caa a22 4500 445840099 CHVBK 20180317145341.0 cr unu---uuuuu 170323e20110101xx s 000 0 eng 10.1007/s00709-010-0239-0 doi (NATIONALLICENCE)springer-10.1007/s00709-010-0239-0 Symplasmic transport and phloem loading in gymnosperm leaves [Elektronische Daten] [Johannes Liesche, Helle Martens, Alexander Schulz] Despite more than 130years of research, phloem loading is far from being understood in gymnosperms. In part this is due to the special architecture of their leaves. They differ from angiosperm leaves among others by having a transfusion tissue between bundle sheath and the axial vascular elements. This article reviews the somewhat inaccessible and/or neglected literature and identifies the key points for pre-phloem transport and loading of photoassimilates. The pre-phloem pathway of assimilates is structurally characterized by a high number of plasmodesmata between all cell types starting in the mesophyll and continuing via bundle sheath, transfusion parenchyma, Strasburger cells up to the sieve elements. Occurrence of median cavities and branching indicates that primary plasmodesmata get secondarily modified and multiplied during expansion growth. Only functional tests can elucidate whether this symplasmic pathway is indeed continuous for assimilates, and if phloem loading in gymnosperms is comparable with the symplasmic loading mode in many angiosperm trees. In contrast to angiosperms, the bundle sheath has properties of an endodermis and is equipped with Casparian strips or other wall modifications that form a domain border for any apoplasmic transport. It constitutes a key point of control for nutrient transport, where the opposing flow of mineral nutrients and photoassimilates has to be accommodated in each single cell, bringing to mind the principle of a revolving door. The review lists a number of experiments needed to elucidate the mode of phloem loading in gymnosperms. The Author(s), 2010 Bundle sheath nationallicence Endodermis nationallicence Plasmodesmata nationallicence Transfusion tissue nationallicence Strasburger cells nationallicence Xylem nationallicence Liesche Johannes Department of Plant Biology and Biotechnology, University of Copenhagen, Thorvaldsensvej 40, DK-1871, Frederiksberg C, Denmark aut Martens Helle Department of Plant Biology and Biotechnology, University of Copenhagen, Thorvaldsensvej 40, DK-1871, Frederiksberg C, Denmark aut Schulz Alexander Department of Plant Biology and Biotechnology, University of Copenhagen, Thorvaldsensvej 40, DK-1871, Frederiksberg C, Denmark aut Protoplasma Springer Vienna 248/1(2011-01-01), 181-190 0033-183X 248:1<181 2011 248 709 https://doi.org/10.1007/s00709-010-0239-0 text/html Onlinezugriff via DOI 1 review-article jats NATIONALLICENCE 856 40 https://doi.org/10.1007/s00709-010-0239-0 text/html Onlinezugriff via DOI NATIONALLICENCE 700 1- Liesche Johannes Department of Plant Biology and Biotechnology, University of Copenhagen, Thorvaldsensvej 40, DK-1871, Frederiksberg C, Denmark aut NATIONALLICENCE 700 1- Martens Helle Department of Plant Biology and Biotechnology, University of Copenhagen, Thorvaldsensvej 40, DK-1871, Frederiksberg C, Denmark aut NATIONALLICENCE 700 1- Schulz Alexander Department of Plant Biology and Biotechnology, University of Copenhagen, Thorvaldsensvej 40, DK-1871, Frederiksberg C, Denmark aut NATIONALLICENCE 773 0- Protoplasma Springer Vienna 248/1(2011-01-01), 181-190 0033-183X 248:1<181 2011 248 709 Metadata rights reserved Springer special CC-BY-NC licence nationallicence BK010053 XK010053 XK010000 NATIONALLICENCE NATIONALLICENCE NL-springer