Effect of Main Root Decapitation on Root Branching in Flax Seedlings

Root branching patterns in intact and decapitated flax (Linum usitatissimumL.) roots were compared. The number of initiated primordia in the control and decapitated roots was similar, but decapitated roots produced an increased number of lateral roots owing to an increase in the number of primordia developed into the laterals. It is suggested that the apical meristem influences lateral root development only at the stage of root emergence from the parent root.     

Cell cycle modulation in the response of the primary root of Arabidopsis to salt stress

Salt stress inhibits plant growth and development. We investigated the importance of cell cycle regulation in mediating the primary root growth response of Arabidopsis to salt stress. When seedlings were transferred to media with increasing concentrations of NaCl, root growth rate was progressively reduced. At day 3 after transfer of seedlings to growth medium containing 0.5% NaCl the primary roots grew at a constant rate well below that prior to the transfer, whereas those transferred to control medium kept accelerating. Kinematic analysis revealed that the growth reduction of the stressed roots was due to a decrease in cell production and a smaller mature cell length. Surprisingly, average cell cycle duration was not affected. Hence, the reduced cell production was due to a smaller number of dividing cells, i.e. a meristem size reduction. To analyze the mechanism of meristem size adaptation prior to day 3, we investigated the short-term cell cycle events following transfer to saline medium. Directly after transfer cyclin-dependent kinase (CDK) activity and CYCB1;2 promoter activity were transiently reduced. Because protein levels of both CDKA;1 and CDKB1;1 were not affected, the temporary inhibition of mitotic activity that allows adaptation to the stress condition is most likely mediated by posttranslational control of CDK activity. Thus, the adaptation to salt stress involves two phases: first, a rapid transient inhibition of the cell cycle that results in fewer cells remaining in the meristem. When the meristem reaches the appropriate size for the given conditions, cell cycle duration returns to its default.     

Dependence of Root Cell Growth and Division on Root Diameter

Primary roots of 98 species from different families of monocotyledonous and dicotyledonous plants and adventitious roots obtained from bulbs and rhizomes of 24 monocot species were studied. Root growth rate, root diameter, length of the meristem and elongation zones, number of meristematic cells in a file of cortical cells, and length of fully elongated cells were evaluated in each species after the onset of steady growth. The mitotic cycle duration and relative cell elongation rate were calculated. In all species, the meristem length was approximately equal to two root diameters. When comparing different species, the rate of root growth increased with a larger root diameter. This was due to an increase in the number of meristematic cells in a row and, to a lesser degree, to a greater length of fully elongated cells. The duration of the mitotic cycle and the relative cell elongation rate did not correlate with the root diameter. It is suggested that the meristem size depends on the level of nutrient inflow from upper tissues, and is thereby controlled during further growth.     

The seminal root primordia in barley and the participation of their non-meristematic cells in root construction

In addition to the primary seminal primordium, the so-called secondary seminal root primordia are also initiated in a barley embryo. The primary root primordium is developmentally most advanced. It is formed by root meristem covered with the root cap, and by a histologically determined region with completed cell division. On germination, the restoration of growth processes begins in this non-meristematic region of root primordium by cell elongation, with the exception of the zone adjacent to the scutellar node, the cells of which do not elongate but continue differentiating. In the root primordia initiated later, the zone with completed cell division is relatively shorter, in the youngest primordia the non-meristematic cells may be lacking. The root meristem is reactivated after the primary root primordium has broken through the sheath-like coleorrhiza and emerges from the caryopsis as the primary root. The character of root meristem indicates a reduced water content at the embryonic development of root primordium. With progressing growth the root apex becomes thinner, the meristematic region becomes longer, and the differences in the extent of cell division between individual cell types increase. — The primary root base is formed of cells pre-existing in the seminal root primordium. Upon desiccation of caryopsis in maturation, and subsequent quiescent period, their development was temporarily broken, proceeding with the onset of germination. The length of this postembryonically non-dividing basal zone is different in individual cell types. The column of central metaxylem characteristic of the smallest number of cell cycles, has, under the given conditions, a mean length of about 22 mm, whereas the pericycle, as the tissue with most prolonged cell division, has a mean length of about 6 mm. In the seminal root primordia initiated later the non-dividing areas are relatively shorter. The basal region of seminal roots thus differs in its ontogenesis from the increase which is formed “de novo” by the action of root meristem upon seed germination.     

