Journal Article

Frequencies of plasmodesmata in Allium cepa L. roots: implications for solute transport pathways

Fengshan Ma and Carol A. Peterson

in Journal of Experimental Botany

Published on behalf of Society for Experimental Biology

Volume 52, issue 358, pages 1051-1061
Published in print May 2001 | ISSN: 0022-0957
Published online May 2001 | e-ISSN: 1460-2431 | DOI:
Frequencies of plasmodesmata in Allium cepa L. roots: implications for solute transport pathways

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Plasmodesmatal frequencies (PFs) were analysed in Allium cepa L. roots with a mature exodermis (100 mm from the tip). For all interfaces within the root, the numbers of plasmodesmata (PD) μm−2 wall surface (Fw) were calculated from measurements of 60 walls on ultrathin sections. For tissues ranging from the epidermis up to the stelar parenchyma, the frequencies were also expressed as total PD numbers mm−1 root length (Fn), which is most instructive for considering the radial transport of ions and photosynthates (because the tissues were arranged in concentric cylinders). The Fn values were constantly high at the interfaces of exodermis–central cortex, central cortex–endodermis and endodermis–pericycle (4.05×105, 5.13×105, and 5.64×105, respectively). If the plasmodesmata are functional, a considerable symplastic transport pathway exists between the exodermis and pericycle. Two interfaces had especially low PFs: epidermis–exodermis (Fn=8.96×104) and pericycle–stelar parenchyma (Fn=6.44×104). This suggests that there is significant membrane transport across the interface of epidermis–exodermis (through short cells) and direct transfer of ions from pericycle to protoxylem vessels. In the phloem, the highest PF was detected at the metaphloem sieve element–companion cell interface (Fw=0.42), and all other interfaces had much lower PFs (around 0.10). In the pericycle, the radial walls had a high PF (Fw=0.75), a feature that could permit lateral circulation of solutes, thus facilitating ion (inward) and photosynthate (outward) delivery.

Keywords: Allium cepa L.; phloem unloading; plasmodesmata; plasmodesmatal frequency; root; symplastic transport; transmission electron microscopy.; Fn, number of plasmodesmata mm−1 root length (at individual tissue interfaces); Ft, number of plasmodesmata μm−2 tissue interface; Fw, number of plasmodesmata μm−2 wall surface; PD, plasmodesmata; PF, plasmodesmatal frequency; TEM, transmission electron microscopy.

Journal Article.  7557 words.  Illustrated.

Subjects: Plant Sciences and Forestry

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