Root meristems cycle

Figure 32-8 (A) The life cycle of a flowering plant with emphasis on egg-cell formation and seed development. (B) Some further details of embryo development. T, terminal cell B, basal cell C, cotyledon A, axis SC, seed coat En, endosperm EP, embryo proper S, suspensor SM, shoot meristem Pd, protoderm RM, root meristem. From Goldberg et al.i66 with modification.

Mucilage has protective functions for the root meristem and improves root-soil contact by inclusion and aggregation of soil particles. It may also contribute to P desorption and to the exclusion of toxic elements (Al, Cd, Pb) by complexation with galacturonates, mainly in exchange with Ca2+ (Neumann and Romheld, 2002). Secreted enzymes contribute to the extracellular enzyme pool it has been shown that the activity of extracellular enzymes, such as phophatases, proteases, and aryl-sulfatases, exhibit more activity in the rhizosphere relative to the bulk soil and may have a dramatic effect on the cycling of nutrients such as P, N, and S (Badalucco and Nannipieri, 2007). 

Once clear inhibitory effects were detected on germination and radicle growth of lettuce plants exposed to BOA,12,35 our group developed methodology to show whether there is a detectable BOA effect on the cell cycle progression in lettuce root meristems.13 This allows a more detailed characterization of the putative direct or indirect effect on the meristems of BOA-treated seedlings. 

When the HU synchronization period was finished, and the HU was removed, the seedlings were immediately incubated with 1 mM BOA or water (controls), and the cell cycle was analyzed at different times. Samples (nuclear suspensions) are prepared from root meristems and analyzed by flow cytometry every 2 h during a time period of 12-14 h.15 Comparison of the synchronized-cell cycle progression of BOA-treated plants with that of the corresponding controls allows for detection of partial or total inhibition of the cell cycle. 

So, these results showed a double effect of BOA on lettuce meristems an increasing significant delay in the cell cycle progression and a decrease in the mitotic index. Flow cytometry analysis showed a weak effect of BOA at cell cycle level in the step from G2 to M. BOA effect appeared to retard the cell cycle progression of treated-root meristems, which was very clear after 10 h BOA exposition (Fig. 12.5), and as an inhibition of the number of cells undergoing cell division. 

Dolezel, J., Cihalikova, J., Weiserova, J., and Lucretti, S. 1999. Cell cycle synchronization in plant root meristems. Meth. Cell Sci. 21, 95-107... 

Cytokinins are reported to affect both the and G2 phases of the cell cycle of root meristems, causing a reduction in G (MacLeod 1968) and a lengthening of G2 (MacLeod 1968, Van t Hof 1968). This indicates that even though cytokinins may act most directly on a particular step of the cell cycle... 

Although auxin is required for cell division, it increases the duration of the mitotic cycle in the root meristem of Vicia faba (MacLeod and Davidson 1966, Davidson and MacLeod 1966) and at higher concentrations suppresses mitosis completely in onion root meristem (Nagl 1972). In the liverwort, Riella helicophylla, anti-auxin treatment accelerates division, presumably by reducing the effective endogenous level of auxin (Grotha 1976). 

Barlow PW (1973) Mitotic cycles in root meristems. In Balls M, Billet FS (eds) The cell cycle in development and differentiation. Cambridge Univ Press, Cambridge, pp 133-165... 

Barlow PW, Macdonald PDM (1973) An analysis of the mitotic cell cycle in the root meristem of Zea mays. Proc Roy Soc Lond B 183 385-398 Basrur SV, Fletcher RA, Basrur PK (1976) In vitro effects of 2,4-dichlorophenoxy acetic acid (2,4-D) on bovine cells. Can J Comp Med 40 408-415 Batra MW, Edwards KL, Scott TK (1975) Auxin transport in roots Its characteristics and relationships to growth. In Torrey JG, Clarkson DT (eds) The development and function of roots. Academic Press, London New York, pp 299-325 Beasley CA, Ting IP (1973) The effects of plant growth substances on in vitro fiber development from fertilized cotton ovules. Am J Bot 60 130-139 Beasley CA, Ting IP (1974) Phytohormone effects on in vitro cotton seed development. 

MacLeod RD (1968) Changes in the mitotic cycle in lateral root meristems of Vida faba following kinetin treatment. Chromosoma 24 177-187 MacLeod RD (1971) The response of apical meristems of primary roots of Vida faba L. 

