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Plant Physiol, May 2000, Vol. 123, pp. 125-138
Sieve Tube Unloading and Post-Phloem Transport of Fluorescent
Tracers and Proteins Injected into Sieve Tubes via Severed Aphid
Stylets1
Donald B.
Fisher* and
Cora E.
Cash-Clark2
Department of Botany, Washington State University, Pullman,
Washington 99164-4238
A variety of fluorescent tracers and proteins were injected via
severed aphid stylets into the sieve tubes of wheat (Triticum aestivum L.) grains to evaluate the dimensions of plasmodesmal channels involved in sieve element/companion cell (SE/CC)
unloading and post-phloem transport. In the post-phloem pathway, where
diffusion is the predominant mode of transport, the largest molecule to show mobility was 16-kD dextran, with a Stokes radius of 2.6 nm. This
suggests that the aqueous channels for cell-to-cell transport must be
about 8 nm in diameter. Even the largest tracer injected into the sieve
tubes, 400-kD fluorescein-labeled Ficoll with a Stokes radius of about
11 nm, was unloaded from the SE/CC complex. However, in
contrast to smaller tracers ( 3 kD, with a Stokes radius 1.2 nm), the unloading of fluorescein-labeled Ficoll and other large
molecules from the SE/CC complex showed an irregular, patchy distribution, with no further movement along the post-phloem pathway. Either the plasmodesmal channels involved in SE/CC
unloading are exceptionally large (perhaps as much as 42 nm in
diameter), with only a very small fraction of plasmodesmata being
conductive, or the larger tracers damage the plasmodesmata in some way,
enlarging smaller channels.
1
This work was supported by the National Science
Foundation (grant no. IBN-9514188).
2
Present e-mail address:
tclark2{at}iusb.edu.
*
Corresponding author; e-mail dbfisher{at}wsu.edu.
© 2000 American Society of Plant Physiologists
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