Agriculture Reference
In-Depth Information
movement or export signal. Unfortunately, these authors did not test to see if the basal
SEL of plasmodesmata was increased in the presence of LEAFY, and therefore it is
difficult to ascertain if the movement of the LEAFY protein is truly non-selective.
Haywood
et al.
(2002) have compared cell-to-cell transport of proteins through
plasmodesmata to import of protein into organelles, which is known to involve
exposure of a targeting motif, binding to a translocation receptor complex, protein
unfolding and/or structural modification to the translocation complex. Kragler
et al.
(2000) also remark that this process shares features in common with intracellular
translocation mechanisms in which the transfer of macromolecules occurs in either
a folded or unfolded state (Subramani, 1996; Kragler
et al.
, 1998; Schatz, 1998).
Few potential cellular factors that control regulation of selective plasmodesmal
gating have been identified (Ding
et al.
, 2003). There has been some debate in
the literature as to whether viral MPs may have been plant proteins that became
incorporated into the viral genome to facilitate viral movement (Lucas & Gilbertson,
1994; Maule, 1994; Mezitt & Lucas, 1996). Viral MPs may use an endogenous
plant pathway for intercellular trafficking of macromolecules (Jackson, 2001), and
have provided clues about the factors that potentiate selective gating
in planta
(Ding
et al.
, 2003). The MPs of TMV,
Turnip vein-clearing virus
(TVCV) and
Cauliflower
mosaic virus
(CaMV) have been shown to interact/bind with pectin methylesterase
(PME) (Dorokhov
et al.
, 1999; Chen
et al.
, 2000). Chen
et al.
(2000) found that
deletion of a PME-binding domain in TMV MP prevents cell-to-cell movement of
TMV. PME is an enzyme that modifies pectin in plant cell walls and has been shown
to have RNA-binding properties (Dorokhov
et al.
, 1999). PME has been shown to
have a heterogeneous cellular distribution and has been localised to microdomains of
the cell wall, the plasma membrane and the endoplasmic reticulum of flax hypercotol
cortical cells (Morvan
et al.
, 1998). Oparka (2004) has suggested that MPs may
interact with PME before its delivery to the cell wall, effectively hijacking the PME
in order to 'piggy back' on the cells endogenous macromolecular transport pathway
to plasmodesmata.
Citovsky
et al.
(1993) showed that the MP of TMV is phosphorylated
in vitro
at its C-terminal serine and threonine residues by a cell-wall-associated protein ki-
nase. These authors concluded that phosphorylation of TMV MP may represent a
mechanism for the host plant to sequester MP following its localisation to cell walls.
In vivo
studies of TMV MP phosphorylation showed that this process may negatively
regulate the effect of MP on plasmodesmal permeability (Waigmann
et al.
, 2000).
Phosphorylation of other viral MPs has also been demonstrated (Sokolova
et al.
,
1997; Matsushita
et al.
, 2000, 2002). Waigman
et al.
(2000) also found that activity
of this MP-binding kinase required Mg
2
+
but not Ca
2
+
cations; however, a calcium-
dependent protein kinase has been localised to the cell wall in a plasmodesmata-like
distribution in
Arabidopsis
(Yahalom
et al.
, 1998). Blackman and Overall (2001)
suggest that localisation of ATPase activity to plasmodesmata indicates that post-
translational modification of proteins by kinases and phosphatase plays a role in the
control of cell-to-cell communication. Schulz (1999) also hypothesised that a unify-
ing mechanism of non-selective and selective gating of plasmodesmata may be due
