Agriculture Reference
In-Depth Information
by degradation of plasmodesmal proteins via the ubiquitin degradation pathway
(Ehlers
et al.
, 1996; Fisher, 2000; Roberts & Oparka, 2003). Along with removal,
plasmodesmata can also become occluded. For example, numerous plasmodesmata
are found in the pits that connect immature xylem elements to mesophyll cells
of
Sorbus torminalis.
However, during the final stages of programmed cell death,
the pits become sealed off by deposition of cell wall material across both ends of
the plasmodesmal pores (Lachaud & Maurousset, 1996). Callose and/or deposition
of cell wall material has also been shown to block plasmodesmata at the bundle
sheath-vascular parenchyma cell wall interface of a sucrose deficient (
sxd1
) mu-
tant of maize that is unable to load sucrose into minor veins (Russin
et al.
, 1996;
Botha
et al.
, 2000). The most extreme case of plasmodesmal 'downregulation' oc-
curs around stomatal complexes (Oparka & Roberts, 2001). Guard cells are initially
symplastically coupled to adjoining epidermal cells. However, as the guard cells
mature new wall material is deposited across plasmodesmata, rendering them non-
functional and symplastically isolating the mature guard cells from the surrounding
epidermal cells (Wille & Lucas, 1984; Palevitz & Hepler, 1985). Lack of symplastic
interconnection by plasmodesmata, such as that observed at the interface between
maternal and embryonic tissue, suggests a compulsory apoplastic step in the trans-
port pathway that can be precisely controlled (Ehlers
et al.
, 1999). In a developing
embryo, symplastic discontinuity ensures autonomous development, restricting the
entry of macromolecular complexes such as RNA or plant transcription factors that
might influence the development of the genetically distinct embryo (Lucas, 1995;
Ehlers & van Bel, 1999). The ability to add or lose plasmodesmata, both spatially
and temporally, reveals the highly dynamic nature of intercellular communication
in plants (Botha & Evert, 1988; Brown
et al.
, 1995).
5.2.4 Plasmodesmal components
Studies to elucidate the molecular structure of plasmodesmata have proven difficult
because of the small size of plasmodesmata and their inaccessibility within the cell
wall. Consequently, much of what we know comes from a combination of techniques
such as immunological localisation and the random biochemical identification of
novel proteins and polysaccharides. To date, relatively few plasmodesmal proteins
have been positively identified (Roberts & Oparka, 2003).
One approach has been to use proteases and detergent treatments to remove
components of plasmodesmata, although results from such studies have been con-
tradictory. The desmotubule in the plasmodesmata of fern (
Onoclea sensibilis
)ga-
metophytes was removed by digestion with papain, a protease, which also left the
plasma membrane delineating the plasmodesmata swollen and irregular in profile.
However, treatment with Triton X-100 did solubilise the plasma membrane limit-
ing the plasmodesmata but the desmotubule was left intact (Tilney
et al.
, 1991).
The authors of this study concluded that the desmotubule provides a stabilising cy-
toskeletal element for each plasmodesma. In maize root-tip wall fragments, Turner
et al.
(1994) found that light protease treatment (with trypsin or chymotrypsin)