Regeneration of roots from callus reveals stability of the developmental program for determinate root growth in Sonoran Desert Cactaceae

In some Sonoran Desert Cactaceae the primary root has a determinate root growth: the cells of the root apical meristem undergo only a few cell division cycles and then differentiate. The determinate growth of primary roots in Cactaceae was found in plants cultivated under various growth conditions, and could not be reverted by any treatment tested. The mechanisms involved in root meristem maintenance and determinate root growth in plants remain poorly understood. In this study, we have shown that roots regenerated from the callus of two Cactaceae species, Stenocereus gummosus and Ferocactus peninsulae, have a determinate growth pattern, similar to that of the primary root. To demonstrate this, a protocol for root regeneration from callus was established. The determinate growth pattern of roots regenerated from callus suggests that the program of root development is very stable in these species. These findings will permit future analysis of the role of certain Cactaceae genes in the determinate pattern of root growth via the regeneration of transgenic roots from transformed calli.     

Diversity of cell lengths in terminal portions of roots: implications to cell proliferation

Terminal meristems are responsible for all primary growth of roots. It has been asserted that all cells of root meristems are actively dividing (no cells cycle slowly or arrest in the cycle) and stem cell populations expand exponentially. Because cells do not slide relative to each other in roots, relative cell lengths may be used to determine relative cell cycle durations and/or proportions of cells actively dividing in root tissues. If all cells are cycling, no interphase cells should be longer than critical length (length of longest mitotic cell in the meristem) and cells should exhibit an exponential cell-age distribution. Lengths of all cells were obtained radially across entire median longitudinal root sections at 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 mm from the founder cell/root cap boundary for five plant species to estimate percentages of cells longer than critical length. For example, up to 15 and 90% of all interphase cells were longer than critical length in 0.5 and 2.0 mm tissues, respectively, indicating that slow-cycling and/or non-proliferative cells are present in such tissues. In order to determine if the distribution of cell lengths in 0.5 mm segments approximated an exponential cell-age distribution, lengths of interphase cells less than critical length were determined. Such interphase cells were placed into ten groups according to cell length and percentages of cells in each group were compared with percentages of cells in groups calculated from an exponential cell-age distribution. Percentages of cells were significantly different from predicted percentages of between 6 and 9 out of ten groups - cell lengths were not distributed exponentially. Because there are significant numbers of interphase cells longer than critical length and since lengths of interphase cells shorter than critical length do not resemble an exponential cell-age distribution, it must be concluded that not all cells in root segments from 0.5 to 3.0 mm root segments are actively dividing. Heretofore, no databases of cell lengths have been used to test these assertions.     

Root growth, developmental changes in the apex, and hydraulic conductivity for Opuntia ficus-indica during drought

Developmental changes in the root apex and accompanying changes in lateral root growth and root hydraulic conductivity were examined for Opuntia ficus-indica (L.) Miller during rapid drying, as occurs for roots near the soil surface, and more gradual drying, as occurs in deeper soil layers. During 7 d of rapid drying (in containers with a 3-cm depth of vermiculite), the rate of root growth decreased sharply and most root apices died; such a determinate pattern of root growth was not due to meristem exhaustion but rather to meristem mortality after 3 d of drying. The length of the meristem, the duration of the cell division cycle, and the length of the elongation zone were unchanged during rapid drying. During 14 d of gradual drying (in containers with a 6-cm depth of vermiculite), root mortality was relatively low; the length of the elongation zone decreased by 70%, the number of meristematic cells decreased 30%, and the duration of the cell cycle increased by 36%. Root hydraulic conductivity ( L _P) decreased to one half during both drying treatments; L _P was restored by 2 d of rewetting owing to the emergence of lateral roots following rapid drying and to renewed apical elongation following gradual drying. Thus, in response to drought, the apical meristems of roots of O. ficus-indica near the surface die, whereas deeper in the substrate cell division and elongation in root apices continue. Water uptake in response to rainfall in the field can be enhanced by lateral root proliferation near the soil surface and additionally by resumption of apical growth for deeper roots.     

