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

A neutral invertase controls cell division besides hydrolysis of sucrose for nutrition during germination and seed setting in rice

Sucrose is the transport form of carbohydrate in plants serving as signal molecule besides nutrition, but the signaling is elusive. Here, neutral invertase 8 (OsNIN8) mutated at G461R into OsNIN8m, which increased its charge and hydrophobicity, decreased hydrolysis of sucrose to 13% and firmer binding to sucrose than the wildtype. This caused downstream metabolites and energy accumulation forming overnutrition. Paradoxically, division of subinitials in longitudinal cell lineages was only about 15 times but more than 100 times in wildtype, resulting in short radicle. Further, mutation of OsNIN8 into deficiency of hydrolysis but maintenance of sucrose binding allowed cell division until ran out of energy showing the association but not hydrolysis gave the signal. Chemically, sucrose binding to OsNIN8 was exothermic but to OsNIN8m was endothermic. Therefore, OsNIN8m lost the signal function owing to change of thermodynamic state. So, OsNIN8 sensed sucrose for cell division besides hydrolyzed sucrose.

The quiescent centre of the root apical meristem: conceptual developments from Clowes to modern times

In this review we discuss the concepts of the quiescent centre (QC) of the root apical meristem (RAM) and their change over time, from their formulation by F.A.L. Clowes to the present. This review is dedicated to the 100th anniversary of the birth of Clowes, and we present his short biography and a full bibliography of Clowes' work. Over time, the concept of the QC proved to be useful for the understanding of RAM organization and behaviour. We focus specifically on conceptual developments, from the organization of the QC to understanding its functions in RAM maintenance and activity, ranging from a model species, Arabidopsis thaliana, to crops. Concepts of initial cells, stem cells, and heterogeneity of the QC cells in the context of functional and structural stem cells are considered. We review the role of the QC in the context of cell flux in the RAM and the nature of quiescence of the QC cells. We discuss the origin of the QC and fluctuation of its size in ontogenesis and why the QC cells are more resistant to stress. Contemporary concepts of the organizer and stem cell niche are also considered. We also propose how the stem cell niche in the RAM can be defined in roots of a non-model species.

Origin of the concept of the quiescent centre of plant roots

Concepts in biology feed into general theories of growth, development and evolution of organisms and how they interact with the living and non-living components of their environment. A well-founded concept clarifies unsolved problems and serves as a focus for further research. One such example of a constructive concept in the plant sciences is that of the quiescent centre (QC). In anatomical terms, the QC is an inert group of cells maintained within the apex of plant roots. However, the evidence that established the presence of a QC accumulated only gradually, making use of strands of different types of observations, notably from geometrical-analytical anatomy, radioisotope labelling and autoradiography. In their turn, these strands contributed to other concepts: those of the mitotic cell cycle and of tissue-related cell kinetics. Another important concept to which the QC contributed was that of tissue homeostasis. The general principle of this last-mentioned concept is expressed by the QC in relation to the recovery of root growth following a disturbance to cell proliferation; the resulting activation of the QC provides new cells which not only repair the root meristem but also re-establish a new QC.

Redox regulation of root apical meristem organization: connecting root development to its environment

Post-embryonic root growth relies on the proliferative activity of the root apical meristem (RAM), consisting, in part, of cells with juvenile characteristics (stem cells). It is generally, but erroneously held that the RAM indefinitely produces new cells throughout the lifespan of a plant, resulting in indeterminate root growth. On the contrary, convincing data, mainly from the lab of Thomas L. Rost, show in all species analyzed so far, including Arabidopsis, that RAM organization changes over time in parallel with both a cessation of the production of new cells, and a consequent reduction in root growth, even under optimal conditions. In addition, RAM organization evolved to become highly plastic and dynamic in response to environmental triggers (e.g. water and nutrient availability, pollutants). Under unfavourable conditions, the RAM is rapidly reorganized, and, as a result of the cessation of new cell production at the root tip, root growth is altered, and lateral root production is enhanced, thus providing the plant additional strategies to overcome the stress. It is now becoming increasingly clear that this environment-responsive developmental plasticity is linked to reactive oxygen/nitrogen species, antioxidants, and related enzymes, which form part of a complex signalling module specifically operating in the regulation of RAM functioning, in strict relationship with hormonal control of root development exerted by auxin, gibberellins and cytokinins. In turn, such redox/hormone crosstalk regulates gene expression.

Oxidative stress and formation and maintenance of root stem cells

The hypothesis of L. Feldman and his coworkers, according to which a more oxidizing environment in the cells of root quiescent center results from high activity of ascorbate oxidase activated by indoleacetic acid (IAA) accumulating in these cells, is discussed. The high activity of ascorbate oxidase is responsible for lowered concentrations of the reduced form of ascorbic acid and glutathione and high content of reactive oxygen species in quiescent center cells. The oxidative stress represses proliferation of the cells. Inhibitors of IAA transport attenuate the oxidative stress, thus suggesting a role of IAA as an activator of ascorbate oxidase. Interestingly, the high concentration of IAA in dividing cap cells adjacent to the quiescent center cells did not cause retardation of cell proliferation and oxidative state in these cells.

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.

WHAT CHIMERAS CAN TELL US ABOUT PLANT DEVELOPMENT

▪ Abstract  The generation and analysis of plant chimeras and other genetic mosaics have been used to deduce patterns of cell division and cell fate during plant development and to demonstrate the existence of clonally distinct cell lineages in the shoot meristems of higher plants. Cells derived from these lineages do not have fixed developmental fates but rely on positional information to determine their patterns of division and differentiation. Chimeras with cells that differ genetically for specific developmental processes have been experimentally generated by a variety of methods. This review focuses on studies of intercellular interactions during plant development as well as of the coordination of cells during meristem function and organogenesis. Recent experiments combining mosaic analysis with molecular analysis of developmental mutants have begun to shed light on the nature of the signals involved in these processes and the mechanisms by which they are transmitted and received among cells.

