Potassium (K) and nitrogen (N) are two of the most important macroelements needed by plants. After the absorption through roots, nitrate (NO3-) and K+ undergo the long-distance root-to-shoot transport. Xylem loading is a critical step for the root-to-shoot translocation of nutrients. In Arabidopsis thaliana, NPF7.3/NRT1.5 and SKOR are responsible for the xylem loading of NO3- and K+, respectively. Our group has demonstrated the K concentration in shoots of nrt1.5 knockout mutants was dramatically decreased compared to wild type at low NO3- supply, suggesting NRT1.5 is involved in K+ transport in a NO3--dependent manner. The aim of this work is to investigate the role of NRT1.5 in the root-to-shoot translocation of K+ in Arabidopsis. In this work, we show that NRT1.5 is important for the lateral root development of Arabidopsis at K+ deprivation conditions. Lateral root density of nrt1.5 knockout mutants was significantly reduced by K+ limitation compared to wild type. No K+ transport activity of NRT1.5 was not observed in Saccharomyces cerevisiae. However, yeast BYT12 cells expressing NRT1.5 were more sensitive to hygromycin B. Similar and the opposite responses to HygB were also observed for Arabidopsis 35Sp::NRT1.5 overexpression lines and nrt1.5 mutant plants, respectively. These results indicate that expression of NRT1.5 results in the hyperpolarization of the plasma membrane in yeast and in Arabidopsis. Phenotypical and the elemental analysis with single and the double mutant plants suggest a NO3--dependent job-sharing of NRT1.5 and SKOR in K+ root-to-shoot translocation: NRT1.5 is important when external NO3- amount is limited, whereas SKOR is predominant at high NO3- concentrations. The protein-protein interaction between NRT1.5 and SLAH1, SLAH3 and AHA2 were verified in Nicotiana benthamiana by BiFC. Moreover, the split-ubiquitin assay showed the interaction between NRT1.5 and the CIPK23-CBL1/CBL9 complex in yeast. To investigate the physiological meanings of the interaction, double mutants nrt1.5/slah3 and nrt1.5/aha2 were generated and grown under various NO3- and K+ supply conditions for phenotypical and physiological studies. At last, the expression profiles of several nitrate and potassium transporters in response to various phytohormone treatments were studied. ABA is probably in favor of the retention of nutrients in roots through upregulating nitrate and potassium importers (NRT1.1, HAK5) and inhibiting the expression of transporters for xylem loading (SKOR, NRT1.5). Cytokinin BAP inhibited expression of all tested nitrate transporters and SKOR but stimulated HAK5. SLAH1 expression was strongly impaired by SA treatment. These observations will be helpful to understand the NO3- and K+ transport mediated by phytohormone signals.
