Agriculture Reference
In-Depth Information
3 NA as a signalling molecule
Patrice Dunoyer and Olivier Voinnet
3.1
Intercellular movement of plant mRNAs
3.1.1 Cell-to-cell movement of plant mRNAs
3.1.1.1 Plant plasmodesmata
In all multicellular organisms, the orchestration of key biological functions often
involves intercellular communication. In animals, this process relies mainly on se-
creted signalling peptides interacting with specific cell surface receptors. Although
plant cells can also communicate via secreted molecules (Bergey et al. , 1996;
Fletcher et al. , 1999; McCarty & Chory, 2000; Clark, 2001; Matsubayashi et al. ,
2001), they differ from their animal counterparts in that they are connected with
each other via specific channels known as plasmodesmata (see also Chapter 5).
Cell connections via plasmodesmata create a cytoplasmic continuum known as the
symplasm . Plasmodesmata are used for transport of small molecules, such as ions,
metabolites and hormones (Robards & Lucas, 1990), but these structures are also
involved in the selective trafficking of macromolecules such as proteins and RNAs,
providing the plant with a powerful communication system that allows control of
many developmental and physiological processes.
Plasmodesmata can be simply defined as membrane-lined pores connecting two
adjacent cells, which can arise during cytokinesis or between existing cell walls
(Lucas, 1995; Crawford & Zambryski, 1999; Haywood et al. , 2002; Wu et al. , 2002).
The two main structural components of plasmodesmata are an external membrane
and a central desmotubule that derive from the plasma membrane and the endoplas-
mic reticulum, respectively, both of which are contiguous between connected cells.
Plasmodesmata are typically defined by their size exclusion limit (or SEL), corre-
sponding to the upper limit in the size of macromolecules that can freely diffuse
from one cell to another. Several studies have identified two modes of cell-to-cell
transport through plasmodesmata. Non-targeted movement appears to be based on
passive diffusion and relies therefore solely on the intrinsic SEL of the plasmos-
desmata, which has been shown to vary greatly depending on the age of the tissues
analysed. For instance, the progression of tobacco leaves from carbon sink tissues
to carbon source tissues is accompanied by an important decrease in SEL that cor-
relates with a change in plasmodesmata ultrastructure, from simple to branched.
Non-targeted movement of macromolecules through plasmodesmata has been so
far documented only for proteins (Imlau et al. , 1999; Oparka et al. , 1999; Crawford
& Zambryski, 2000).
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