Genetic and molecular analysis of tissue-culture-derived Ac elements

Our previous experiments on maize (Zea mays L.) plants regenerated from tissue culture revealed genetic activity characteristic of the transposable element Activator (Ac) in the progeny of 2-3% of the plants tested, despite the lack of Ac activity in the progenitor plants. The objective of the present study was to determine whether the presence of Ac activity in tissue-culture-derived plants was associated with changes in the number or structure of Ac-homologous DNA sequences. Families segregating for Ac activity were obtained by crossing plants heterozygous for Ac activity onto Ac-responsive tester plants. A DNA probe derived from a previously isolated Ac sequence was used to examine the Ac-homologous sequences within individual progeny seedlings of segregating families and noncultured control materials. All plants tested had six or more Ac-homologous DNA sequences, regardless of whether Ac activity was present. In the segregating progeny of one tissue-culturederived plant, a 30-kb Ac-homologous SstI restriction fragment and a 10-kb Ac-homologous BglII restriction fragment were found to cosegregate with Ac activity. We propose that these fragments contained a previously silent Ac sequence that had been activated during tissue culture. Although one or more Ac sequences were often hypomethylated at internal PvuII and HpaII sites in plants with Ac activity, hypomethylation was not a prerequisite for activity. Reduced methylation at these sites may have been a result rather than a cause of Ac activity.