The transposition frequency of Tag1 elements is increased in transgenic Arabidopsis lines

Tag1 was identified as a highly active endogenous transposable element in transgenic Arabidopsis thaliana Landsberg erecta plants carrying the maize transposable element Activator (Ac). Here, we describe experiments designed to determine the basis for the high activity of Tag1. The frequency of transposition of Tag1 elements was compared in lines containing or lacking Ac transposase to assess the effect of Ac transposase on Tag1 activity. Three populations of nontransgenic plants, including nontransformed regenerants, were also analyzed. The high level of activity of Tag1 did not correlate with the presence or absence of Ac transposase but was significantly higher in transgenic lines. This result was maintained through at least six generations after transformation. These data suggest that Tag1 transposition is stimulated by processes that occur during the Agrobacterium transformation and that thereafter remain active. Two Tag1 elements are tightly linked in the Landsberg erecta genome and map to the lower arm of chromosome 1. Tag1 elements were found in only a few A. thaliana ecotypes but were present in four other Arabidopsis species.