Involved in the regulation of sugar, amino acid and some primary metabolite export from companion cells (CCs) to sieve elements (SEs) in phloem. Required for apoplastic phloem sugar loading in source leaves in order to transport it to sink tissues. Required for correct sugar partitioning between source leaves and sink organs. Expressed in the vasculature of roots, rosette leaves, stems, cauline leaves and flowers. Specifically expressed in phloem. Induced by senescence (PubMed:14617064). Down-regulated by sucrose, glucose and fructose (PubMed:23715470). No visible phenotype under normal growth conditions. Plants over-expressing NHL26 exhibit slow growth, accumulate high levels of carbohydrates in mature leaves, have increased shoot biomass, decreased root biomass and decreased seed yield.