Gene interactions and epigenetic variation in transgenic plants

Abstract

Unusual gene interactions were observed in several doubly transformed tobacco plants which were obtained following sequential transformation steps using two T‐DNAs encoding different selection and screening markers. The expression of T‐DNA‐I, which encoded kanamycin resistance (Kan^r) and nopaline synthase (NOS), was suppressed in some, but not all, of the double transformants after the introduction of T‐DNA‐II, which encoded hygromycin resistance (Hyg^r) and octopine synthase (OCS). Double transformants in which T‐DNA‐I had been inactivated could produce Kan^rNOS⁺ progeny, but these were shown to lack T‐DNA‐II, thus establishing the role of this T‐DNA in the suppression of T‐DNA‐I. Reversible cytosine methylation of the promoters of T‐DNA‐I genes was shown to correlate with their activation/inactivation cycle. In this paper we pursue further the questions of the mechanism of suppression of T‐DNA‐I genes by T‐DNA‐II, and also the timing and extent of demethylation of T‐DNA‐I promoters in Kan^r progeny following the loss of T‐DNA‐II. We propose that the suppression is due to the competition between homologous regions on each T‐DNA for binding to nuclear sites with fixed locations. We further suggest that incomplete demethylation patterns of T‐DNA‐I promoters in Kan^r progeny reflect the existence in the shoot apex meristem of two cell populations, which have either methylated or unmethylated T‐DNA‐I promoters, respectively. Thus, Kan^r progeny are epigenetic chimeras with respect to the expression of T‐DNA‐I genes.