Soil salinity adversely affects the growth and yield of crops, including cucumber, one of the most important vegetables in the world. Grafting with salt-tolerant pumpkin as the rootstock effectively improves the growth of cucumber under different salt conditions by limiting Na⁺ transport from the pumpkin rootstock to the cucumber scion. High-affinity potassium transporters (HKTs) are crucial for the long distance transport of Na⁺ in plants, but the function of pumpkin HKTs in this process of grafted cucumber plants remains unclear. In this work, we have characterized CmHKT1;1 as a member of the HKT gene family in Cucurbita moschata and observed an obvious upregulation of CmHKT1;1 in roots under NaCl stress conditions. Heterologous expression analyses in yeast mutants indicated that CmHKT1;1 is a Na⁺-selective transporter. The transient expression in tobacco epidermal cells and in situ hybridization showed CmHKT1;1 localization at plasma membrane, and preferential expression in root stele. Moreover, ectopic expression of CmHKT1;1 in cucumber decreased the Na⁺ accumulation in the plants shoots. Finally, the CmHKT1;1 transgenic line as the rootstock decreased the Na⁺ content in the wild type shoots. These findings suggest that CmHKT1;1 plays a key role in the salt tolerance of grafted cucumber by limiting Na⁺ transport from the rootstock to the scion and can further be useful for engineering salt tolerance in cucurbit crops.
Keywords: grafted cucumber; high-affinity potassium transporter (HKT); pumpkin; salt tolerance; sodium transport.