Journal Article

Evidence for functional heterogeneity of sieve element–companion cell complexes in minor vein phloem of Alonsoa meridionalis

Olga V. Voitsekhovskaja, Elena L. Rudashevskaya, Kirill N. Demchenko, Marina V. Pakhomova, Denis R. Batashev, Yuri V. Gamalei, Gertrud Lohaus and Katharina Pawlowski

in Journal of Experimental Botany

Published on behalf of Society for Experimental Biology

Volume 60, issue 6, pages 1873-1883
Published in print April 2009 | ISSN: 0022-0957
Published online March 2009 | e-ISSN: 1460-2431 | DOI: https://dx.doi.org/10.1093/jxb/erp074
Evidence for functional heterogeneity of sieve element–companion cell complexes in minor vein phloem of Alonsoa meridionalis

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Two modes of phloem loading have been proposed, apoplastic and symplastic, depending on the structure of sieve element–companion cell complexes (SE–CCCs) in minor vein phloem. Species are usually classified as either apoplastic or symplastic loaders although the cytology of SE–CCCs in minor veins of the majority of plants indicates that both mechanisms can be simultaneously involved in phloem loading. The functions of structurally different SE–CCCs in minor veins of the stachyose-translocating plant Alonsoa meridionalis were examined. A stachyose synthase gene, AmSTS1, was expressed in intermediary cells but not in the ordinary companion cell of the same vein. In contrast, sucrose transporter AmSUT1 protein was present in ordinary companion cells but not in the neighbouring intermediary cells. These data reveal the principles of phloem sap formation in A. meridionalis and, probably, in many other dicots. The two types of SE–CCCs within one and the same minor vein load different carbohydrates, using contrasting mechanisms for their delivery into the phloem. Lateral sieve pores in the minor vein phloem lead to mixing of the carbohydrates soon after loading. While symplastic and apoplastic pathways can function simultaneously during phloem loading, they are separated at the level of different SE–CCCs combined in phloem endings.

Keywords: Apoplast; companion cells; phloem loading; stachyose synthase; sucrose transporter; symplast

Journal Article.  7130 words.  Illustrated.

Subjects: Plant Sciences and Forestry

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