Agriculture Reference
In-Depth Information
deplasmolysis and some pharmcological treatments have been found to increase
the SEL. Schulz (1999) hypothesised that the prime mechanism for control of non-
selective trafficking is the rapid and transient conformational change in plasmodes-
mal proteins brought about by changes in calcium and/or ATP levels. Comparatively
small and local changes (elevation) in Ca
2
+
concentration will cause a decrease in
plasmodesmal SEL. However, longer term blockage caused by wounding may de-
pend on a massive influx of calcium from the apoplast or the vacuole. Conformational
changes in plasmodesmal proteins are rapidly reversible, whereas callose synthesis
and breakdown is much slower. Conversely, low levels of ATP, that can be caused
by anaerobic stress or induced by azide treatment, dilate plasmodesmata, allowing
increased intercellular exchange (Schulz, 1999). However, there have been contra-
dictory reports on the effect of turgor on plasmodesmal SEL. Roberts and Oparka
(2003) propose that sudden pressure differentials between adjacent cells, as might
occur during wounding, will cause isolation of plant cells from their neighbours,
whereas an overall drop in tissue turgor, characteristic of water stress, will lead to
an increase in SEL.
5.3.4 Fine regulation of plasmodesmal SEL - role of the cytoskeleton
Zambryski and Crawford (2000) have suggested that constriction and relaxation
may occur along the entire length of the pore, correlated to the distribution of actin
and myosin in plasmodesmata. Other authors (Radford & White, 1998; Reichelt
et al.
, 1999) have suggested that the helically arranged 'spokes' that connect the
desmotubule to the plasma membrane (Overall & Blackman, 1996) may provide
a possible contractile mechanism for controlling the aperture of the cytoplasmic
sleeve. In mammalian cells, some myosins have a role in generating tension be-
tween adjacent membranes (Kussel-Andermann
et al.
, 2000). Myosin VIII has been
implicated in plasmodesmata function (Baluska
et al.
, 2000, 2001) and has a char-
acteristic motor-domain region and a C-terminal structure that includes a probable
phosphorylation site for protein kinases, as well as four calmodulin-binding motifs
(Reichelt & Kendrick-Jones, 2000). Myosin VIII has therefore been proposed to
function as a calcium-regulated plasmodesmal motor protein that traffics macro-
molecular cargo along the actin filaments in plasmodesmata (Roberts & Oparka,
2003; Oparka, 2004).
Regulation of plasmodesmal aperture may also occur at the neck region of the
plasmodesma. Blackman and Overall (2001) have suggested that centrin, a calcium-
binding contractile protein found in the neck region, may have a role in regulating
plasmodesmal aperture. Dephosphorylation of this protein, caused by an increase in
cytosolic calcium, induces the centrin nanofilaments to rapidly contract, potentially
closing the plasmodesmata (Martindale & Salisbury, 1990; Blackman
et al.
, 1999).
The ATPase inhibitor 2,3-butanedione 2-monoxime (BDM), which is known to stim-
ulate calcium release from cardiac sarcoplasmic reticulum (Phillips & Altschuld,
1996) and inhibit myosin by preventing microfilament depolymerisation, was found
to contract the neck region of plasmodesmata in
A. cepa
,
Z. mays
and
H. vulgare
