A DNA segment with DNA methylation site was detected from rice callus with or without 5-azaC treatment by MSAP. This segment was located on the first exon of gene OsMAPK2 and its 5` non-coding region. Gene OsMAPK2 had a CpG island in the 5` region and was homologous to AtMAPK12. Real-time quantitative PCR and Hpa II-McrBC PCR were conducted to detect the gene expression and DNA methylation of OsMAPK2 in the process of callus formation. The results showed that the DNA methylation was able to control the expression of OsMAPK2. The additon of 2.0 mg/L 2,4-D could induce DNA demethylation in the 5` region and activate the expression of OsMAPK2 gene. However, after long-time stimu-lation (100 h), the gene was methylated again, and the gene expression level was decreased. The trends of DNA methylation and gene expression stimulated by low concentrations of 2,4-D (0.5 and 1.0 mg/L) were similar to 2.0 mg/L 2,4-D, but the expression levels in each time point were low. On the other hand, high concentration of 2,4-D (5.0 mg/L) completed the processes of induction and suppression of the gene in a shorter time.
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