Characterization of Alkali Stress-Responsive Genes of the CIPK Family in Sweet Sorghum [Sorghum bicolor (L.) Moench]

Calcineurin B-like protein (CBL) interacting protein kinases (CIPKs) are critical components in various stress signal transduction pathways. In previous studies, 32 putative CIPK genes (SbCIPKs) were identified in sweet sorghum [Sorghum bicolor (L.) Moench]. In the present study, a conserved sucrose nonfermenting 1 (SNF1)-like kinase domain in the N terminus and a NAF motif (a 24-amino-acid domain with the conserved amino acids N, A, and F required for the CBL-CIPK interaction) in C terminus were identified in all SbCIPKs using sequence analysis. Stress-and light-responsive cis-elements were also identified in the promoter regions of sorghum CIPKs. The real-time polymerase chain reaction analysis revealed different expression patterns among the 32 SbCIPKs under sodium carbonate (Na2CO3) stress in sweet sorghum seedlings. Based on the expression patterns, six SbCIPKs were upregulated >10-fold at the transcriptional level. Interestingly, these six CIPKs showed leaf-specific expression under normal growth conditions, suggesting a relationship between various regulations in leaf function and alkali stress responses in sweet sorghum. Systemic yeast two-hybrid assay demonstrated the specific interaction between multiple SbCIPKs and SbCBLs. Thus, this study provides the first characterization of the role of the CIPK family proteins in response to alkali stress.