Symplasm

Van Bel AJE, Gamelai YV, Ammerlan A, Bik LPM. Dissimilar phloem loading in leaves with symplasmic and apoplasmic minor-vein configurations. Planta 1992 186 518-525. 

An ion entering a root may immediately enter the symplast by crossing the plasma membrane of an epidermal cell, or it may remain in the apoplasm and diffnse throngh cell walls. It may snbseqnently enter the symplasm by crossing... 

Edgington and Peterson (4) have subdivided apoplastic xeno-biotics into two classes. Euapoplastic (only transported in the apoplast) and pseudoapoplastic (transport occurs mainly in the xylem but entry into the symplast occurs). Most traditional "apoplastic" chemicals are now known to really be pseudoapoplastic chemicals, e.g., atrazine, diuron, oxamyl, etc. The unresolved question is why don t these pseudoapoplastic chemicals which cross the cell membranes and enter the symplast remain in the symplasm of the phloem There have been numerous studies focusing on the molecular requirements for phloem mobility (1-5), In general, there is not a good correlation between phloem mobility and water solubility, metabolism of the xenobiotic, or the presence of various substitution groups in a molecule. 

Let us next consider the flux for the symplasm. In the aqueous part of the plasmodesmata, Dghlcose should be similar to its value in water, 0.7 x 10-9 m2 s-1 (Table 1-1), and KgXncosfi is 1- We will let the plasmodesmata be 0.5 p,m long. The flux density in the pores thus is... 

Figure 1-4 presents longitudinal and cross-sectional views near the tip of a root, indicating the types of cells that can act as barriers to flow. Water may fairly easily traverse the single cell layer of the root epidermis to reach the cortex (see Chapter 1, Section 1.1D). The root cortex often consists of 5 to 10 cell layers, with the cytoplasm of adjacent cells being continuous because of plasmodesmata (Fig. 1-14). The collective protoplasm of interconnected cells is referred to as the symplasm (see Chapter 1, Section 1.5B). In the...

In fact, membranes generally serve as the main barrier to water flow into or out of plant cells. The interstices of the cell walls provide a much easier pathway for such flow, and hollow xylem vessels present the least impediment to flow (such as up a stem). Consequently, the xylem provides a plant with tubes, or conduits, that are remarkably well suited for moving water over long distances. The region of a plant made up of cell walls and the hollow xylem vessels is often called the apoplast, as noted above (Chapter 1, Section 1.1D and in Section 9.4A). Water and the solutes that it contains can move fairly readily in the apoplast, but they must cross a membrane to enter the symplast (symplasm), the interconnected cytoplasm of the cells. 

Plant iron uptake can be divided into two distinct families, with quite distinct strategies. Strategy I plants reduce Fe + to Fe + outside of the roots, and then take up the Fe +. In contrast. Strategy II plants solubilise Fe + by excreting Fe +phytosiderophores, which are taken up by specihc transporters and the iron is then reduced to Fe " " in the symplasm of the root cell (Fig. 7.14). In Strategy I plants, (dicotyledons such as Arabidopsis pea. 

The Al concentration in the inner cell layers and root cell cytoplasm (symplasm) of plants was lower than in the surrounding medium (Kochian and Jones). Between 30% and 90% of absorbed Al was localized in the apoplast (Mossor-Piettraszewska 2001). Al and silicon appear to be co-localized in root cell walls, perhaps reflecting the primary internal sites of aluminosilicate formation and Al detoxification (Hodson and Sangster 1999). 

The internal pH (- 7) of cells, compared to the much lower pH around the plant root, changes Al species. During uptake, Al has been calculated to be associated with charged organic compounds present in symplasm, predominantly citrate, but also ATP and GTP, thereby reducing free cytoplasmic Al to picomolar concentrations (Kochian and Jones 1997). These do not appear to be toxic Al species, but may permit intracellular Al redistribution. 

Two ways of water transport in a plant have been recognized apoplasmic and symplasmic (Figure 32.4). It is generally agreed that the cell walls provide the major pathway of water movement in plant material. The ratio of volume flows in the apoplasmic and symplasmic (vacuole-to-vacuole) pathways is of the order of 50 1 in leaf tissue [1]. For the root cortex, the ratio is lower. 

Change of osmotic pressure in xylem and phloem and the dewatering of the cell walls will initiate the symplasmic movement of water in the material. The dehydration of the cells will take place and plasmol-ysis will be induced. 

R.M. Spanswick, Symplasmic transport in tissues. In Encyclopedia of Plant Physiology. Vol. 2. Transport in Plants II, Part B. Tissues and Organs (U. Luttge and M.G. Pitman, eds.). Springer Verlag, Berlin, Germany, 1976, p. 35. 

F.J. Molz, Water transport through plant tissue The apoplasm and symplasm pathways, J. Theor. Biol., 59 277 (1976). 

The deposition of callose in the vicinity of plasmodesmata disturbs symplasmic communication (this will be described in detail below) between cells and - in this manner -influences the exchange of signals through plasmodesmata (Fig. 10 A). When callose is deposited in the cell wall it can interrupt the exchange of signals through the apoplast (Fig. 10 B). 

Fig. 12 The distribution of the symplasmic tracer (HPTS -8-hydroxypyrene-l,3,6-trisulfonic acid) within the protodermal cells ol Arabidopsis explants. A - stomata as an example of the permanent symplasmic domain. B-C examples of the temp)oral symplasmic domains composed of a few cells (C) or in a single cell (D as to B, note the imequal distribution of fluorochromes in the daughter cells after a division - arrow bar = 10 pm).

Symplasmic communication within explant cells during the initiation and development of somatic embryos was not intensively studied. Analysis of the distribution of CFDA (fluorescent tracer 5-(and-6) Carboxyfluorescein Diacetate) during the DSE in Arabidcrpsis explants showed the presence of the fluorochrome only in the protodermis and subprotodermis of the explants, indicating that the downregulation of plasmodesmata connection within an explant took place (Kurczyhska et al., 2007). 

Studies on the zygotic embryos of Eleutherococcus senticosus as explants showed that the disruption of plasmodesmata between explants cells promotes the formation of somatic embryos even on the medium without auxin (You et al., 2006). The interpretation of these results is as follows the interruption of symplasmic communication stimulates the reprogramming of cells into cells competent for the embryogenic pathway (You et al., 2006). 

However, not all of the results described so far are in agreement with those presented above. In the case of Pineapple guava symplasmic, isolation was not detected during the formation of somatic embryos (Canhoto et al., 1996). Plasmodesmata were present between the cells of the embryo, but also between the embryo and the surrounding cells. This suggests that symplasmic isolation is not a prerequisite for somatic embryo formation (Canhoto et al., 1996). In other tissue culture systems, the same conclusion was drawn (Jasik et al., 1995 Thorpe, 1980 Williams Meheswaran, 1986). 

Symplasmic communication within somatic embryos is also not well-described. It was shown for barley androgenic embryos that the symplasmic barrier exists between protodermis and the rmderlying tissues up to the late globular stage, in the isolation of... 

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