Pericycle cell proliferation and lateral root initiation in Arabidopsis

In contrast with other cells generated by the root apical meristem in Arabidopsis, pericycle cells adjacent to the protoxylem poles of the vascular cylinder continue to cycle without interruption during passage through the elongation and differentiation zones. However, only some of the dividing pericycle cells are committed to the asymmetric, formative divisions that give rise to lateral root primordia (LRPs). This was demonstrated by direct observation and mapping of mitotic figures, cell-length measurements, and the histochemical analysis of a cyclin-GUS fusion protein in pericycle cells. The estimated duration of a pericycle cell cycle in the root apical meristem was similar to the interval between cell displacement from the meristem and the initiation of LRP formation. Developmentally controlled LRP initiation occurs early, 3 to 8 mm from the root tip. Thus the first growth control point in lateral root formation is defined by the initiation of primordia in stochastic patterns by cells passing through the elongation and young differentiation zones, up to where lateral roots begin to emerge from the primary root. Therefore, the first growth control point is not restricted to a narrow developmental window. We propose that late LRP initiation is developmentally unrelated to the root apical meristem and is operated by a second growth control point that can be activated by environmental cues. The observation that pericycle cells divide and lateral root primordia form without intervening mitotic quiescence suggests that lateral organ formation in roots and shoots might not be as fundamentally different as previously thought.     

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.     

Effect of 2,4-D on cell proliferation and elongation in the roots of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>

We investigated the effect of 2,4-D (2,4-dichlorophenoxyacetic acid) at concentrations of 1.5, 15, 30, and 60 nM on the growth of the main root of 3–7-d-old plants of Arabidopsis thaliana L. On the basis of measurements of the rate of root growth, lenght of fully elongated cells, and the number of cells in the meristem and elongation zone, we calculated the rates of cell proliferation and their transition to elongation, duration of cell cycle, and life span of cells in the meristem. At a concentration of 1.5 nM, 2,4-D did not affect these characteristics. At concentrations above 1.5 nM, 2,4-D noticeably retarded root growth, which was accounted for by a reduction in the length of cells that completed elongation, deceleration of cell proliferation and their transition to elongation, and prolongation of cell cycle and life span of the cells in the meristem. Thus, auxin decelerated root growth not only as a result of suppression of cell elongation but also at the higher concentrations via retardation of cell divisions in the meristem and their transition to elongation.     

ON THE CORRELATION OF PRIMARY ROOT LENGTH,MERISTEM SIZE AND PROTOXYLEM TRACHEARY ELEMENT POSITION IN PEA SEEDLINGS

Pea seedlings (Pisum sativum L. cv. Alaska) were darkgrown in vermiculite. Roots of various lengths were cleared, stained and measured to determine the relative meristem height (MH), width and volume and the distance to the most proximal protoxylem tracheary element (PTE). A correlation was found between root length, MH and PTE position as follows: in roots from 4–40 mm as the root elongated the MH lengthened and PTE position increased its distance from the body/cap juncture; in roots 40–80 mm MH and PTE position remained approximately constant; in longer roots (80–120 mm) MH became shorter, and PTE position closer to the tip as the root elongated. The relationship, using our measurement procedure was that for every 0.19 mm change in MH, the PTE position changed by 1 mm. This response was partially growth rate dependent since short roots (4–80 mm) grew at a constant rate and longer roots (80–140 mm) grew slower. Root manipulations and trifluralin treatment, to inhibit cell division, caused tip swelling and modulated the position of the PTE toward the root tip. The control of the spatial relationship between meristem size and maturation position is discussed.     

Effect of Water Stress on Cortical Cell Division Rates within the Apical Meristem of Primary Roots of Maize

We characterized the effect of water stress on cell division rates within the meristem of the primary root of maize (Zea mays L.) seedlings. As usual in growth kinematics, cell number density is found by counting the number of cells per small unit length of the root; growth velocity is the rate of displacement of a cellular particle found at a given distance from the apex; and the cell flux, representing the rate at which cells are moving past a spatial point, is defined as the product of velocity and cell number density. The local cell division rate is estimated by summing the derivative of cell density with respect to time, and the derivative of the cell flux with respect to distance. Relatively long (2-h) intervals were required for time-lapse photography to resolve growth velocity within the meristem. Water stress caused meristematic cells to be longer and reduced the rates of cell division, per unit length of tissue and per cell, throughout most of the meristem. Peak cell division rate was 8.2 cells mm-1 h-1 (0.10 cells cell-1 h-1) at 0.8 mm from the apex for cells under water stress, compared with 13 cells mm-1 h-1 (0.14 cells cell-1 h-1) at 1.0 mm for controls.     

