Agriculture Reference
In-Depth Information
and characterised. Some genes have been identified that affect plasmodesmata func-
tion but do not localise to plasmodesmata. For example, the
sxd1
mutation in maize
(Russin
et al.
, 1996; Botha
et al.
, 2000) affects plasmodesmal development, but its
primary role is in chloroplast-to-nucleus signalling (Mezitt Provencher
et al.
, 2001).
Similarly, in transgenic plants expressing a yeast acid invertase gene, complex sec-
ondary plasmodesmata formation between mesophyll cells was found to be arrested
during maturation (Ding
et al.
, 1993). These studies point to a complex interplay
between basic metabolic processes and the formation of plasmodesmata.
In two recent studies, cDNA::GFP fusions have been used to probe for protein
localisation and function
in planta
. Cutler
et al.
(2000) created a large number of
Arabidopsis
transgenic plants that expressed random GFP::cDNA fusions. Medina
Escobar
et al.
(2003) took this high-throughput screening approach a stage fur-
ther by using a Tobacco mosaic virus (TMV) vector to express libraries of random
partial cDNAs fused to GFP. Over 20,000 infection foci expressing independent
cDNA::GFP fusions were examined, and this screen isolated 12 putative plasmod-
esmal proteins. Further studies will reveal whether these proteins are structural
components of plasmodesmata or whether they play a role in regulating transit
through plasmodesmata (Ding
et al.
, 2003).
5.2.5 Passage through the cytoplasmic sleeve
The consensus in the literature is that diffusion of many small molecules such as sug-
ars, metabolites, ions and amino acids are all thought to move by diffusion through
the cytoplasmic sleeve of plasmodesmata (see Lucas
et al.
, 1993). Small fluorescent
dyes that have been loaded passively (Duckett
et al.
, 1994; Roberts
et al.
, 1997) or
microinjected (Goodwin, 1983; Erwee
et al.
, 1985; Madore & Lucas, 1986; Oparka
et al.
, 1991) utilise this pathway. Recent studies suggest that larger molecules, in-
cluding GFP, may also move via the cytoplasmic sleeve (Roberts & Oparka, 2003).
Plasmodesmata models derived from high-resolution electron micrographs show
gaps in the range of 3 nm between the inner-plasmodesmal proteins that form the
individual channels within the cytoplasmic sleeve (Ding
et al.
, 1991; Lucas
et al.
,
1993; Overall & Blackman, 1996; Fisher, 1999; Blackman & Overall, 2001), con-
sistent with basal SEL of about 1 kDa (Tucker, 1982; Goodwin, 1983). However, a
recalculation of data for diffusion through plasmodesmata (Terry & Robards, 1987)
by Fisher (1999) showed that the basal SEL might be nearer to 4 nm, which signifi-
cantly increases the potential SEL of the pore. As Roberts and Oparka (2003) note,
an increase from 3 to 4 nm would mean a substantial increase in the Stokes radius
for macromolecules that might diffuse through the cytoplasmic sleeve.
5.3
Macromolecular trafficking
There is now a considerable body of evidence to support the concept that plas-
modesmata have the capacity to mediate cell-to-cell trafficking of endogenous
