Comparative analysis of cytokinin response factors in Brassica diploids and amphidiploids and insights into the evolution of Brassica species.

Background: Cytokinin is a classical phytohormone that plays important roles in numerous plant growth and development processes. In plants, cytokinin signals are transduced by a two-component system, which involves many genes, including cytokinin response factors (CRFs). Although CRFs take vital part in the growth of Arabidopsis thaliana and Solanum lycopersicum, little information of the CRFs in the Brassica U-triangle species has been known yet.

Results: We identified and compared 141 CRFs in the diploids and amphidiploids of Brassica species, including B. rapa, B. oleracea, B. nigra, B. napus, and B. juncea. For all the 141 CRFs, the sequence and structure analysis, physiological and biochemical characteristics analysis were performed. Meanwhile, the Ka/Ks ratios of orthologous and paralogous gene pairs were calculated, which indicated the natural selective pressure upon the overall length or a certain part of the CRFs. The expression profiles of CRFs in different tissues and under various stresses were analyzed in B. oleracea, B. nigra, and B. napus. The similarities and differences in gene sequences and expression profiles among the homologous genes of these species were discussed. In addition, AtCRF11 and its ortholog BrCRF11a were identified to be related to primary root growth in Arabidopsis.

Conclusion: This study performed a genome-wide comparative analysis of the CRFs in the diploids and amphidiploids of the Brassica U-triangle species. Many similarities and differences in gene sequences and expression profiles existed among the CRF homologous genes of these species. In the bioinformatics analysis, we found the close relativity of the CRF homologous genes in the Brassica A and C genomes and the distinctiveness of those in the B genome, and the CRF homologous genes in B subgenome were considerably influenced by the A subgenome of B. juncea. In addition, we identified a new function of the Clade V CRFs related to root growth, which also clarified the functional conservation between Arabidopsis and B. rapa. These results not only offer useful information on the functional analysis of CRFs but also provide new insights into the evolution of Brassica species.