Abstract
Trees are generally assumed to be symplastic phloem loaders. A typical feature for most wooden species is an open minor vein structure with symplastic connections between mesophyll cells and phloem cells, which allow sucrose to move cell-to-cell through the plasmodesmata into the phloem. Fraxinus excelsior (Oleaceae) also translocates raffinose family oligosaccharides in addition to sucrose. Sucrose concentration was recently shown to be higher in the phloem sap than in the mesophyll cells. This suggests the involvement of apoplastic steps and the activity of sucrose transporters in addition to symplastic phloem-loading processes. In this study, the sucrose transporter FeSUT1 from F. excelsior was analysed. Heterologous expression in baker's yeast showed that FeSUT1 mediates the uptake of sucrose. Immunohistochemical analyses revealed that FeSUT1 was exclusively located in phloem cells of minor veins and in the transport phloem of F. excelsior. Further characterization identified these cells as sieve elements and possibly ordinary companion cells but not as intermediary cells. The localization and expression pattern point towards functions of FeSUT1 in phloem loading of sucrose as well as in sucrose retrieval. FeSUT1 is most likely responsible for the observed sucrose gradient between mesophyll and phloem. The elevated expression level of FeSUT1 indicated an increased apoplastic carbon export activity from the leaves during spring and late autumn. It is hypothesized that the importance of apoplastic loading is high under low-sucrose conditions and that the availability of two different phloem-loading mechanisms confers advantages for temperate woody species like F. excelsior.
Highlights
Deciduous trees are highly integrated systems of different and roots, the major sinks in trees include wood tissue that source tissues and of competing carbohydrate sinks
Heterologous expression of FeSUT1 in yeast showed that it codes for a functional SUT
The Km value of FeSUT1 was typical for Group II SUTs
Summary
Deciduous trees are highly integrated systems of different and roots, the major sinks in trees include wood tissue that source tissues and of competing carbohydrate sinks. Whereas exhibits annual cycles of carbohydrate storage and remobithe major sinks in herbaceous annuals are generative organs lization for growth. Phloem loading, and delivery through the phloem are different options that can be regulated to cope with environmental and seasonal changes. This is especially true for woody species because of their long lifespan and exposure to potentially extreme climate changes during annual cycles in temperate forests (De Schepper et al, 2013). The regulation processes involved in carbon partitioning in trees are far from understood because most research regarding phloem loading as a regulatory step in carbohydrate delivery has been performed in herbaceous species
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