Increased Expression of a Phloem Membrane Protein Encoded by NHL26 Alters Phloem Export and Sugar Partitioning in Arabidopsis

Abstract

The complex process of phloem sugar transport involves symplasmic and apoplasmic events. We characterized Arabidopsis thaliana lines ectopically expressing a phloem-specific gene encoding NDR1/HIN1-like26 (NHL26), a putative membrane protein. NHL26 overexpressor plants grew more slowly than wild-type plants, accumulated high levels of carbohydrates in mature leaves, and had a higher shoot biomass, contrasting with slower root growth and a lower seed yield. Similar effects were observed when NHL26 was overexpressed in companion cells, under the control of a companion cell-specific promoter. The soluble sugar content of the phloem sap and sink organs was lower than that in the wild type, providing evidence of a sugar export defect. This was confirmed in a phloem-export assay with the symplastic tracer carboxyfluorescein diacetate. Leaf sugar accumulation was accompanied by higher organic acid, amino acid, and protein contents, whereas analysis of the metabolite profile of phloem sap exudate revealed no change in amino acid or organic acid content, indicating a specific effect on sugar export. NHL26 was found to be located in the phloem plasmodesmata and the endoplasmic reticulum. These findings reveal that NHL26 accumulation affects either the permeability of plasmodesmata or sugar signaling in companion cells, with a specific effect on sugar export. INTRODUCTIONPlants are photoautotrophic organisms, producing carbohydrates in photosynthetic organs and distributing them to heterotrophic organs. The phloem manages the partitioning of sugars in the plant, controlling the entry of sugars into the translocation stream, sugar transport from source to sink organs, and delivery to the various competing sink organs (van Bel, 2003). Sugar transport involves the delivery of these molecules from photosynthetic cells to the phloem sieve elements (SEs) (Sjölund, 1997;Lalonde et al., 2003). Three transport strategies have been described in different species: active loading from the apoplasm, passive diffusion via the symplasm, and passive symplasmic transfer followed by polymer trapping (Rennie and Turgeon, 2009). Plasmodesmata (PD) are directly involved in the symplasmic steps, whereas Suc symporters at the plasma membrane of the phloem cells are involved in apoplasmic loading (Dinant and Lemoine, 2010;Ayre, 2011). The sugar loading process comprises several steps (Lalonde et al., 2003;Ayre, 2011), including efflux into the apoplasm by transporters from the SWEET family (Chen et al., 2012) and influx into the companion cell (CC)-SE complex by transporters from the SUC/SUT family (Sauer, 2007). However, other aspects, such as the sugar signaling pathways involved in coordinating the symplasmic or apoplasmic steps and the mechanisms preventing the flow of Suc back through PD in various cell types (Turgeon, 2006) remain unclear. It is also possible that both the apoplasmic and symplasmic pathways contribute to photoassimilate flux (Turgeon and Ayre, 2005). A recent study showed that, in cantaloupe (Cucumis melo), a species define...

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