Phloem sugar transport is a complex process involving symplasmic and apoplasmic events. We characterized Arabidopsis overexpressor lines accumulating high levels of carbohydrates in mature leaves due to the ectopic expression of a phloem-specific gene encoding NHL26, a putative membrane protein. NHL26 overexpressor plants grew more slowly than wild-type plants and had a higher shoot biomass, contrasting with slower root growth and a lower seed yield. Interestingly, vein density was also modified in the source leaves, indicating downstream effects on vein development. 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 CFDA. Leaf sugar accumulation was accompanied by higher organic acid, amino-acid and protein contents, whereas analysis of the metabolite profile by GC/MS of phloem sap exudate revealed no change in amino-acid or organic acid content, indicating a specific effect on the symplasmic export of sugars. 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.< Réduire