Genome-Wide Identification of the CBL-CIPK Gene Family in the Ice Plant and Functional Analysis of Salt Stress Tolerance

<b>Background</b>: The ice plant (<i>Mesembryanthemum crystallinum</i> L.) is a typical halophyte with remarkable stress resistance traits, including salinity and alkalinity tolerance. As a crucial signaling transduction pathway for plant responses to environmental stress, the CBL-CIPK signaling system plays a key role in regulating plant stress resistance. <b>Methods</b>: This study systematically analyzed the composition characteristics of the CBL and CIPK gene families across 24 plant species, including the ice plant, using comparative genomics approaches. <b>Results</b>: A total of 297 CBL and 561 CIPK gene family members were identified across the 24 species. Within the ice plant genome, 9 CBL and 18 CIPK genes were identified. Compared to model plants like <i>Arabidopsis thaliana</i>, the ice plant possesses a relatively higher number of CIPK genes, which may be related to its specific adaptation to saline-alkaline environments. Phylogenetic analysis indicated that the ice plant CBL and CIPK genes could be classified into three and four subfamilies, respectively. Expression analysis revealed that several genes (e.g., <i>McCBL1</i>, <i>McCBL4</i>, <i>McCIPK1</i>, <i>McCIPK2</i>) were significantly upregulated under salt stress, suggesting their important roles in the salt stress response. Notably, ice plant CBL and CIPK genes exhibit significant structural diversity. For instance, <i>McCBL3</i> contains significantly more CDS regions than other members, while CIPK genes can be divided into two types: single-CDS type and multi-CDS type. This structural variation may be associated with functional divergence during the evolution of the gene family. Furthermore, three-dimensional (3D) structure prediction showed that CBL proteins primarily consist of EF-hand domains and α-helices, whereas CIPK proteins additionally contain β-sheet domains, implying that this structural difference may be related to their distinct regulatory mechanisms. <b>Conclusions</b>: This study provides an important theoretical basis for a deeper understanding of the molecular mechanisms underlying the CBL-CIPK signaling pathway in the saline-alkaline stress response of the ice plant.