Van Staden J, Wareing PF (1972) The effect of light on endogenous cytokinin levels in seeds of Rumex obtusifolius. Planta 104 126-133 Van Staden J, Olatoye ST, Hall MA (1973) Effect of light and ethylene upon cytokinin levels in seed of Spergula arvensis. J Exp Bot 24 662-666 Van t Hof J (1965) Relationships between mitotic cycle duration, S period duration and the average rate of DNA synthesis in the root meristem cells of several plants. Exp Cell Res 39 48-58... 

Van t Hof J (1968) The action of lAA and kinetin on the mitotic cycle of proliferative and stationary phase excised root meristems. Exp Cell Res 51 167-176 Van t Hof J (1973) Two principal points of control in the mitotic cycle of pea meristem cells energy considerations, characterization, and radiosensitivity. In Duplan JF, Chapiro A (eds) Advances in radiation research. Biology and medicine Vol 2. Gordon and Breach, London, pp 881-894... 

Cell Division Inhibitors. The most common mode of action of soil-appHed herbicides is growth inhibition, primarily through dkect or indkect interference with cell division (163). Such growth inhibitory activity is the basis for most pre- or post-emergent herbicides intended to control germinating weed seeds. In germinating seeds, cell division occurs in the meristems of the root and the shoot. Meristematic cells go through a cycle... 

Comparative cell cycle analysis of 6 h hydroxyurea-blocked root cells from 1 mM BOA-treated and control lettuce meristems. Seedlings were exposed to treatment after HU release, and then nuclear suspensions were prepared with 40 meristems per treatment and analyzed by flow cytometry every 2 h. (From Coba de la Pena, T. and Sanchez-Moreiras, A. 2001, Handbook of Plant Ecophysiology Techniques, Kluwer Academica Publishers, Dordrecht, the Nederlands, pp. 65-68. With permission). 

An example of the time devoted to each phase of the cell cycle in the apical meristem of Vida faba roots is shown in Fig. 1, though these values vary somewhat in reports from different laboratories (MacLeod 1971). Cells which are not committed to nuclear division are sometimes viewed as entering a state Go (Gould et al. 1974, Peaud-Lenoel 1977), different from those shown in Fig. 1. Cells which begin differentiation leave the cell cycle and initiate a specialized pathway of development. 

It is difficult to relate the effects of GA on division to a direct role of GA in the control of the cell cycle or even to prove that the enhancement of cell division by GA is not an indirect effect arising from enhancement of cell elongation (Jones 1973). GA increases the mitotic index in meristematic cells of the leaf and root of barley and in both cases a simultaneous increase in DNA synthesis occurs (Svarinskaya and Gavrilova 1976). GA may have some selective effect on the cell cycle in the barley root in this case since it reduces the duration of G, G2, and M. GA decreases the cell cycle time in the subapical meristem of dwarf waterm elon seedlings (Liu and Loy 1976) where it decreases time spent in S. The primary effect of GA in apical buds of Rud-... 

Friedberg SH, Davidson D (1970) Duration of S phase and cell cycles in diploid and tetraploid cells of mixoploid meristems. Exp Cell Res 61 216-218 Fuchs Y, Lieberman M (1968) Effects of kinetin, lAA, and gibberellin on ethylene production, and their interactions in growth of seedlings. Plant Physiol 43 2029-2036 Gaither DH, Lutz DH, Forrence LE (1975) Abscisic acid stimulates elongation of excised pea root tips. Plant Physiol 55 948-949... 

Watanabe T, Kondo N (1976) Isolation of a proteinaceous inhibitor of ethylene biosynthesis from marine algae. Agric Biol Chem 40 1877-1878 Webster PL (1967) Cell cycle kinetics in meristems Effects of colchicine and lAA. PhD Thesis, Case-Western Res Univ, Cleveland, Ohio Webster PL, Langenauer HD (1973) Experimental control of the activity of the quiescent centre in excised root tips of Zea mays. Planta 112 91-100 White KL, Hill AC, Bennett JH (1974) Synergistic inhibition of apparent photosynthesis rate of alfalfa by combinations of sulfur dioxide and nitrogen dioxide. Environ Sci Technol 8 574-576... 

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