Plasmodesmata (singular plasmodesma) are membrane-lined pores that interconnect plant cells to form a functional cytoplasmic continuum known as the symplasm. Although first described in the context of cell-to-cell communication by Eduard Tangl in 1879 (cited by Carr 1976), it was not until relatively recently that the importance of the endoplasmic reticulum (ER) within plasmodesmata became appreciated. Whilst the exact role of the ER is only now being characterised, it is recognised that the ER is an essential component of the structure and function of plasmodesmata.

Neighbouring cells within all multicellular eukaryotes must communicate to allow coordinated differentiation (Hashimoto and Inze 2003). From recent ultrastructural, physiological, biochemical and molecular studies it is now clear that plasmodesmata are not merely static channels that allow the movement of low molecular weight solutes, but rather function as flexible structures that are able selectively to control the cell-to-cell movement of proteins and RNAs (Lucas et al. 1993; Blackman and Overall 2001; Ehlers and Kollmann 2001; Roberts and Oparka 2003; Ding et al. 2003). As will be described, the ER forms an essential component of plasmodesmata, and can also determine where they are formed within the cell wall. The ER is inti mately involved in the transfer of molecules to and through plasmodesmata, providing a number of pathways for movement between cells, and has been implicated in the mechanisms that control transport. A number of reviews have appeared recently providing a comprehensive description of plasmodes-mata and their function, and the reader is referred to these for more detailed discussions (Haywood et al. 2002; Roberts and Oparka 2003; Oparka 2004; Ruiz-Medrano et al. 2004; Heinlein and Epel 2004; Gillespie and Oparka 2005; Roberts 2005). In this chapter we will focus on the ER and its role in plasmod-esmatal formation and function.

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