Agriculture Reference
In-Depth Information
A
B
Figure 5.2
Schematic representation of a simple plasmodesmata in longitudinal (A) and transverse (B)
sections. CW, cell wall; D, desmotubule; CR, central rod; CS, cytoplasmic sleeve; PM, plasma
membrane; SP, spoke-like extensions; PMP, plasma-membrane-embedded proteins; DP,
desmotubule-embedded proteins. Figure adapted from Roberts and Oparka (2003) and based on the
model in Ding
et al.
(2003).
sleeve (Overall & Blackmann, 1996). In some cases this constriction is thought to be
an artefact owing to physical wounding and glutaraldehyde fixation. Radford
et al.
(1998) showed that treatment with 2-deoxy-D-glucose (an inhibitor of callose syn-
thesis) caused the neck of
Allium cepa
L. plasmodesmata to become funnel-shaped
rather than constricted. When viewed in cross section, the cytoplasmic sleeve is
seen to be partially occluded by the globular subunits surrounding the desmotubule
(Ding
et al.
, 1992a; Overall & Blackmann, 1996). These subunits are thought to
have a helical arrangement that divide the space in the cytoplasmic sleeve into a
number of spiralling channels (Zee, 1969; Robards, 1976; Olesen, 1979; Overall
et al.
, 1982; Wolf
et al.
, 1989; Olesen & Robards, 1990; Robards & Lucas, 1990;
Ding
et al.
, 1991; Robinson-Beers & Evert, 1991; Lucas
et al.
, 1993; Lucas & Wolf,
1993; Overall & Blackman, 1996; Waigman
et al.
, 1997). The functional diameter
of these channels has been calculated to be between 3 and 4 nm (Terry & Robards,
1987; Fisher, 1999). An electron-dense ring of proteinaceous material in the wall
surrounding the neck region has also been seen in electron micrographs (Olesen,
1979; Olesen & Robards, 1990; Beebe & Turgeon, 1991; Badelt
et al.
, 1994; Turner
et al.
, 1994). This wall collar, or 'sphincter', is most consistently visualised when
tannic acid is included during fixation (Fisher, 2000). The collar can be closely
associated with the plasma membrane, or it can be expanded out from the plasma
membrane and connected to it via spokes or strings (Overall, 1999). The actual func-
tion of these structures in plasmodesmal regulation has not been verified; however,
most authors believe them to be involved in modulating the outer dimensions of the
plasmodesmal pore.
5.2.2 Primary and secondary; simple or branched
Plasmodesmata that develop between daughter cells during cell division are termed
primary plasmodesmata (Jones, 1976).
Primary plasmodesmata
are formed across
