title: Phloem unloading in Arabidopsis roots is convective and regulated by the phloem-pole pericycle.
abstract: In plants, a complex mixture of solutes and macromolecules is transported by the phloem. Here, we examined how solutes and macromolecules are separated when they exit the phloem during the unloading process. We used a combination of approaches (non-invasive imaging, 3D-electron microscopy, and mathematical modelling) to show that phloem unloading of solutes in Arabidopsis roots occurs through plasmodesmata by a combination of mass flow and diffusion (convective phloem unloading). During unloading, solutes and proteins are diverted into the phloem-pole pericycle, a tissue connected to the protophloem by a unique class of 'funnel plasmodesmata'. While solutes are unloaded without restriction, large proteins are released through funnel plasmodesmata in discrete pulses, a phenomenon we refer to as 'batch unloading'. Unlike solutes, these proteins remain restricted to the phloem-pole pericycle. Our data demonstrate a major role for the phloem-pole pericycle in regulating phloem unloading in roots.
authors.affiliation: School of Biological Sciences, Washington State University, Pullman, United States.
authors.forename: Timothy J
authors.identifier: 0000-0002-0991-9588
authors.initials: TJ
authors.lastname: Ross-Elliott
authors.affiliation: Department of Physics, Technical University of Denmark, Lyngby, Denmark.
authors.forename: Kaare H
authors.identifier: 0000-0003-0787-5283
authors.initials: KH
authors.lastname: Jensen
authors.affiliation: Department of Physics, Technical University of Denmark, Lyngby, Denmark.
authors.forename: Katrine S
authors.initials: KS
authors.lastname: Haaning
authors.affiliation: School of Biological Sciences, Washington State University, Pullman, United States.
authors.forename: Brittney M
authors.identifier: 0000-0001-9679-659X
authors.initials: BM
authors.lastname: Wager
authors.affiliation: School of Biological Sciences, Washington State University, Pullman, United States.
authors.forename: Jan
authors.identifier: 0000-0002-8952-3961
authors.initials: J
authors.lastname: Knoblauch
authors.affiliation: School of Biological Sciences, Washington State University, Pullman, United States.
authors.forename: Alexander H
authors.identifier: 0000-0001-6735-0660
authors.initials: AH
authors.lastname: Howell
authors.affiliation: School of Biological Sciences, Washington State University, Pullman, United States.
authors.forename: Daniel L
authors.initials: DL
authors.lastname: Mullendore
authors.affiliation: Department of Biological and Medical Sciences, Oxford Brookes University, Headington, Oxford, United Kingdom.
authors.forename: Alexander G
authors.identifier: 0000-0003-1731-0446
authors.initials: AG
authors.lastname: Monteith
authors.affiliation: Institute of Molecular Plant Science, University of Edinburgh, Edinburgh, United Kingdom.
authors.forename: Danae
authors.initials: D
authors.lastname: Paultre
authors.affiliation: Sainsbury Laboratory, University of Cambridge, Cambridge, United Kingdom.
authors.forename: Dawei
authors.identifier: 0000-0001-8256-0279
authors.initials: D
authors.lastname: Yan
authors.affiliation: Sainsbury Laboratory, University of Cambridge, Cambridge, United Kingdom.
authors.forename: Sofia
authors.identifier: 0000-0001-9409-8544
authors.initials: S
authors.lastname: Otero
authors.affiliation: Sainsbury Laboratory, University of Cambridge, Cambridge, United Kingdom.
authors.forename: Matthieu
authors.initials: M
authors.lastname: Bourdon
authors.affiliation: Department of Plant and Soil Sciences, Delaware Biotechnology Institute, University of Delaware, Newark, United States.
authors.forename: Ross
authors.initials: R
authors.lastname: Sager
authors.affiliation: Department of Plant and Soil Sciences, Delaware Biotechnology Institute, University of Delaware, Newark, United States.
authors.forename: Jung-Youn
authors.initials: JY
authors.lastname: Lee
authors.affiliation: Sainsbury Laboratory, University of Cambridge, Cambridge, United Kingdom.
authors.forename: Ykä
authors.initials: Y
authors.lastname: Helariutta
authors.affiliation: School of Biological Sciences, Washington State University, Pullman, United States.
authors.forename: Michael
authors.identifier: 0000-0003-0391-9891
authors.initials: M
authors.lastname: Knoblauch
authors.affiliation: Institute of Molecular Plant Science, University of Edinburgh, Edinburgh, United Kingdom.
authors.forename: Karl J
authors.identifier: 0000-0002-8035-5076
authors.initials: KJ
authors.lastname: Oparka
mesh_terms: D017360:Arabidopsis / Q000378:metabolism*; D001692:Biological Transport; D008962:Models, Theoretical; D061848:Optical Imaging; D052585:Phloem / Q000378:metabolism*; D018517:Plant Roots / Q000378:metabolism*; D031425:Plasmodesmata / Q000378:metabolism
keywords: A. thaliana; funnel plasmodesmata; long distance signaling; phloem; phloem pole pericycle; phloem unloading; plant biology; protophloem