title: Phloem ultrastructure and pressure flow: Sieve-Element-Occlusion-Related agglomerations do not affect translocation.
abstract: Since the first ultrastructural investigations of sieve tubes in the early 1960s, their structure has been a matter of debate. Because sieve tube structure defines frictional interactions in the tube system, the presence of P protein obstructions shown in many transmission electron micrographs led to a discussion about the mode of phloem transport. At present, it is generally agreed that P protein agglomerations are preparation artifacts due to injury, the lumen of sieve tubes is free of obstructions, and phloem flow is driven by an osmotically generated pressure differential according to Münch's classical hypothesis. Here, we show that the phloem contains a distinctive network of protein filaments. Stable transgenic lines expressing Arabidopsis thaliana Sieve-Element-Occlusion-Related1 (SEOR1)-yellow fluorescent protein fusions show that At SEOR1 meshworks at the margins and clots in the lumen are a general feature of living sieve tubes. Live imaging of phloem flow and flow velocity measurements in individual tubes indicate that At SEOR1 agglomerations do not markedly affect or alter flow. A transmission electron microscopy preparation protocol has been generated showing sieve tube ultrastructure of unprecedented quality. A reconstruction of sieve tube ultrastructure served as basis for tube resistance calculations. The impact of agglomerations on phloem flow is discussed.
authors.affiliation: School of Biological Sciences, Washington State University, Pullman, Washington 99164-4236, USA.
authors.forename: Daniel R
authors.initials: DR
authors.lastname: Froelich
authors.forename: Daniel L
authors.initials: DL
authors.lastname: Mullendore
authors.forename: Kåre H
authors.initials: KH
authors.lastname: Jensen
authors.forename: Tim J
authors.initials: TJ
authors.lastname: Ross-Elliott
authors.forename: James A
authors.initials: JA
authors.lastname: Anstead
authors.forename: Gary A
authors.initials: GA
authors.lastname: Thompson
authors.forename: Hélène C
authors.initials: HC
authors.lastname: Pélissier
authors.forename: Michael
authors.initials: M
authors.lastname: Knoblauch
mesh_terms: D017360:Arabidopsis / Q000235:genetics / Q000254:growth & development* / Q000378:metabolism; D003001:Cloning, Molecular; D005456:Fluorescent Dyes / Q000378:metabolism; D017110:Freeze Substitution; D017343:Genes, Plant; D005822:Genetic Vectors / Q000235:genetics / Q000378:metabolism; D007091:Image Processing, Computer-Assisted; D046529:Microscopy, Electron, Transmission; D016254:Mutagenesis, Insertional; D052585:Phloem / Q000254:growth & development / Q000378:metabolism / Q000648:ultrastructure*; D059828:Plant Cells / Q000378:metabolism; D018521:Plant Physiological Phenomena; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism*; D018517:Plant Roots / Q000235:genetics / Q000254:growth & development / Q000378:metabolism; D030821:Plants, Genetically Modified / Q000235:genetics / Q000254:growth & development / Q000378:metabolism; D032107:Populus / Q000254:growth & development / Q000378:metabolism; D011312:Pressure; D021381:Protein Transport; D014026:Nicotiana / Q000254:growth & development / Q000378:metabolism; D014170:Transformation, Genetic
chemical_list: D005456:Fluorescent Dyes; D010940:Plant Proteins