Journal Article

Hexoses as phloem transport sugars: the end of a dogma?

Aart J. E. van Bel and Paul H. Hess

in Journal of Experimental Botany

Published on behalf of Society for Experimental Biology

Volume 59, issue 2, pages 261-272
Published in print February 2008 | ISSN: 0022-0957
Published online February 2008 | e-ISSN: 1460-2431 | DOI:

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According to most textbooks, only non-reducing carbohydrate species such as sucrose, sugar alcohols, and raffinose-family sugars function as phloem translocates. Occasional abundance of reducing sugar species (such as hexoses) in sieve-tube sap has been discarded as an experimental artefact. This study, however, discloses a widespread occurrence of hexoses in the sieve-tube sap. Phloem exudation facilitated by EDTA provided evidence that many of the members of two plant families (Ranunculaceae and Papaveraceae) investigated translocate >80% of carbohydrates in the form of hexoses. Representatives of other families also appear to translocate appreciable amounts of hexoses in the sieve tubes. Promoting effects of EDTA, activities of sucrose-degrading enzymes, and sugar uptake by micro-organisms on hexose contents of phloem exudates were checked. The rate of sucrose degradation is far too low to explain the large proportions of hexoses measured in phloem exudates; nor did other factors tested seem to stimulate the occurrence of hexoses. The validity of the approach is further supported by the virtual absence of hexoses in exudates from species that were known as exclusive sucrose transporters. This study urges a rethink of the existing views on carbohydrate transport species in the phloem stream. Hexose translocation is to be regarded as a normal mode of carbohydrate transfer by the phloem equivalent to that of sucrose, raffinose-family sugars, or sugar alcohols.

Keywords: Carbohydrate translocation; family-related transport sugars; hexose; Papaveraceae; phloem transport; Ranunculaceae; sieve tubes

Journal Article.  7390 words.  Illustrated.

Subjects: Plant Sciences and Forestry

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