,on the other. Zea mays, maize, root, primordium, lateral, indol-3-yl acetic acid, meristematic activity     

Cell Cycle Duration in the Root Meristem of Sonoran Desert Cactaceae as Estimated by Cell-flow and Rate-of-cell-production Methods

Slow rates of cactus growth in the Sonoran Desert and high productivityof some Cactaceae under cultivation suggest that relativelylow growth rates are not the consequence of a long cell divisioncycle but of short optimal periods for growth and adverse environmentalfactors. To verify this hypothesis, the duration of the celldivision cycle (T)in the root apical meristem of seedlings ofthree sympatric species from the Sonoran Desert [Ferocactuspeninsulae(F. A. C. Weber) Britton & Rose ‘Townsendianus’(Britton & Rose) N. P. Taylor, stat. nov.,Stenocereus gummosus(Engelm.)Gibson & Horak andPachycereus pringlei(S. Watson) Britton& Rose] was estimated with the rate-of-cell-production (RCP)and the cell-flow (colchicine) methods. Both methods were appliedduring the steady-state growth phase, which was relatively shortin the first two species because of the determinate patternof root growth. The RCP method permitted estimation ofTin eachroot individually. Durations of the cell division cycle wereinversely proportional to the rate of root growth (r²rangedfrom 0.42 to 0.88,P<0.05).T,determined by the cell-flow method,ranged from 14.4 to 19.3 h in these species and was within thesame range asTdetermined by the RCP method. The averageTdeterminedby the RCP method was 67 to 75% of that determined by the cell-flowmethod. Results obtained with both methods are compared andanalysed. The proposed hypothesis appears to be correct, indicatingthat these species can be more productive under cultivationthan in the wild due to the relatively short duration of thecell division cycle. Adaptive features of these findings arealso considered.Copyright 1998 Annals of Botany Company Cactaceae, cell division cycle, root growth, root meristem, Sonoran Desert     

Morphogenesis in Yucca schidigera Roezl. Root Organ Cultures

Root development in suspension cultures of Yucca schidigerawas light-mediated. The green cultures consisted of roots, smalltissue aggregates and suspension cells. Roots possessed an apicalmeristem with a root cap, meristematic region and region ofdifferentiating tissues. Phloem, xylem vessels and tracheidsoccurred in discrete polyarch vascular bundles. Xylary wallthickening was reticulate, and endodermis and pericycle werepresent. Roots of intact Y. schidigera plants had a similardistribution of vascular tissues. Dark-grown cultures were cream-colouredand contained only lobed tissue aggregates and suspension cells. Yucca schidigera Roezl., tissue cultures, morphogenesis, root organ, light/dark     

Leaf Extension in Zea mays: II. LEAF EXTENSION IN RESPONSE TO INDEPENDENT VARIATION OF THE TEMPERATURE OF THE APICAL MERISTEM, OF THE AIR AROUND THE LEAVES, AND OF THE ROOT-ZONE

The temperatures of the roots, the apical meristem, and theshoots of Zea mays plants were varied independently of eachother and the rates of leaf extension were measured. When thetemperature of the apical meristem and region of cell expansionat the base of the leaf was kept at 25 °C, changes of leafextension in response to changes of root and shoot temperatureswere less pronounced. When the temperature of the meristematicregion was changed by increments of 5 or 10 °C from 0 to40 °C, and the root and shoot temperatures were kept at25 °C, rapid changes in leaf extension occurred. It was concluded that the rates of leaf extension were controlledat root-zone temperatures of 5 to 35 °C by heating or coolingof the meristematic region. Changes in rates of leaf extensionin response to changes in air temperature were attributed todirect effects on the temperature of the meristematic regionand on the physiology of the leaf.