Dissecting plasmodesmata molecular composition by mass spectrometry-based proteomics

Magali S Salmon,Emmanuelle M F Bayer

doi:10.3389/fpls.2012.00307

Abstract

In plants, the intercellular communication through the membranous channels called plasmodesmata (PD; singular plasmodesma) plays pivotal roles in the orchestration of development, defence responses, and viral propagation. PD are dynamic structures embedded in the plant cell wall that are defined by specialized domains of the endoplasmic reticulum (ER) and the plasma membrane (PM). PD structure and unique functions are guaranteed by their particular molecular composition. Yet, up to recent years and despite numerous approaches such as mutant screens, immunolocalization, or screening of random cDNAs, only few PD proteins had been conclusively identified and characterized. A clear breakthrough in the search of PD constituents came from mass-spectrometry-based proteomic approaches coupled with subcellular fractionation strategies. Due to their position, firmly anchored in the extracellular matrix, PD are notoriously difficult to isolate for biochemical analysis. Proteomic-based approaches have therefore first relied on the use of cell wall fractions containing embedded PD then on “free” PD fractions whereby PD membranes were released from the walls by enzymatic degradation. To discriminate between likely contaminants and PD protein candidates, bioinformatics tools have often been used in combination with proteomic approaches. GFP fusion proteins of selected candidates have confirmed the PD association of several protein families. Here we review the accomplishments and limitations of the proteomic-based strategies to unravel the functional and structural complexity of PD. We also discuss the role of the identified PD-associated proteins.

Highlights

In plants, intercellular communication must overcome the rigid pectocellulosic wall that encompasses all cells
These technologies have transformed what in the past could only be the result of laborious sequencing of few selected proteins enriched in wall or PD fractions, into a non-targeted approach whereby most, if not all, proteins present in a given sample could be identified
An alternative strategy consists on downstream analysis of the proteomic datasets generated using bioinformatic tools, databases, and literature sources. This approach was employed by the Maule laboratory following the establishment of Arabidopsis cell wall proteome, where PD components accounted for a small proportion of total proteins (Bayer et al, 2006)

Summary

Introduction

Intercellular communication must overcome the rigid pectocellulosic wall that encompasses all cells. The first attempts to identify PD-associated proteins from purified cell walls, relied on plant tissues known to be rich in PD (Monzer and Kloth, 1991; Kotlizky et al, 1992; Turner et al, 1994; Epel et al, 1995, 1996). Combined with the development of subcellular fractionation strategies these approaches have permitted the identification of an unprecedented number of PD-associated proteins.

Results

Conclusion