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.

The xyloglucan endotransglycosylase/hydrolase (XTH) gene family plays a crucial role in plant cell wall remodeling, facilitating growth and structural changes. However, the divergence of paralogous genes among different species of Populus remains inadequately understood. This study investigates the phylogenetic relationships and expression characteristics of XTH genes in two Populus species: Populus trichocarpa and Populus alba × P. glandulosa '84K'. Forty-one XTHs were identified in P. trichocarpa and 38 and 33 members in the subgenome A and G of '84K' poplar, respectively. Gene expression analysis demonstrated differences among paralogous genes within the same subgenome and between orthologous genes across species, likely influenced by variations in promoter regions. Notably, XTH12 showed a specific response to drought stress among various abiotic stresses. In a population of 549 Populus individuals, functional SNPs in XTH12's coding region did not affect its conserved ExDxE catalytic site, highlighting its irreplaceable function. Furthermore, validation through qRT-PCR and ProPagXTH12::GUS activity, alongside PagXTH12-overexpression poplar lines, substantiated the role of PagXTH12 in modulating the balance between plant biomass and drought resistance. Overall, this research provides valuable insights into the biological functions of XTHs in plant environmental adaptability and offers strategies for targeted regulation of tree growth and stress resistance.

Background/Objectives: Cytokinin and auxin are essential for vascular development in plants. This study aims to explore whether these two hormones exhibit crosstalk in the cambium, analogous to that observed in the apical meristem. Methods: Using the hybrid poplar (Populus alba × P. glandulosa clone ‘84K’), we integrated gravitropic induction with transcriptomic analysis and identified the cytokinin biosynthesis gene PagIPT5 as differentially expressed in a tension wood induction system. PagIPT5 overexpression lines were generated and assessed for growth-related phenotypes. The interaction between cytokinin and auxin was investigated via anatomical observation, cell proliferation assays, in situ PCR, and immunofluorescence detection of auxin and cytokinin. Results: Compared with the wild type, PagIPT5 overexpression lines showed growth inhibition and an auxin-deficient phenotype. High PagIPT5 expression in the vascular cambium elevated cytokinin levels while reducing auxin levels, leading to decreased cambial cell proliferation and suppressed xylem development. However, in the tension wood induction system, both auxin and cytokinin levels increased in the vascular cambium of tension wood relative to opposite wood. Treatment with a superoxide anion activator promotes the accumulation of both auxin and cytokinin in 84K plants. Conclusions: These results revealed an antagonistic interaction between the two hormones in the cambium zone. However, this antagonism is attenuated in tension wood, which may be induced by the accumulation of superoxide anion in tension wood.