Agriculture Reference
In-Depth Information
pore (for review see Stone & Clarke, 1992), and callose deposition at plasmodes-
mata has also been confirmed by localisation of antibodies raised against callose
(Northcote
et al.
, 1989) and against a 65-kDa component of the callose synthase
complex (Delmer
et al.
, 1993). Citovsky
et al.
(1993) showed that the activity of
a cell-wall-associated protein kinase was correlated with the developmental mat-
uration of plasmodesmata, and antibodies raised against an
Arabidopsis
calcium-
dependent protein kinase were localised to the cell wall in a plasmodesmata-like
distribution (Yahalom
et al.
, 1998). ATPase activity has been found at plasmodes-
mata in several plant species (Didehvar & Baker, 1986; Robards & Lucas, 1990;
Chauhan
et al.
, 1991). In barley roots, ATPase activity is localised to the neck region
of plasmodesmata, as are calcium-binding sites (Belitser
et al.
, 1982). The calcium-
sequestering protein calreticulin, normally found in the lumen of the endoplasmic
reticulum, has been found to be a component of the cortical endoplasmic reticulum
elements associated with plasmodesmata (Baluska
et al.
, 1999, 2001), and centrin,
a calcium-binding contractile protein, has also been localised to the neck region of
plasmodesmata (Baluska
et al.
, 1999; Blackman
et al.
, 1999).
It is clear that cytoskeletal elements of the actin-myosin families are located
in plasmodesmata. Overall (1999) has suggested that the helically arranged spiral
of electron-lucent particles around the desmotubule is composed of actin micro-
filaments, and in the structural model proposed by Blackman and Overall (2001)
putative myosin spokes radiate out from the actin, physically linking it to the plasma
membrane. Myosins are a large superfamily of motor proteins that, in association
with actin, are involved in intracellular motile processes (Reichelt
et al.
, 1999). It
has been hypothesised that the actin filaments may form a static scaffold within
plasmodesmata along which molecules move using a myosin-based motor (Roberts
& Oparka, 2003).
Other cellular proteins/components that have been localised at or near plasmodes-
mata by cytochemical reactions include a -amylase (Gubler
et al.
, 1987), peroxidase
(Schnepf & Sych, 1983), pectin methylesterase (Morvan
et al.
1998), unesterified
pectin (Casero & Knox, 1995), low-esterified pectin (Roy
et al.
, 1997) and 5
-
nucleotidase (Nougarede
et al.
, 1985). Additional cytochemical studies have shown
plasmodesmata to be the location of high enzyme activity and strong reducing sub-
stances (see review by Olesen, 1979). However, as White
et al.
(1994) note, very
little of this information has been successfully incorporated into a working model
for plasmodesmal regulation.
The creation of mutants with defects in plasmodesmata structure and/or func-
tion, and the subsequent cloning of the genes concerned, is another approach that
is being used to identify unique molecular components of plasmodesmata (Ding,
1998). However, both Ding (1998) and Zambryski (2004) note that the major obsta-
cle to this approach is that alterations to plasmodesmata structure or function may
be lethal or have drastic effects on plant development. To overcome this problem,
Kim
et al.
(2002a) developed a heterozygous embryo-assay system in
Arabidopsis
to screen mutants that have an increased plasmodesmal size exclusion limit (SEL)
at the torpedo stage of embryo development. This screen has so far identified two
ise
(
increased size exclusion
) mutants, although the genes for these are yet to be cloned
