Comparative Expression and Phylogenetic Analysis of Maize Cytokinin Dehydrogenase/Oxidase (CKX) Gene Family

Cytokinin dehydrogenase (CKX) degrades the cytokinin hormone in plants and plays an important role in cytokinin regulatory processes. CKX proteins are encoded by a multigene family with a varying number of members. In this study, 13 maize CKX sequences were collected in which ten transcripts were confirmed by RT-PCR. The tissue- and cytokinin-dependent expression studies indicated that ZmCKX genes exhibit a variety of expression patterns, suggesting diverse functions. Besides 13 maize CKXs, 7 Arabidopsis, 9 poplar, and 11 rice CKX proteins were further used to construct a phylogenetic tree. The CKX members were assigned to six groups, and the intron/exon structures, sequence motifs, and protein properties were conserved within groups. The genome distribution of CKXs supports that segmental duplication contributes to the expansion of the CKX gene family. By quantitative RT-PCR analysis of maize members and digital Northern analysis of Arabidopsis, poplar, and rice members for their tissue expression patterns, highly correlative expression profiles of CKX genes were found among some of the orthologs, whereas different expression manners were found between some of the paralogs. These results suggest functional conservation within each group of the CKX family and provide a clue for transfer of a gene function from one species to the other and further contribute to uncovering the role of CKX genes in planta.