Posttranscriptional Regulation of the Sesbania rostrata Early Nodulin Gene SrEnod2 by Cytokinin

The mRNA from the Sesbania rostrata early nodulin gene SrEnod2 accumulates in response to cytokinin application. Nuclear run-on assays using isolated root nuclei have shown that this accumulation occurs posttranscriptionally, and northern blot analysis of nuclear and total RNA levels revealed that it occurs primarily in the cytoplasm and not in the nucleus. After cytokinin enhancement of SrEnod2 mRNA accumulation and the subsequent removal of cytokinin, the levels of SrEnod2 mRNA did not return to basal levels, but oscillated over a 36-h time course. Application of the translational inhibitor cycloheximide was found to inhibit the enhancement of SrEnod2 mRNA accumulation by cytokinin and to cause its rapid decay. Okadaic acid and staurosporine, inhibitors of protein phosphatases and kinases, respectively, also inhibited cytokinin enhancement of SrEnod2 mRNA accumulation. In addition, okadaic acid was found to cause a decrease in SrEnod2 mRNA levels. These results provide evidence for a posttranscriptional mechanism of cytokinin enhancement of SrEnod2 mRNA accumulation, which appears to require concurrent protein synthesis, to involve protein phosphatases and kinases, and to occur primarily in the cytoplasm of the plant cell.