Soluble and filamentous proteins in Arabidopsis sieve elements

Plant Cell Environ. 2012 Jul;35(7):1258-73. doi: 10.1111/j.1365-3040.2012.02487.x. Epub 2012 Feb 24.


Phloem sieve elements are highly differentiated cells involved in the long-distance transport of photoassimilates. These cells contain both aggregated phloem-proteins (P-proteins) and soluble proteins, which are also translocated by mass flow. We used liquid chromatography-tandem mass spectrometry (LC-MS/MS) to carry out a proteomic survey of the phloem exudate of Arabidopsis thaliana, collected by the ethylenediaminetetraacetic acid (EDTA)-facilitated method. We identified 287 proteins, a large proportion of which were enzymes involved in the metabolic precursor generation and amino acid synthesis, suggesting that sieve tubes display high levels of metabolic activity. RNA-binding proteins, defence proteins and lectins were also found. No putative P-proteins were detected in the EDTA-exudate fraction, indicating a lack of long-distance translocation of such proteins in Arabidopsis. In parallel, we investigated the organization of P-proteins, by high-resolution transmission electron microscopy, and the localization of the phloem lectin PP2, a putative P-protein component, by immunolocalization with antibodies against PP2-A1. Transmission electron microscopy observations of P-proteins revealed bundles of filaments resembling strings of beads. PP2-A1 was found weakly associated with these structures in the sieve elements and bound to plastids. These observations suggest that PP2-A1 is anchored to P-proteins and organelles rather than being a structural component of P-proteins.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Arabidopsis / metabolism*
  • Arabidopsis Proteins / analysis*
  • Arabidopsis Proteins / ultrastructure
  • Chromatography, Liquid
  • Microscopy, Electron, Transmission
  • Phloem / metabolism*
  • Phloem / ultrastructure
  • Plant Exudates / analysis
  • Plant Lectins / analysis
  • Plastids / ultrastructure
  • Proteome / analysis*
  • Proteomics
  • Tandem Mass Spectrometry


  • Arabidopsis Proteins
  • PP2-A1 protein, Arabidopsis
  • Plant Exudates
  • Plant Lectins
  • Proteome