The size of quiescent centre in roots of Allium cepa L. grown with ascorbic acid

Following ascorbic-acid treatment a large number of quiescent centre cells undergo DNA synthesis and, at the same time, the cell proliferation in the entire root meristem of Allium cepa is stimulated. The effects of ascorbic acid on dividing cells in the meristem proper and on the quiescent centre are long-lasting since they are obtained in both short- and long-term experiments. Whatever the time of treatment with ascorbic acid and whatever the starting size of the quiescent centre (450 or 1000 cells), there is always a minimum number of quiescent centre cells (90-100) which remain in the G₁ phase.

CELLULAR PACKETS, CELL DIVISION AND MORPHOGENESIS IN THE PRIMARY ROOT MERISTEM OF ZEA MAYS L

Dormant meristematic cells of the unemerged radicle contained within the caryopsis of Zea mays L. are arrested in either the G₁ or G₂ phase of the mitotic cycle. Following germination and the resumption of root growth, each of these cells divides repeatedly to form multicellular groups, or 'packets'. The thickened cell walls that bound each packet correspond to the walls of the formerly dormant mother cell, while the thinner walls that partition the packet correspond to walls laid down when the successive rounds of division are completed. The relative thickness of these partition walls corresponds to their age, the most recently inserted wall being the thinnest. The packets thus give evidence of not only the number of divisions that have occurred since germination, but also the sequence in which these divisions took place. In addition, the elongation of the packets during root growth allows their displacement away from the root tip into the zone beyond the margin of the meristem to be measured. Using roots fixed at different times during early growth, the kinetics of packet development has been followed in cells occupying different positions within the meristem at the start of root growth. By counting the number of cells in the packets at frequent intervals during root growth, the period between each round of division has been found to be fairly constant, even as the cells are displaced towards the margin of the meristem. Variability in the interdivisional period within a packet is insufficient to cause extensive overlapping of the different rounds of division. Exceptions are found in cortical and stelar cells around the quiescent centre, where the more distal cells in a packet often divide at up to half the rate of the more proximal cells. This is evidence of a steep gradient of cell extension rate near the quiescent centre; such a gradient does not occur along packets elsewhere in the meristem. In the quiescent centre itself, cells of its cortical portion divide more rapidly than cells of its stelar portion. Cells in the cortex (but not in the stele) often divide unequally at their first transverse division; the distal (apical) daughter is usually the longer of the two daughter cells. Asymmetric, transverse division also occurs in some cells during the next rounds of division. The more rapid entry into mitosis of the longer daughter cell results in packets with particular sequences of division. Asymmetric, longitudinal (periclinal) divisions also occur in the cortex, the inner daughter cell being wider than the outer daughter cell. These periclinal divisions occur in the distal portion of the cortex near its inner and outer borders with the stele and epidermis, respectively. At the start of root growth, periclinal divisions commence sooner in the outer cortex than in the inner cortex but do not persist, and the number of cell files across the width of the cortex declines. A concurrent loss of files also occurs in the stele. The first two rounds of periclinal divisions in the innermost file of the cortex show a definite spatial pattern. These divisions intrude into the quiescent centre and may account for the apparently anomalous faster cycling cells that have been reported here. The cellular packets give insights into certain of the fundamental aspects of root morphogenesis, the choice that confronts a cell of whether to divide transversely or longitudinally being of special importance. Particular ranges of values for the ratio between cell length and breadth are associated with these two classes of division.

Cytokinin biosynthesis in roots of corn

Removal of the quiescent center (QC) from the root apex of maize (Zea mays L., cv. Kelvedon 33) initiates a set of events which culiminate in the regeneration of an intact apex with a newly formed QC. Concomitant with the formation of a new QC is a marked reduction in extractable cytokinins in the tissue of the proximal meristem. Replacing the excised QC with a Dowex (acidic cation-exchange resin) bead affects both root growth and QC regeneration. Root growth is inhibited by "plain" Dowex beads and Dowex beads treated with zeatin; this inhibition is reversed if the beads have been treated with CaCl2 (±zeatin). Dowex beads treated with zeatin delay the formation of a new QC; this effect is the same whether or not the beads also contain CaCl2. The results of this investigation support the notions that cytokinin biosynthesis in roots is a result of activities of both the QC and the proximal meristem, and that cytokinins, at least if supplied exogenously, can play a role in root morphogenesis by delaying the regeneration of the QC.

Development of the quiescent center in maturing embryonic radicles of pea (Pisum sativum L. cv. Alaska)

Maturing embryos of pea (Pisum sativum L. cv. Alaska) were treated with an aqueous solution of tritiated thymidine for 1 h, sectioned, and processed for autoradiography. An analysis of the distribution of labelled nuclei and mitotic figures demonstrated the presence of a quiescent center (QC) in the radicles of developing embryos. The QC developed in the radicle during the growth of the embryo. Immature radicles that did not contain a well-formed zone of root-cap initials did not show a QC. In the latter stages of seed ripening, the pattern of arrest of DNA synthesis and mitosis was tissue-specific. Cells within the QC remained inactive. The region lacking labelled nuclei and mitotic figures progressively expanded to include the root cap initials and then the provascular cylinder. Mitosis was arrested before DNA synthesis in the embryonic cortex. Cells within the QC synthesized DNA during the first stages of seed germination.