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.

3D analysis of mitosis distribution highlights the longitudinal zonation and diarch symmetry in proliferation activity of the Arabidopsis thaliana root meristem.

To date CYCB1;1 marker and cortex cell lengths have been conventionally used to determine the proliferation activity of the Arabidopsis root meristem. By creating a 3D map of mitosis distribution we showed that these markers overlooked that stele and endodermis save their potency to divide longer than the cortex and epidermis. Cessation of cell divisions is not a random process, so that mitotic activity within the endodermis and stele shows a diarch pattern. Mitotic activity of all root tissues peaked at the same distance from the quiescent center (QC); however, different tissues stopped dividing at different distances, with cells of the protophloem exiting the cell cycle first and the procambial cells being the last. The robust profile of mitotic activity in the root tip defines the longitudinal zonation in the meristem with the proliferation domain, where all cells are able to divide; and the transition domain, where the cell files cease to divide. 3D analysis of cytokinin deficient and cytokinin signaling mutants showed that their proliferation domain is similar to that of the wild type, but the transition domain is much longer. Our data suggest a strong inhibitory effect of cytokinin on anticlinal cell divisions in the stele.

Global analysis of an exponential model of cell proliferation for estimation of cell cycle duration in the root apical meristem of angiosperms.

Background and Aims: Information on cell cycle duration (T) in the root apical meristem (RAM) provides insight into root growth, development and evolution. We have previously proposed a simple method for evaluating T based on the dynamics of root growth (V), the number of cells in the RAM (Nm) and the length of fully elongated cells (l), which we named the rate-of-cell-production (RCP) method. Here, a global analysis was performed to confirm the reliability of this method in a range of angiosperm species and to assess the advantages of this approach. Methods: We measured V, Nm and l from live or fixed cleared primary roots of seedlings or adventitious roots of bulbs and used this information to estimate the average T values in 73 angiosperm species via the RCP method. The results were then compared with published data obtained using the classical but laborious and time-consuming 3H-thymidine method. Key Results: In most species examined, the T values obtained by the RCP method were nearly identical to those obtained by the 3H-thymidine method. Conclusions: The global analysis demonstrated that the relationship between the variables V, Nm and l in roots in the steady state of growth is correctly described by the equation T = (ln2 Nm l)V-1. Thus, the RCP method enables cell cycle duration in the RAM to be rapidly and accurately determined. This method can be performed using live or fixed roots for each individual cell type. The simplicity of the approach suggests that it will be widely used in phenomics, evolutionary ecology and other plant biology studies.

Longitudinal zonation pattern in Arabidopsis root tip defined by a multiple structural change algorithm.

Background and Aims The Arabidopsis thaliana root is a key experimental system in developmental biology. Despite its importance, we are still lacking an objective and broadly applicable approach for identification of number and position of developmental domains or zones along the longitudinal axis of the root apex or boundaries between them, which is essential for understanding the mechanisms underlying cell proliferation, elongation and differentiation dynamics during root development. Methods We used a statistics approach, the multiple structural change algorithm (MSC), for estimating the number and position of developmental transitions in the growing portion of the root apex. Once the positions of the transitions between domains and zones were determined, linear models were used to estimate the critical size of dividing cells (LcritD) and other parameters. Key Results The MSC approach enabled identification of three discrete regions in the growing parts of the root that correspond to the proliferation domain (PD), the transition domain (TD) and the elongation zone (EZ). Simultaneous application of the MSC approach and G2-to-M transition (CycB1;1DB:GFP) and endoreduplication (pCCS52A1:GUS) molecular markers confirmed the presence and position of the TD. We also found that the MADS-box gene XAANTAL1 (XAL1) is required for the wild-type (wt) PD increase in length during the first 2 weeks of growth. Contrary to wt, in the xal1 loss-of-function mutant the increase and acceleration of root growth were not detected. We also found alterations in LcritD in xal1 compared with wt, which was associated with longer cell cycle duration in the mutant. Conclusions The MSC approach is a useful, objective and versatile tool for identification of the PD, TD and EZ and boundaries between them in the root apices and can be used for the phenotyping of different genetic backgrounds, experimental treatments or developmental changes within a genotype. The tool is publicly available at www.ibiologia.com.mx/MSC_analysis.

Reversible and Irreversible Color Change during Photo and Thermal Degradation of PolyphenyleneSulfide Composite.

The effect of the light spectral composition and temperature on the change of color characteristics and reflection spectra during the irradiation of polyphenylene sulfide reinforced by short glass fibers in the SUNTEST apparatus was analyzed. The scales of reversible color change upon successive exposure to total radiation corresponding to the sunlight spectrum and to visible light wereevaluated and the possible mechanisms for the observed effects are discussed. The features of the color change upon visible light irradiation of previously thermally aged samples wereconsidered. Possible causes for deviation from the Arrhenius law during thermal aging of the composite are discussed. It wasdemonstrated that even with a significant change in color, the physicomechanical and electrotechnical characteristics of the composite only changedslightly or remained virtually at the same high level.

Cytokinins regulate root growth through its action on meristematic cell proliferation but not on the transition to differentiation.

Contrary to the wide-spread view that cytokinins change the rate of root growth and meristem size by regulating the cell transition to elongation (differentiation), our data showed that cytokinins affected the cell cycle duration in the meristem. The rate of meristematic cell transition to elongation itself is regulated by two groups of independent processes, through influence on (i) the life-span of cells in the meristem, and (ii) the cell proliferation rate in the meristem. Trans-zeatin slows down the root growth rate and the cell transition to elongation as a result of prolongation of mitotic cycles. The life-span of cells in the meristem does not change. The number of meristematic cells in one file decreases due to inhibition of cell proliferation but not to an acceleration of cell transition to elongation. Roots of triple mutant ipt3ipt5ipt7, in which cytokinin synthesis is slowed down, behave in an opposite way such that the rate of cell transition to elongation and cell proliferation is speeded up. Their peculiarity is that the life-span of cells in meristem becomes shorter than in control roots. In both cases, a change in concentration of endogenous cytokinin or in its signalling are associated with a change in mitotic cycle duration.

Longitudinal zonation pattern in plant roots: conflicts and solutions.

Despite the relative simplicity of Arabidopsis root organization, there is no general agreement regarding the terminology used to describe the longitudinal zonation pattern (LZP) of this model system. In this opinion article, we examine inconsistencies in the terminology and provide a conceptual framework for the LZP that may be applied to all angiosperms. We propose that the root apical meristem (RAM) consists of the cell-proliferation domain where cells maintain a high probability to divide and the transition domain with a low probability of cell division; in both domains cells grow at the same, relatively low, rate. Owing to stochastic termination of cell proliferation in the RAM, the border between the domains is 'fuzzy'. Molecular markers analyzed together with quantitative growth and cell analyses could help to identify developmental zones along the root and lead to a better understanding of the LZP in angiosperms.