Agriculture Reference
In-Depth Information
organs of the same plant, within different tissues of an organ and between cell types
at different developmental stages (Goodwin & Cantrill, 1999; Oparka
et al.
, 1999).
On the basis of these findings, some authors (Itaya
et al.
, 2000; Blackman & Overall,
2001; Zambryski, 2004) have questioned how specific metabolites, small proteins
and peptides are retained within cells. Importantly, it is not the molecular mass of
a protein that determines its permeability through a plasmodesma, but its Stokes
radius (
R
s
). Terry and Robards (1987) emphasised that the mobility of a molecule
through a plasmodesma is determined by the effective
R
s
, which is governed by
the shape (width and depth) and the hydrodynamic drag of a molecule. Fisher and
Cash-Clark (2000) point out that there is no unique relationship between
M
r
and
molecular dimensions; for example, a 25-kDa dextran has the same
R
s
as a 51-kDa
globular protein (Jørgensen & Møller, 1979). In the case of GFP, its compact beta-
barrel structure (Ormo
et al.
, 1996), and physical dimensions (diameter 3 nm; length
4 nm; Phillips, 1997), rather than its molecular mass, may enable it to traffic passively
through plasmodesmata.
5.3.2 Selective transport and gating: modulation of the SEL
Transport through a plasmodesma is described as selective when molecules larger
than the basal SEL traffic between cells, mediated by conformational changes in
plasmodesmata that permit an increase in the SEL. The term
gating
has generally
been used to describe the specific interaction between a molecule and a plasmod-
esma that leads to dilation of the plasmodesmal pore. However, as Schulz (1999)
remarks the term gating can be ambiguous, as passage of smaller solutes is also
controlled by the width of the cytoplasmic sleeve. Thus, dilation/gating of a plas-
modesma can also be non-selective. Not all authors use the same terminology to
describe selective transport and plasmodesmal gating; for example, Zambryski and
Crawford (2000) describe plasmodesmata as having different functional states, and
comment that 'plasmodesmata can be assumed to be either closed or open'. They
define a closed plasmodesma as not allowing any intercellular exchange, an open
plasmodesma as allowing the movement of molecules below the basal SEL, and a
dilated plasmodesma as allowing the movement of macromolecules larger than the
basal SEL. Zambryski (2004) concludes that non-targeted protein movement may
provide coordination between large groups of cells, whereas proteins with more
limited movement allow the programming of more specific pathways.
5.3.3
Physiological modulation of SEL
Various abiotic and biotic factors have been found to alter the basal SEL of plas-
modesmata (see Table 5.1). Turgor pressure differentials and elevated Ca
2
+
lev-
els have been shown to decrease the SEL, although changes in pH may negate
the effect of elevated calcium (Holdaway-Clarke
et al.
, 2001). Plasmodesmata are
also sensitive to wounding, which induces rapid callose deposition at the neck
of the pore. Decreases in ATP levels, anerobic stress, long-term osmotic stress,
