Agriculture Reference
In-Depth Information
signalling cascades within the cell (Hashimoto & Inze, 2003). In a similar way,
plasmodesmata, which were once thought to be static structures embedded within
the cell wall, are being revealed as dynamic structures that have a fundamental role
in controlling symplastic protein and RNA trafficking.
5.1.2 Plasmodesmata: simple description, complex function
Various descriptions of plasmodesmata have been given over the last century (for
review see Roberts & Oparka, 2003). The historical concept of plasmodesmata as
static, membrane-lined channels that facilitate the cell-to-cell movement of low-
molecular-weight solutes has dominated much of the debate on the structure and
function of plasmodesmata (Lucas
et al.
, 1993; Blackman & Overall, 2001). How-
ever, recent ultrastructural, physiological, biochemical and molecular studies on
plasmodesmata have transformed this opinion, and plasmodesmata are now consid-
ered to be highly flexible and diverse structures that can exert considerable control on
the flux of molecules that pass through them (Lucas
et al.
, 1993; Blackman & Over-
all, 2001; Ehlers & Kollmann, 2001; Roberts & Oparka, 2003). As Lee
et al.
(2000)
comment, the sophistication of cell-to-cell communication through plasmodesmata
may yet rival that of nuclear transport.
5.1.3 Discovery of plasmodesmata
The term
plasmodesmata
(singular, plasmodesma) was first used by Strasburger,
in 1901 (Carr, 1976). However, it was in 1879 that Eduard Tangl first observed
the intercellular striations between cells in the cotyledons of
Strychnos nuxvomica
,
which he interpreted to be protoplasmic contacts (Roberts & Oparka, 2003, citing
Carr, 1976). In the same year Albert Pfeffer commented that '. . . it is quite likely
that the connections could be utilised in the transport of substances and even, in
particular cases, principally or solely for this purpose' (cited by Carr, 1976). Tangl's
work formed the basis of the concept that intercellular communication between
living plant cells is necessary not only for the survival of individual cells, but for
the plant itself. This notion was significant as it contested the then current view
that plant cells functioned as autonomous units (Carr, 1976). It was not until 1930
that Munch took this concept further and used the term
symplasm
to describe the
cytoplasmic continuity that exists between plant cells throughout the whole plant.
Since the work of Tangl, many key papers have increased our understanding of
plasmodesmata, and provided new insights into their structure and function.
5.2
Structure
5.2.1 The general ultrastructure of plasmodesmata
Structural models of plasmodesmata have generally been based on data from trans-
mission electron microscopy. Since the first published model of plasmodesmata
