Plant Molecular Biology 51: 643–650, 2003.
© 2003 Kluwer Academic Publishers. Printed in the Netherlands.
Somatic mobility of the maize element Ac and its utility for gene tagging in
and Matthias Fladung
BFH Institute for Forest Genetics and Forest Tree Breeding, Sieker Land Str. 2, D-22927 Grosshansdorf, Germany
author for correspondence; e-mail: firstname.lastname@example.org)
Received 2 April 2001; Accepted in revised form 20 June 2002
Key words: aberrant transposition, Ac transposon, activation tagging, forest tree
We have investigated the somatic activity of the maize Activator (Ac) element in aspen with the objective of
developing an efﬁcient transposon-based system for gene isolation in a model tree species. The analysis of the new
insertion sites revealed the exact reconstitution of the Ac, however, aberrant transposition events were also found.
Characterization of the genomic sequences ﬂanking the Ac insertions showed that about one third (22/75) of the
sequences were signiﬁcantly similar to sequences represented in public databases and might correspond to genes.
The frequency of Ac landing into coding regions was about two-fold higher when compared to the frequency
of T-DNA hitting the predicted genes (5/32) in the aspen genome. Thus, Ac is demonstrated to be a potentially
useful heterologous transposon tag in a tree species. This is the ﬁrst report on transposon-based gene tagging in a
tree species describing the excision and reinsertion of transposable element into new genomic positions. We also
suggest a heterologous transposon tagging strategy that can be used in aspen somatic cells to obtain dominant gain-
of-function mutants and recessive loss-of-function mutants overcoming the regeneration time barrier of a long-lived
Transposable elements and T-DNAs have proven to be
useful genetic tools in the analysis of the genome of
many species. They have been extensively used as an
effective method for insertion mutagenesis and subse-
quent cloning of genes responsible for mutant pheno-
types in many annual plants (Feldmann, 1991; Koncz
et al., 1989; Martienssen, 1998; Springer, 2000). In-
sertion of transposons or T-DNAs in coding regions
or promoters normally results in recessive loss-of-
function mutants by disrupting the gene. In contrast,
dominant gain-of-function mutants can be obtained
with an insertion mutagenesis system called activa-
tion tagging (Kakimoto, 1996; Weigel et al., 2000).
The observation that maize transposable elements re-
main capable of transposition when transformed into
other plant species suggests the possibility of using
these elements as gene tags in plants without necessar-
ily characterizing endogenous elements (Chuck et al.,
1993; Belzile and Yoder, 1992; Whitham et al., 1994;
James et al., 1995; Lawrence et al., 1995; Greco et al.,
2001; Scholz et al., 2001).
Transposon and T-DNA tagging schemes typically
entail mutagenizing a population of plants with a
transposable element followed by screening a self or
test-cross population for the desired insertional mu-
tations. The screening is performed in one or more
generations after the mutagenesis. These strategies
would be exceedingly time consuming and are im-
practical for gene tagging in forest tree species that
have long regeneration cycles. We are, therefore, ex-
ploring an alternative gene tagging strategy based on
our earlier observations that the maize transposable
element Ac is very active during the somatic devel-
opment of transgenic aspen, a model tree system.
Similar to the results obtained in tobacco (Spena et al.,
1989) and tomato (Jones et al., 1992), rolC gene
from Agrobacterium rhizogenes has also been used as
a phenotypically-visible excision marker to identify