Restoration of cell growth and proliferation in the wheat roots after their inhibition by nickel sulfate

The dynamics of cell growth and proliferation restoration in different tissues and quiescent center (QC) in the wheat (Triticum aestivum L.) seedling roots and also the differentiation of rhizodermal cells and lateral root initiation after 48-h treatment with 100 μM NiSO₄ were studied. Within 24 h after nickel removal from medium, root growth was resumed due to the increase in the rate of cell growth in the meristem and the region where cell elongation started in control roots. Stimulation of cell proliferation was restored in the main part of the meristem and later in the initial cells of the files and QC. Cell proliferation was not observed in the QC. The time of cell proliferation resumption in the roots and in tested tissues depended on the degree of their injury by nickel treatment. In most tested roots, DNA synthesis and cell division were restored in 32 h. In the cells leaving the meristem due to the resumption of their growth and proliferation, growth of root hairs started. In 48 h, the number of roots with perished cells in the rhizodermis in the meristem was sharply increased and the regeneration of the damaged region by the cells of outer cortex was observed. Only after the appearance of root hairs, the cells coming from the meristem started to elongate. In most roots, the formation of the new elongation zone occurred in 56 h. During its formation, the initiation of lateral root primordia was shifted in the basipetal direction. It was concluded that the cessation of cell growth and proliferation under the influence of high concentration of heavy metal (HM) ions is not lethal for the root. At the action of toxic HM concentrations, the plant strategy is the maintenance of meristematic cell capacity for cell growth and proliferation resumption. The cellular mechanism of this capacity maintenance is the transition of meristematic cells from G₁ phase to dormancy due to growth inhibition and the inhibition of the transition to DNA synthesis.     

Consequences of root growth kinetics and vascular structure on the distribution of lateral roots

It has been proposed that the acropetal initiation of lateral roots is a built‐in process specified as part of the general process of cell division and differentiation in the parent root tip. Conversely, it is commonly reported that root branching is essentially a variable feature. In the present study, the interlateral distance along the parent root has been investigated using three banana varieties (Musa spp.) grown in two substrates. The pattern of lateral root initiation was obscured by variations of root growth patterns and vascular structure among roots, genotypes and substrates. A framework model is formulated showing the influence of growth pattern and vascular structure on branching density. The model raises a distinction between growth components which should not affect the branching density (i.e. rate of cell division) and which may affect it (i.e. size of mature cells and number of transverse divisions performed by cells executing their trajectory in the meristem). It appears also that lateral root density and root growth rate might be independently modulated by appropriate changes of root growth patterns, in banana and presumably many other taxa.     

Abscisic acid signal transduction in epidermal cells of Pisum sativum L. Argenteum: both dehydrin mRNA accumulation and stomatal responses require protein phosphorylation and dephosphorylation

The determinate growth of the primary root, its organization and relationship with lateral-root development, and the possible ecological significance of this growth pattern were analyzed in three sympatric species of Cactaceae from the Sonoran Desert, Stenocereus gummosus (Engelm.) Gibson & Horak, S. thurberi (Engelm.) Buxbaum and Ferocactus peninsulae (F.A.C. Weber) Britton & Rose, var. townsendianus (Britton & Rose) N.P. Taylor, stat. nov., Engelm. After seed germination, primary roots of these species commonly grew only for 2–3 d after the start of radicle protrusion (ASRP). This pattern of growth was observed on seedlings growing on filter paper, in vitro under sterile conditions, or in soil. The root-hair zone approached the very tip of the root and meristem exhaustion appeared to be typical in all seedlings of a population in all species. On average, 23 meristematic cells in the epidermal cell file in F. peninsulae were counted during the short steady-state period of growth (12–24 h ASRP). In S. gummosus, the size of the meristem was smaller with the number of epidermal cells in the meristem during the short steady-state growth period (12–36 h ASRP) averaging 13. The dynamics of meristem exhaustion obeyed Ivanov's model of the life span of cells in the meristem that states: if cell division is suppressed, half of the cells present in the meristem at a given time leave the meristem and start elongation during the period equal to the duration of the cell division cycle. It was deduced, on average, three to five cell division cycles in the meristem preceded its exhaustion. The lost meristem integrity can be related to only a few initial cells being found in the radicle. The cessation of meristematic activity in the primary-root apical meristem was directly related to the induction of lateral-root formation. Determinate primary-root growth can be thus viewed as a physiological root-tip decapitation that stops production of a signal inhibiting lateral-root primordia initiation. The time of lateral-root formation in S. gummosus and F. peninsulae was equal to or shorter than in agronomic mezophyte plants. Lateral roots also had determinate growth. The rapidity of root-system development and the ability to stop and to continue growth at any time under unfavorable and favorable conditions suggests the important role of determinate growth in seedling establishment of these Sonoran Desert species. Received: 13 December 1996 / Accepted: 6 January 1997     

