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JXB Advance Access originally published online on March 25, 2009
Journal of Experimental Botany 2009 60(6):1873-1883; doi:10.1093/jxb/erp074
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© The Author [2009]. Published by Oxford University Press [on behalf of the Society for Experimental Biology]. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

RESEARCH PAPER

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

Olga V. Voitsekhovskaja1,2,*, Elena L. Rudashevskaya2 {dagger}, Kirill N. Demchenko3,4, Marina V. Pakhomova2, Denis R. Batashev2, Yuri V. Gamalei2, Gertrud Lohaus1 {ddagger} and Katharina Pawlowski1,5 {ddagger}

1Albrecht-von-Haller-Institute for Plant Sciences, Department of Plant Biochemistry, Justus-von-Liebig-Weg 11, D-37077 Goettingen, Germany
2Komarov Botanical Institute, Russian Academy of Sciences, Laboratory of Plant Ecological Physiology, ul. Professora Popova, 2, 197376 St Petersburg, Russia
3Komarov Botanical Institute, Russian Academy of Sciences, Laboratory of Plant Anatomy and Morphology, ul. Professora Popova, 2, 197376 St Petersburg, Russia
4All-Russia Research Institute for Agricultural Microbiology, Laboratory of Molecular and Cellular Biology, Podbelskogo 3, Pushkin 8, 196608, St Petersburg, Russia
5Department of Botany, Stockholm University, 10691 Stockholm, Sweden

* To whom correspondence should be addressed. E-mail: ovoitse{at}yandex.ru

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.

Key words: Apoplast, companion cells, phloem loading, stachyose synthase, sucrose transporter, symplast

{dagger} Present address: Department of Plant Biology and Biotechnology, University of Copenhagen, Thorvaldsensvej 40, 1871 Frederiksberg, Denmark.

{ddagger} These authors contributed equally to this work.

Received 2 October 2008; Revised 18 February 2009 Accepted 23 February 2009


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