Meristematic cells

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... 

It may be that the presence of this isoform promotes the strengthening of calli cell walls through a special mechanism, since cultured cells have mainly undifferentiated cell walls missing lignification and suberin deposition, similar to meristematic cells in plants. 

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. 

The differentiation of cells occurs concomitantly to modifications of wall components. The nature of the pectins of the walls changes under the action of enzymes, among which esterases, secreted between the apical meristematic cells and the more basal differentiated cells. The apposition of new layers of pectins with different compositions at the inner surface of the walls is another mechanism by which the cells adapt their immediate environment. Using the 2F4 antibody, we have observed, in plant suspensions as well as in tissues, a third mechanism involved in wall modification. Numerous invaginations of the... 

Proplastids are small colorless or pale green undifferentated plastids that occur in the meristematic cells of roots and stems. Proplastids are precursors of more highly differentiated plastids. Etioplasts are proplastids containing prolemellar bodies and are precursors of chloroplasts developmentally arrested by low light levels. 

Meristematic Meristematic cells Cell division and growth Increase in length Increase in girth... 

Apical or primary Meristematic cells Produce primary body of root-and-shoot system... 

Aminouracil produces a block in the mitotic cycle of various plants. For example, cessation of mitosis occurred in Vida faba roots incubated 24 h with this compound [392—394]. Depending on different experimental conditions, thymidine or thymidylic acid may or may not alleviate these effects [392—394]. It was concluded that 5-aminouracil depressed the rate of DNA synthesis, which led to an accumulation of cells in the S phase. After removal of the agent, DNA synthesis resumed. Similar results have been observed with Allium cepa and Haplopappus gracilis [395, 396]. Inhibition of guanosine incorporation into RNA of meristematic cells in Vida faba by 5-aminouracil was also reported [397]. 

Figure 2. Meristematic cell of the root-tip of Phaseolus vulgaris. The section was treated with an antibody specific for L-arabinofuranose and stained with gold-labelled goat-antirabbit serum. The label is seen at the developing cell plate (cp) and the young wall (cw) of the mother cell.

The elongation/differentiation zone of the principal root includes small, densely meristematic cells that are in continuous metabolic activity and are more susceptible... 

Pericambium of one or two layers of meristematic cells whose walls are extremely thin. 

Pericambium, of a layer of actively growing meristematic cells, which has the power of producing lateral rootlets. 

Wood Cell Production. The site of wood cell production, the vascular cambium, is illustrated in Figure 3B. Technically, it is a microscopic sheath of meristematic cells. However, the exact circumferential line of cambial cells is very difficult to locate precisely, particularly during the tree s growing season, because of the presence of recent xylem and phloem derivatives. Therefore, it is more common to reference this lateral meristem as the cambial zone (2). 

Fig. 4. Effects of high (10 ppm and 100 ppm) ethylene treatment (24 h) on MTs in cells of cv. LG-11 maize roots. 10 ppm treatment (a-e) has no effects in cells of the quiescent centre (QC) (a) and in meristematic cells of root cap (RC) (a.b), but it depletes MTs from developmentally older root cap cells (b). In addition this ethylene treatment randomizes CMTs in all postmitotic cells of the root proper. For the outer cortex (CXI) and epidermis (EP) cells, this effect is only slight (c). However, cells of the inner cortex (d,e) are strongly affected in this respect and the transverse CMTs are replaced by completely random CMTs. 100 ppm ethylene treatment (f,g) disintegrates MTs from most root cells with the exception of the distal part of meristem and root cap initials and quiescent centre cells (not shown). In the root proper, the epidermis (EP) and outer cortex (OC) cells are the only ones to preserve their MTs which, in the case of epidermis, are still transverse and well-ordered (f). In contrast, the inner cortex cells (IC) are devoid of MTs (g). For more details on ethylene treatments see [157]. Bar represents 10 pm.

It is noteworthy that the cells just behind the shoot and root meristems, and adjacent to the cambium and phellogen, which are so frequently reported as accumulating alkaloids, are in a stage of active vacuolation. The most detailed study up to the present has been made by Chaze (13, 30). in the meristems and flower buds of the tobacco plant. In very young radicles at the outset of germination, the meristematic cells were observed... 

Rhizobia are released from the tip of the infection thread into the cytoplasm of the polyploid meristematic cells by endocytosis (Goodchild and Bergersen, 1966 Dart, 1977 Goodchild, 1978). This phagocytotic process... 

The question now arises whether meristematic cells are histologically, morphologically and ultrastructurally equal to embryogenic ones Next, how can we recognise meristematic and embryogenic explant cells ... 

Histological and ultrastructural analysis during the SE of pineapple guava showed that meristematic cells are rich in cytoplasm and containing many ribosomes, some amyloplasts and numerous mitochondria (Canhoto Cruz, 1996 Canhoto et al., 1996). In this system, meristematic cells were similar on the ultrastructural level to embryogenic (proembryo) cells, with the only exception that the meristematic cells were more vacuolated. In the case of coconut, the meristematic cells were also characterised by dense cytoplasm, many ribosomes, reduced vacuole and a voluminous central nucleus with one or two nucleoli (Fig. 5 A Verdeil et al., 2001). Cells with the same characteristics were described for Carya (Rodriguez Wetzstein, 1998). 

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