On the role of sugar compartmentation and stachyose synthesis in symplastic phloem loading

Abstract

Compartmentation of phloem-translocated carbohydrates and localization of the synthesis of one specific transport form, stachyose, was studied in leaves of symplastic phloem loaders. In these plants, mesophyll and phloem are directly connected via highly developed plasmodesmata which allow an entirely symplastic loading of assimilates from the mesophyll into the phloem, in contrast to apoplastic phloem loaders where no plasmodesmal connections are present between mesophyll and phloem. The presence of plasmodesmata makes it necessary for the plant to develop an effective mechanism to sustain a concentration gradient of translocated carbohydrates between mesophyll and phloem in spite of the symplastic continuity of these tissues. This thesis aimed to elucidate whether and how the compartmentation of translocated carbohydrates can be used by a plant to establish such a mechanism. First, the distribution and concentrations of carbohydrates in subcellular compartments of mesophyll cells were analysed in two model plant species, the symplastic phloem loader Alonsoa meridionalis and the apoplastic loader Asarina barclaina (Scrophulariaceae). Second, the capacity of plasmamembranes of leaf cells to exchange different carbohydrates between the apoplast and the cytosol was compared in leaves of a number of symplastic and apoplastic phloem loaders. Third, the expression pattern of stachyose synthase from leaves of Alonsoa meridionalis (AmSTS) was studied at the cellular level. The regulation of AmSTS expression was further investigated by the isolation of the AmSTS promoter and the construction of transgenic Arabidopsis thaliana plants expressing an AmSTS-GUS fusion. The results showed that the subcellular compartmentation of carbohydrates in mesophyll cells of the symplastic phloem loader Alonsoa does not differ from that in apoplastic loaders. The study of uptake of different carbohydrates from the apoplast into the cytosol of leaf cells did not reveal any differences between the groups of apoplastic and symplastic phloem loaders. The companion cells (intermediary cells) of the symplastic loader Alonsoa were shown to be the sites of expression of AmSTS and thereby of stachyose synthesis. Furthermore, the expression of AmSTS was shown to be confined to companion cells of source organs only, and to be regulated by sugars and/or osmotic pressure. It is speculated that in intermediary cells, stachyose is localized within an endomembrane compartment which prevents the elimination of the stachyose concentration gradient between mesophyll and intermediary cells via plasmodesmata.