title: Soluble and filamentous proteins in Arabidopsis sieve elements.
abstract: 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.
authors.affiliation: UMR1332, Biologie du Fruit et Pathologie, INRA, F-33140 Villenave d'Ornon, France.
authors.forename: Brigitte
authors.initials: B
authors.lastname: Batailler
authors.forename: Thomas
authors.initials: T
authors.lastname: Lemaître
authors.forename: Françoise
authors.initials: F
authors.lastname: Vilaine
authors.forename: Christian
authors.initials: C
authors.lastname: Sanchez
authors.forename: Denis
authors.initials: D
authors.lastname: Renard
authors.forename: Thibaud
authors.initials: T
authors.lastname: Cayla
authors.forename: Julie
authors.initials: J
authors.lastname: Beneteau
authors.forename: Sylvie
authors.initials: S
authors.lastname: Dinant
mesh_terms: D017360:Arabidopsis / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000032:analysis* / Q000648:ultrastructure; D002853:Chromatography, Liquid; D046529:Microscopy, Electron, Transmission; D052585:Phloem / Q000378:metabolism* / Q000648:ultrastructure; D053147:Plant Exudates / Q000032:analysis; D037121:Plant Lectins / Q000032:analysis; D018087:Plastids / Q000648:ultrastructure; D020543:Proteome / Q000032:analysis*; D040901:Proteomics; D053719:Tandem Mass Spectrometry
chemical_list: D029681:Arabidopsis Proteins; C552547:PP2-A1 protein, Arabidopsis; D053147:Plant Exudates; D037121:Plant Lectins; D020543:Proteome