Isolation and characterization of two putative cytokinin oxidase genes related to grain number per spike phenotype in wheat

Cytokinin oxidases are involved in the regulation of plant cytokinin levels, which are important in regulating plant growth and development, and may affect the yield of cereals. Here, we report the isolation and characterization of two putative cytokinin oxidase genes, TaCKX2.1 and TaCKX2.2, from wheat. Both TaCKX2.1 and TaCKX2.2 are mapped to the 0.24-0.55 region of the short arm of wheat chromosome 3D and their coding proteins are most closely related to OsCKX2. Phylogenetic tree analysis reveals that TaCKX2.1 and TaCKX2.2 belong to the clustered clade I of monocot plants. Tissue expression pattern show that both TaCKX2.1 and TaCKX2.2 genes are highly expressed in young spikes and culms of wheat. The detailed spatial expression pattern of TaCKX2.1 were further conducted by in situ hybridization and promoter-fused GUS expression in Arabidopsis experiments. A collection of 12 typical common wheat varieties exhibiting grain number per spike ranging from 31 to 139 were used for the transcription abundance detection of two TaCKX2 genes. A significantly positive correlation between expression level of two TaCKX2 genes and grain number per spike suggests that TaCKX2.1 and TaCKX2.2 on wheat chromosome 3DS may play an important role in wheat spike morphogenesis.