Carbohydrate metabolism and transport in apple roots under nitrogen deficiency

Soluble sugars play important roles in plant development and stress response, and the nitrogen supply level can affect the among-organ distribution and metabolism of sugar in plants and, in turn, plant growth. To explore the adaptive response of apple root growth to nitrogen supply and its relationship with sugar metabolism, we used a hydroponic culture system to study how the nitrogen supply affects soluble sugar concentrations and sugar metabolism in apple roots. In hydroponic seedlings of Malus hupehensis, low nitrogen application caused rapid and vigorous proliferation of lateral roots, and the transcript levels of MdSOT1 and MdSUT3, which are involved in photoassimilate unloading in roots, were upregulated. The accumulation of sorbitol and sucrose in the fine roots was higher, and the activities of sucrose synthase, invertase and sorbitol dehydrogenase, which are involved in the degradation of sucrose and sorbitol, were significantly increased under a low nitrogen supply. Genes involved in sugar degradation, such as MdSDH1, MdSuSy5, and MdNINV3, play important roles in the efficient use of sorbitol and sucrose under nitrogen deficiency. Additionally, the activity of fructokinase and hexokinase, which are involved in hexose phosphorylation, and transcript levels of MdFRK2 and MdHK3 were significantly upregulated under nitrogen deficiency, and the hexose phosphate products F6P and G6P accumulated greatly in the roots. These results showed that the sugar metabolism capability and sink strength of the roots increased under low nitrogen, indicating that low nitrogen promotes the utilization of sugar in the roots to meet the demand for sugar under rapid root growth.