Whole-genome identification and salt- and ABA-induced expression trends of the Nicotiana tabacum CKX gene family

Cytokinin hormones are indispensable for plant growth and development. Cytokinin oxidase/dehydrogenase (CKX) helps regulate the dynamic balance of endogenous cytokinin levels. However, little is known about the CKX genes of Nicotiana tabacum (NtCKXs). In silico analyses were used to isolate, characterize, and elucidate the phylogenetic relationships of 15 NtCKX genes. Multi-species phylogenetic tree construction placed NtCKX1–15 on five of the eight branches of the CKX phylogenetic tree. Protein structure and network analyses revealed that NtCKX genes located on the same phylogenetic branch generally contained several conserved motifs and have highly similar structures, with structural domains related to flavin adenine dinucleotide (FAD) and cytokinin-binding found on all of the predicted NtCKX-encoded proteins. The upstream promotor region of NtCKX genes contained many abiotic stress-responsive cis-acting elements, including DRE, ERE, MBS, MYB, and MYC. Gene expression analysis revealed that each NtCKX gene responded differently to salt stress and exogenous abscisic acid (ABA) treatment. Four NtCKX genes exhibited ABA-induced expression trends with varying peak times. Under salt stress, NtCKX expression was significantly suppressed in two genes and upregulated in five others. In summary, we provided basic information regarding the CKX gene family of N. tabacum and elucidated their gene expression patterns under abiotic stresses, including ABA treatment and salt stress. The findings of this work can serve as a foundation for future study of the functions of NtCKX genes.