Low frequency of T‐DNA based activation tagging in Arabidopsis is correlated with methylation of CaMV 35S enhancer sequences

Abstract
A powerful system to create gain‐of‐function mutants in plants is activation tagging using T‐DNA based vehicles to introduce transcriptional enhancer sequences. Large Arabidopsis populations of individual plants carrying a quadruple cauliflower mosaic virus (CaMV) 35S enhancer are frequently used for mutant screenings, however the frequency of morphological mutants remains very low. To clarify this low frequency we analyzed a subset of lines generated by this method. The correlation between the number of T‐DNA insertion sites, the methylation status of the 35S enhancer sequence and 35S enhancer activity was determined. All plants containing more than a single T‐DNA insertion showed methylation of the 35S enhancer and revealed a dramatic decrease in 35S enhancer activity. The results support the notion that in a large proportion of the T‐DNA based activation tagged lines the 35S transcriptional enhancer is silenced due to methylation, which is induced by multiple T‐DNA integrations.