Somatic Excision of the Ac Transposable Element in Transgenic Arabidopsis thaliana after 5-Azacytidine Treatment

We have introduced the maize Ac transposable element in Arabidopsisthaliana and found that after three selfing generations, theelement is immobile and extensively methylated. Moreover, thenopaline synthase (nos) gene present on the same transferredT-DNA, was active early after transformation and regeneration,but inactive in most of the S1 progeny. We used 5-azacytidine(5AzaC) to determine whether a reduction in the methylationwould affect both Ac transposition and expression of the nosgene. After treatment with 5AzaC doses from 0.3 mM to 1.0 mM,approximately 25% of the plants produced detectable amountsof nopaline, indicating that the nos gene was reactivated. Usingthe polymerase chain reaction (PCR) to detect the empty donorsite left by Ac transposition, we demonstrated that 5AzaC alsoactivates Ac excision in the transgenic plants. Approximately13% of the 5AzaC treated plants (doses from 0.1 mM to 1.0 mM)were shown to have empty donor sites due to Ac excision. Noneof the plants cultivated in the absence of 5AzaC showed evidencefor Ac transposition or reactivation of the nos gene. Furtheranalysis using Southern blot indicate that some de-methylationocurred in the genome of individual plants. These results mayrepresent demethylation in few cells during development whichmay be sufficient to reactivate in these cells the expressionof the nos and Ac transposase transgenes, the latter promotingAc transposition in somatic cells. (Received July 16, 1996; Accepted January 8, 1997)