Opportunities and successes in the search for plasmodesmal proteins

The proteinaceous composition of plasmodesmata (PDs) is a puzzle for which pieces have proven particularly difficult to find. This review describes the numerous approaches that have been undertaken in the search for PD-associated proteins and what each has contributed to our understanding of PD structure and function. These approaches include immunolocalisation of known proteins, proteomic characterisation of PD-enriched tissue fractions, high-throughput screens of random cDNAs and mutant screens. In addition to components of the cytoskeleton, novel proteins with predicted or unknown functions have been identified. Many of these have properties that relate to the symplastic and/or apoplastic faces of the plasma membrane. Mutant screens have identified proteins involved in previously unconnected cell pathways such as ROS signalling, implicating ROS in PD formation and regulation. Proteins associated with callose synthesis and degradation have also been identified and characterised, providing considerable weight to the hypothesis that callose deposition around the neck of the PD pore is one mechanism by which the PD aperture is regulated. The techniques described in this review have been developed such that it is to be expected that a considerable number of new PD proteins will be identified in coming years to fill in further detail of the structure and functional mechanisms of these dynamic pores.