Functional Characterization of a New Salt Stress Response Gene, PeCBL4, in Populus euphratica Oliv

Populus euphratica is a typical stress-resistant tree species that provides valuable natural genetic resources for breeding salt-tolerant plants. The calcineurin B-like (CBL)-interacting protein kinase (CIPK) network plays an important role in regulating plant responses to abiotic stresses. The aim of this study was to characterize the function of a new CBL member, PeCBL4, in response to abiotic stresses. PeCBL4 was cloned, and sequence analysis was performed. The subcellular localization of PeCBL4 was determined using the fusion expression vector of GFP. Yeast two-hybrid assays and bimolecular fluorescence complementation were performed to identify PeCIPK members that interacted with PeCBL4. PeCBL4 was then transformed into the corresponding Arabidopsis thaliana mutants. Na+ and K+ content as well as their net fluxes were determined under high salt stress and low K+ stress. Phylogenetic tree analysis showed that PeCBL4 was clustered together with PtCBL4 and belonged to the same subgroup as AtCBL4. Subcellular localization indicated that PeCBL4 was expressed on the plasma membrane. Yeast two-hybrid assays and bimolecular fluorescence complementation showed that PeCBL4 interacted with PeCIPK24 and PeCIPK26. In addition, under high salt stress, the Na+ efflux capacities of seedlings decreased in sos3 mutants, and transgenic plants of PeCBL4 enhanced efflux capacities. In addition, the overexpression of PeCBL4 negatively influenced the influx capacity of K+. PeCBL4 interacts with PeCIPK24 and PeCIPK26 and regulates Na+/K+ balance under low K+ and high salt stress.