Meristem

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... 

Those herbicides that block mitotic entry decrease or prevent the formation of mitotic figures in meristems. Amino acid, protein, RNA, DNA, and ATP synthesis and/or utilization can all attest cell growth (163,166). Although not registered as herbicides, cycloheximide [66-81-9] inhibits mitotic entry by inhibiting protein synthesis (167) hydroxyurea/727-(97-/7 inhibits DNA synthesis (168) and actinomycin D [50-76-0] nh2oix.s RNA synthesis (167). 

Crevecoeur M. Pinedo M. Greppin H. Penel C. (1997) Peroxodase activity in shoot apical meristem from Spimcia / / Acta Histochem. V. 99(2). P. 177-186. 

Surprisingly, very little physiological work has been done to understand the nature and processes of plant recovery from extreme drought stress, especially in relation to plant production (Chapter 7). In order for the plant to recover properly from severe water stress, its various meristems must survive. The association between severe plant stress and the factors that affect meristem survival and function upon rehydration are unclear though osmoregulation may have a possible protective role and as a potential source of carbon for recovery. Active plant apices generally excel in osmoregulation and do not lose much water upon plant dehydration (Barlow, Munns Brady, 1980). 

Barlow, E.W.R., Munns, R.E. Brady, C.J. (1980). Drought responses of apical meristems. In Adaptation of Plants to Water and High Temperature Stress, ed. N.C. Turner and P.J. Kramer, pp. 191-206. New York Wiley Interscience. 

Immunogold localization of the pectic epitope has been performed on different types of cells cell suspensions, roots, shoots, meristems, coleoptiles, pollen grains, protoplasts from different species carrot, sugar beet, tobacco, oat... The pattern of labeling was always the same polygalacturonic acid was essentially located on the material expanded at three-way junctions between cells or lining intercellular space, but was not found in primary walls. No epitope could be located close to the plasma membrane (Fig. lO.a). Middle lamellae far from junction zones and walls of meristematic cells were never labeled. 

Aluminium toxicity is a major stress factor in many acidic soils. At soil pH levels below 5.0, intense solubilization of mononuclear A1 species strongly limits root growth by multiple cytotoxic effects mainly on root meristems (240,241). There is increasing evidence that A1 complexation with carboxylates released in apical root zones in response to elevated external Al concentration is a widespread mechanism for Al exclusion in many plant species (Fig. 10). Formation of stable Al complexes occurs with citrate, oxalate, tartarate, and—to a lesser extent— also with malate (86,242,243). The Al carboxylate complexes are less toxic than free ionic Al species (244) and are not taken up by plant roots (240). This explains the well-documented alleviatory effects on root growth in many plant species by carboxylate applications (citric, oxalic, and tartaric acids) to the culture media in presence of toxic Al concentrations (8,244,245) Citrate, malate and oxalate are the carboxylate anions reported so far to be released from Al-stressed plant roots (Fig. 10), and Al resistance of species and cultivars seems to be related to the amount of exuded carboxylates (246,247) but also to the ability to maintain the release of carboxylates over extended periods (248). In contrast to P deficiency-induced carboxylate exudation, which usually increases after several days or weeks of the stress treatment (72,113), exudation of carboxylates in response to Al toxicity is a fast reaction occurring within minutes to several hours... 

W. J. Horst, A. Wagner, and H. Marschner, Mucilage protects root meristems from aluminium injury. Z. Pflanzenphys. 105 435 (1982). 

Most plants exhibit "apical dominance" which means that the presence of a terminal (distal) meristem tends to suppress lateral shoot initiation (11). Since lateral shoot production is an important characteristic to assess in hydrilla, the frequency of shoot production was determined in sequentially cut (distal to proximal) explants (Table VI). Even though the 4 cm apical meristem contained several nodes, almost none of these produced new shoots. However, nearly half the 2-node explants subtending the cut apical meristem produced new shoots. There was no apparent difference in percent of new shoots produced once the apical meristem was removed. 

Results presented in Table VII show that sufficient light is needed for new shoot formation on explants and that the herbicide fluridone causes chlorosis in new growth just as in whole plants (12,13). These data also confirm that the apical explant, which contains the terminal meristem, is a poor system for assaying inhibitors of new shoot production. 

FIG. 1. Shoot apical meristem structure. The shoot apical meristem consists of three clonally distinct cell layers (LI, L2, L3). LI and L2 represent tunica layers, L3 represents the corpus. Cell divisions in LI and L2 are exclusively anticlinal, cell divisions in the L3 occur in all planes. The central zone harbours the stem cells and is surrounded by the peripheral zone where organ primordia are initiated. 

CLHi mRNA is found primarily in the LI and L2 layer of the central zone of shoot and floral meristems (Fig. 3), while the RNA of CLH/ is found mostly in an... 

Transgenic plants expressing WUS in a broader domain of the meristem resembled civ mutants with large and fasciated meristems (Schoof et al 2000). When WUS was expressed from the ANT promoter in organ primordia, these primordia were converted into shoot meristems that expressed CL.V3,... 

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