Phloem export, sugar partitioning and senescence

Early steps of leaf senescence are associated to a rise in sugar content and conversely elevated sugar accumulation can trigger leaf senescence (Wringler et al., 2006). Carbohydrate partitioning is regulated by the phloem that secures long distance transport and local adjustments. Phloem sugar transport is a complex process involving both symplasmic and apoplasmic events. The phloem also plays an important role in the partitioning of amino acids and other organic compounds. In addition, the phloem sieve elements contain a range of enzymes involved in glycolysis, citric acid cycle and conversion of amino acids (Batailler et al., 2012; Dinant and Lucas, 2012), potentially contributing to local adjustments. 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 member of the Hin/NDR1 family (Vilaine et al., 2013). NHL26 OEX plants grew more slowly than wild-type plants and had a higher shoot biomass, contrasting with slower root growth and a lower seed yield. They also showed significantly delayed senescence. NHL26 was found to be located in the phloem plasmodesmata, suggesting that the overaccumulation of NHL26 affects the permeability of plasmodesmata and symplasmic sugar export. Sugar accumulation in mature leaves was accompanied by higher organic acid, amino-acid and protein contents, and transcriptional reprogramming of genes encoding enzymes involved in nitrogen remobilization and assimilation appeared. The soluble sugar content of the phloem sap was lower. By contrast the analysis of the metabolite profile of phloem sap exudate revealed no change in amino-acid or organic acid content, indicating a specific effect on the symplasmic export of sugars. This suggests that the mechanisms of export of sugars and amino acids within the phloem involved different regulatory pathways that should enable the plant to cope with excess sugar