Agriculture Reference
In-Depth Information
5
Plasmodesmata - gateways for intercellular
communication in plants
Trudi Gillespie and Karl J. Oparka
5.1
Introduction
The aim of this chapter is to provide an overview of the structure and function
of plasmodesmata, highlighting their major role in controlling the flux of solutes,
signals and proteins that pass from cell to cell in the growing plant. Several major
pieces of research have been conducted on plasmodesmata, and as our knowledge
and understanding of the spatial and temporal changes that occur within plasmodes-
mata grows, numerous theories are being developed to explain how plasmodesmata
function in intercellular communication. Some of this work will be brought together
in this chapter, with an emphasis on those critical pieces of work that have made a
significant contribution to our current understanding of plasmodesmata. Inevitably,
for a chapter of this type, it will be impossible to do justice to the complete field,
and the reader will be directed to key texts for additional information.
5.1.1 Plasmodesmata - key components of the symplast
One feature of plant cells that makes them unique from other eukaryotic cells is
the cell wall. To be able to respond to environmental stimuli and coordinate differ-
entiation and growth, neighbouring plant cells (and often distant cells) must com-
municate (Hashimoto & Inze, 2003). Communication and nutrient transfer between
higher plant cells can occur by two parallel aqueous pathways. The aqueous phase
of the apoplast lies outside the plasma membrane and consists of the cell walls and
conducting cells of the xylem (Fisher, 2000). Molecules such as hormones and se-
creted protein ligands are known to carry information between cells apoplastically,
and some transcription factors may travel in the apoplast to subsequently regulate
gene expression (Hashimoto & Inze, 2003). This apoplastic pathway is best char-
acterised by CLAVATA-mediated signalling, which has been shown to regulate the
stem cell population in the shoot apical meristem (SAM) of
Arabidopsis
(Doerner,
2003). However, as Ding (1998) comments, higher plants have evolved a unique
intercellular organelle, the plasmodesma, that permits direct cell-to-cell communi-
cation of molecules through a cytoplasmic channel traversing the wall (for discussion
see Fisher, 2000). Recent research has shown that traditional views of the role of
the plant cell wall as simply providing mechanical strength and acting as a barrier
to pathogens need to be reconsidered. Plant cells are now thought to be capable
of monitoring changes in wall composition, and using this information to activate
