Apoplastic and symplastic phloem loading in Quercus robur and Fraxinus excelsior

Soner Öner-Sieben,Gertrud Lohaus

doi:10.1093/jxb/eru066

Abstract

Whereas most of the research on phloem loading is performed on herbaceous plants, less is known about phloem loading strategies in trees. In this study, the phloem loading mechanisms of Quercus robur and Fraxinus excelsior were analysed. The following features were examined: the minor vein structure, the sugar concentrations in phloem sap by the laser-aphid-stylet technique, the distribution of photoassimilates in the mesophyll cells by non-aqueous fractionation, gradients of sugar concentrations and osmotic pressure, and the expression of sucrose transporters. The minor vein configurations of Q. robur and F. excelsior belong to the open type. Quercus robur contained companion cells in the minor veins whereas F. excelsior showed intermediary cells in addition to ordinary companion cells. The main carbon transport form in Q. robur was sucrose (~1M). In F. excelsior high amounts of raffinose and stachyose were also transported. However, in both tree species, the osmolality of phloem sap was higher than the osmolality of the mesophyll cells. The concentration gradients between phloem sap and the cytoplasm of mesophyll cells for sucrose were 16-fold and 14-fold for Q. robur and F. excelsior, respectively. Independent of the type of translocated sugars, sucrose transporter cDNAs were cloned from both species. The results indicate that phloem loading of sucrose and other metabolites must involve active loading steps in both tree species. Quercus robur seems to be an apoplastic phloem loader while F. excelsior shows indications of being a symplastic or mixed symplastic-apoplastic phloem loader.

Highlights

The sharing of work between autotrophic source tissues and heterotrophic sink organs in higher plants makes an efficient vascular system necessary to transport carbohydrates and other metabolites
The results indicate that phloem loading of sucrose and other metabolites must involve active loading steps in both tree species
The minor vein structure was analysed to confirm the classifications into different minor vein types of Q. robur and F. excelsior performed by Gamalei (1989)

Summary

Introduction

The sharing of work between autotrophic source tissues and heterotrophic sink organs in higher plants makes an efficient vascular system necessary to transport carbohydrates and other metabolites. It was postulated that the size exclusion limit of plasmodesmata connecting ICs to the BSCs enables the passage of disaccharides such as sucrose from the mesophyll into the phloem, whereas the tri- and tetrasaccharides raffinose and stachyose remain trapped in the phloem Both types of phloem loading, apoplastic and symplastic, result in much higher sugar concentrations in the phloem sap compared with the MCs and phloem loading takes place against a concentration gradient (Lohaus et al, 1995; Voitsekhoskaja et al, 2006). Several results indicated that at least some plants harbour redundant phloem loading strategies by the combination of active symplastic and apoplastic and/or passive loading mechanisms (Fisher, 1986; van Bel et al, 1992; Voitsekhovskaja et al, 2006; Slewinski et al, 2013)

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Conclusion