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Symplasmic Constriction and Ultrastructural Features of the Sieve
Element/Companion Cell Complex in the Transport Phloem of
Apoplasmically and Symplasmically Phloem-Loading
Species1
Ronald Kempers*,
Ankie Ammerlaan, and
Aart J.E. van Bel
Transport Physiology Research Group, Department of Plant Ecology
and Evolutionary Biology, Utrecht University, Sorbonnelaan 16, NL-3584
CA Utrecht, The Netherlands (R.K., A.A.); and Institut für
Allgemeine Botanik und Pflanzenphysiologie, Justus-Liebig
Universität Giessen, Senckenbergstrasse 17, D-35390 Giessen,
Germany (A.J.E.v.B.)
The ultrastructural features of the
sieve element/companion cell complexes were screened in the stem phloem
of two symplasmically loading (squash, [Cucurbita
maxima L.] and Lythrum salicaria L.) and two
apoplasmically loading (broad bean [Vicia faba L.] and Zinnia elegans L.) species. The distinct ultrastructural
differences between the companion cells in the collection phloem of
symplasmically and apoplasmically phloem-loading species continue to
exist in the transport phloem. Plasmodesmograms of the stem phloem
showed a universal symplasmic constriction at the interface between the sieve element/companion cell complex and the phloem parenchyma cells.
This contrasts with the huge variation in symplasmic continuity between
companion cells and adjoining cells in the collection phloem of
symplasmically and apoplasmically loading species. Further, the
ultrastructure of the companion cells in the transport phloem faintly
reflected the features of the companion cells in the loading zone of
the transport phloem. The companion cells of squash contained numerous
small vacuoles (or vesicles), and those of L. salicaria contained a limited number of vacuoles. The companion cells of broad
bean and Z. elegans possessed small wall protrusions.
Implications of the present findings for carbohydrate processing in
intact plants are discussed.
1
A part of this study was subsidized by NWO
(Dutch Organization for Scientific Research).
*
Corresponding author; e-mail r.kempers{at}boev.biol.ruu.nl; fax
31-30-251-8366.
Plant Physiol. (1998) 116: 271-278
Copyright Clearance Center: 0032-0889/98/116/0271/08
© 1998 American Society of Plant Physiologists
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