Effects and Absorption of Sethoxydim in Cell Cycle Progression of Corn (Zea mays) and Pea (Pisum sativum)

The mechanisms of selective herbicidal action of sethoxydim were investigated by using cultured root tips of corn (Zea mays L. cv Goldencrossbantam) and pea (Pisum sativum L. cv Alaska). Meristematic cells in the cultured roots were arrested in G₁ and G₂ of the cell division cycle by sucrose starvation and resumed growth and cell division (proliferation) when sucrose was provided. Corn root growth after sucrose addition was inhibited by sethoxydim at concentrations of 0.01 micromolar and greater when roots were treated in the presence of sucrose but was not inhibited at 10 micromolar sethoxydim when they were treated during sucrose starvation. Greater absorption of [¹⁴C]sethoxydim into the meristematic region of corn roots was observed when cells were in proliferative condition but not when they were arrested by sucrose starvation, whereas no greater absorption of the herbicide into pea meristems was observed in either growth condition. In the cell cycle study, greater absorption of [¹⁴C]sethoxydim into the corn root meristem was observed at a certain limited time before S (DNA synthesis) stage. The physiological effects and the greater absorption of sethoxydim clearly depended on cell cycle progression of corn root meristem, whereas fatty acid synthesis, as well as its inhibition by sethoxydim, was not associated with either cell cycle progression or greater absorption of the herbicide.     

The involvement of wheat and common bean lectins in the control of cell division in the root apical meristems of various plant species

The effects of wheat germ agglutinin (WGA) and phytohemagglutinin (PHA) at the concentration of 1 mg/l on the rate of cell division in the root apical meristem of wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), rice (Oryza sativa L.), and common bean (Phaseolus vulgaris L.) seedlings were compared. WGA enhanced cell division in the roots of barley and rice approximately similarly as in wheat roots but did not affect division of meristematic cells in the roots of common bean seedlings. In contrast PGA enhanced mitotic activity in the root apical meristem of common bean seedlings but did not affect division in the wheat and barley roots. Seedling treatment with lectins shifted the hormonal balance in them toward accumulation of growth activators (IAA and cytokinins). The relationship between lectin and hormonal systems in the control of cell division is discussed.     

Growth components in Allium roots

Summary This paper reports results of a study of root growth, duration of the division cycle, and cell size for different intervals of root length, associated with different phases of growth, namely, from the onset of growth to the achievement of a steady state characterized by a constant rate of growth. The material is roots of Allium cepa.Steady-state kinetics of growth are achieved only when all components of growth reach a constant value. Although the cell flow remains constant from a root length of 8 mm onwards, the steady condition is only observed when the final size of the epidermal cells reaches its maximum and constant value (at about 12 mm of root length).The equation G=24×10^-5 \N\·L is proposed to analyze the growth of Allium roots in its components. G is the growth rate (mm/24 hr), N the number of cells per column in the meristematic ribs, the rate of the cell cycle (the reciprocal of its duration x100), and L the average final size of the epidermal cells (in ).     

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.     

CORTICAL ONTOGENY IN ROOTS. I. ZEA MAYS

Serial growth stages of young Zea mays primary roots were analyzed for patterns of ground meristem ontogeny. The number of cell layers in the cortex decreases from approximately 15 to 11 during early root growth. The cortex arises mostly by periclinal divisions in the outer portions of the ground meristem at levels 50–150 μm from the meristem tip, although some layers of outer cortex arise beyond 150 μm. The proendodermis contributes 3–5 cell layers to the cortex, but this contribution diminishes during early seedling growth as anticlinal divisions occur in the proendodermis. The relationship between the ground meristem and protoderm changes at the tip of the meristem during root elongation.