Agriculture Reference
In-Depth Information
plasmodesmata are established (Ehlers & Kollmann, 2001). During tissue develop-
ment, neighbouring primary plasmodesmata may develop into complex plasmodes-
mal morphotypes, with median branching planes and central cavities. For example,
during the sink-source transition in leaves, simple plasmodesmata are converted
to branched plasmodesmata that contain a central cavity aligned along the middle
lamella of the cell wall (Oparka
et al.
, 1999; Roberts
et al.
, 2001). This transforma-
tion is thought to occur via an H-shaped intermediate that appears to form by the
introduction of a new protoplasmic bridge between neighbouring pairs of simple
plasmodesmata (Roberts & Oparka, 2003). At present, the genetic determinants of
plasmodesmata formation are unknown.
5.2.3
Plasmodesmal frequency and distribution: gain and loss
Plasmodesmata are found between most living cells of higher plants, but are absent
from key developmental interfaces, e.g. between sporophyte and gametophyte, be-
tween guard cells and epidermal cells, and between maternal tissues and embryos.
In other tissues, plasmodesmal frequencies are within the range of 0.1-10 mm
−
2
of
cell wall, although exceptions can be found on either side of this range (Robards
& Lucas, 1990). However, the relationship between plasmodesmal frequency and
capacity for intercellular trafficking is not straightforward; plasmodesmata within
the plant are not uniform and their transport capacity can vary considerably between
cells and tissues (van Bel & Oparka, 1995). Thus, a paucity of plasmodesmata does
not necessarily mean impaired transport (Fisher, 2000). As cells expand and differ-
entiate, their fate determines the extent to which their cytoplasmic connectivity to
other cells is maintained (Mezitt & Lucas, 1996). Various studies have found that
plasmodesmal densities do not decline significantly as a result of cell elongation and
wall expansion (Schnepf & Sych, 1983; Seagull, 1983). Such constancy in density
can only be accounted for by the formation of secondary plasmodesmata. It is gener-
ally agreed that the formation of secondary plasmodesmata is an integral component
of normal plant development (Jones, 1976; Robards & Lucas, 1990; Lucas
et al.
,
1993). However, loss or restriction (occlusion) of plasmodesmata also appears to be
common during differentiation. For example, in vascular tissue some cells greatly
reduce the number of plasmodesmata in their adjoining cell walls (Gamalei, 1989).
In plant species with a putative apoplastic loading mechanism, plasmodesmata in the
vascular tissue are almost completely eliminated, with only a small number between
the bundle sheath/phloem parenchyma cells and the sieve element-companion cell
(SE-CC) complexes remaining (Oparka & Santa Cruz, 2000). During the sink-
source transition in tobacco leaves, when there is a rapid phase of cell expansion in
the spongy-mesophyll cells, primary plasmodesmata are thought to be lost, dramat-
ically reducing the numbers of simple plasmodesmata (Roberts
et al.
, 2001). At the
same time, branched plasmodesmata are thought to be formed through the merging
of any remaining simple plasmodesmata (Oparka
et al.
, 1999). The immunocytolog-
ical localisation of ubiquitin in those plasmodesmata destined for removal has led
some authors to suggest that the removal process is accomplished, at least in part,
