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PLoS One. 2015 Feb 25;10(2):e0118122. doi: 10.1371/journal.pone.0118122. eCollection 2015.

Live imaging of companion cells and sieve elements in Arabidopsis leaves.

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Institut Jean-Pierre Bourgin, INRA-AgroParisTech, UMR1318, ERL CNRS 3559, Saclay Plant Sciences, Versailles, France.
Univ. Bordeaux, CNRS, INSERM, Bordeaux Imaging Center, UMS 3420, Bordeaux, France; INRA, Univ. Bordeaux, UMR1332 de Biologie du Fruit et Pathologie, Villenave d'Ornon, France.
INRA, Univ. Bordeaux, UMR1332 de Biologie du Fruit et Pathologie, Villenave d'Ornon, France.
College of Agricultural Sciences, The Pennsylvania State University, University Park, PA, United States of America.
Plateforme de Cytologie et Imagerie Végétale, UMR1318, INRA-AgroParisTech, Versailles, France.


The phloem is a complex tissue composed of highly specialized cells with unique subcellular structures and a compact organization that is challenging to study in vivo at cellular resolution. We used confocal scanning laser microscopy and subcellular fluorescent markers in companion cells and sieve elements, for live imaging of the phloem in Arabidopsis leaves. This approach provided a simple framework for identifying phloem cell types unambiguously. It highlighted the compactness of the meshed network of organelles within companion cells. By contrast, within the sieve elements, unknown bodies were observed in association with the PP2-A1:GFP, GFP:RTM1 and RTM2:GFP markers at the cell periphery. The phloem lectin PP2-A1:GFP marker was found in the parietal ground matrix. Its location differed from that of the P-protein filaments, which were visualized with SEOR1:GFP and SEOR2:GFP. PP2-A1:GFP surrounded two types of bodies, one of which was identified as mitochondria. This location suggested that it was embedded within the sieve element clamps, specific structures that may fix the organelles to each another or to the plasma membrane in the sieve tubes. GFP:RTM1 was associated with a class of larger bodies, potentially corresponding to plastids. PP2-A1:GFP was soluble in the cytosol of immature sieve elements. The changes in its subcellular localization during differentiation provide an in vivo blueprint for monitoring this process. The subcellular features obtained with these companion cell and sieve element markers can be used as landmarks for exploring the organization and dynamics of phloem cells in vivo.

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