Promotive effect of brassinosteroids on cell division involves a distinct CycD3-induction pathway in Arabidopsis

Brassinosteroids (BRs) are steroid hormones that play an essential role in plant growth and development. However, the contradictory results of previou s studies make their role in cell division unclear. Using a cDNA array, we identified genes that respond to BR in the det2 suspension culture of Arabi dopsis, and found that epi-brassinolide upregulated transcription of the Cy cD3, a D-type plant cyclin gene through which cytokinin activates cell divi sion. RNA gel-blot analysis and cell culturing showed that epi-brassinolide may promote cell division through CycD3, and can substitute cytokinin in c ulturing of Arabidopsis callus and suspension cells. The CycD3 induction by epi-brassinolide was further shown to involve de novo protein synthesis, b ut no protein phosphorylation or dephosphorylation. Induction was also foun d to occur in cells of a BR-insensitive mutant, bri1, suggesting that BR in duces CycD3 transcription through a previously unknown signal pathway in pl ants.