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.

Grafted plant is a chimeric organism formed by the connection of scion and rootstock through stems, so stem growth and development become one of the important factors to affect grafted plant state. However, information regarding the molecular responses of stems secondary growth after grafting is limited. A grafted Rosa plant, with R. rugosa ‘Rosea’ as the scion (Rr_scion) grafted onto R. multiflora ‘Innermis’ as the stock (Rm_stock), has been shown to significantly improve stem thickness. To elucidate the molecular mechanisms of stem secondary growth in grafted plant, a genome-wide transcription analysis was performed using an RNA sequence (RNA-seq) method between the scion and rootstock. Comparing ungrafted R. rugosa ‘Rosea’ (Rr) and R. multiflora ‘Innermis’ (Rm) plants, there were much more differentially expressed genes (DEGs) identified in Rr_scion (6887) than Rm_stock (229). Functional annotations revealed that DEGs in Rr_scion are involved in two Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways: the phenylpropanoid biosynthesis metabolism and plant hormone signal transduction, whereas DEGs in Rm_stock were associated with starch and sucrose metabolism pathway. Moreover, different kinds of signal transduction-related DEGs, e.g., receptor-like serine/threonine protein kinases (RLKs), transcription factor (TF), and transporters, were identified and could affect the stem secondary growth of both the scion and rootstock. This work provided new information regarding the underlying molecular mechanism between scion and rootstock after grafting.