Minor vein structure and sugar transport in Arabidopsis thaliana |
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Authors: | Edith Haritatos Richard Medville Robert Turgeon |
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Institution: | (1) Section of Plant Biology, Cornell University, Ithaca, NY 14853, USA, US;(2) Electron Microscopy Services and Consultants, 18407 North 12th Place, Phoenix, AZ 85022, USA, US |
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Abstract: | Leaf and minor vein structure were studied in Arabidopsis thaliana (L.) Heynh. to gain insight into the mechanism(s) of phloem loading. Vein density (length of veins per unit leaf area) is
extremely low. Almost all veins are intimately associated with the mesophyll and are probably involved in loading. In transverse
sections of veins there are, on average, two companion cells for each sieve element. Phloem parenchyma cells appear to be
specialized for delivery of photoassimilate from the bundle sheath to sieve element-companion cell complexes: they make numerous
contacts with the bundle sheath and with companion cells and they have transfer cell wall ingrowths where they are in contact
with sieve elements. Plasmodesmatal frequencies are high at interfaces involving phloem parenchyma cells. The plasmodesmata
between phloem parenchyma cells and companion cells are structurally distinct in that there are several branches on the phloem
parenchyma cell side of the wall and only one branch on the companion cell side. Most of the translocated sugar in A. thaliana is sucrose, but raffinose is also transported. Based on structural evidence, the most likely route of sucrose transport is
from bundle sheath to phloem parenchyma cells through plasmodesmata, followed by efflux into the apoplasm across wall ingrowths
and carrier-mediated uptake into the sieve element-companion cell complex.
Received: 5 October 1999 / Accepted: 20 November 1999 |
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Keywords: | :Arabidopsis (sucrose transport) – Minor vein – Phloem loading – Plasmodesmata – Raffinose – Sucrose transport |
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