From the evidence gathered so far it is clear that the selective transport of proteins and RNAs through plasmodesmata is complex and highly regulated. Many current models of cell-to-cell transport through plasmodesmata implicate the cortical ER and desmotubule in trafficking NCAP through plasmod-esmata. The greatest advances at present are emerging from new discoveries at the protein level using genomic and proteomic techniques (Cutler et al. 2000; Escobar et al. 2003; Tian et al. 2004; Oparka 2004). These should allow the identification of the receptor, docking and regulatory proteins at present hypothesised to be part of the pore and increase our understanding of the role of the ER within plasmodesmata.

Acknowledgement The authors thank T. Gillespie for assistance with the electron microscopy images. This work was supported by the Scottish Executive Environment and Rural Affairs Department.

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