Cytokinin regulates the expression of a soybean β-expansin gene by a post-transcriptional mechanism

The cytokinin-inducible soybean mRNA Cim1 accumulates 20-60-fold upon cytokinin addition to cytokinin-starved soybean suspension cultures. In this report, we demonstrate that cytokinin-induced stability of the Cim1 mRNA plays an important role in the accumulation of the message. We also present evidence that cytokinin-induced Cim1 stability is blocked by the addition of the protein phosphatase inhibitor okadaic acid. Thus, we suggest that protein phosphatase activity is required for the cytokinin-induced stability and subsequent accumulation of Cim1 in soybean cells. The deduced amino acid sequence of the Cim1 protein product is similar to the group I pollen allergens from various plants, which constitute a subfamily of expansin proteins. The relatedness between Cim1 and the expansins supports our hypothesis that the protein product of Cim1 is localized to the cell wall and suggests a role for Cim1 in cytokinin-regulated cell wall expansion. Thus, post-transcriptional regulation of Cim1 by cytokinin may represent a molecular link between cytokinin and changes in cell shape and size.