T-DNA and transposon tagging in aspen.

: We have investigated the somatic activity of the maize Activator (Ac) element in haploid and diploid aspen with the objective of developing an efficient transposon-based system for gene isolation in the model tree species Populus. It was shown that Ac is reinserted, frequently into or near coding regions in aspen, and therefore can be used for gene tagging studies. A number of phenotypic variants were also found following transformation of constructs harbouring the rolC gene. Comparative analyses of T-DNA flanking regions of variants and wild type lines indicate that T-DNA insertion has occurred in or near coding regions. However, the frequency of T-DNA insertion into genes is about one half of the frequency of Ac insertion hitting coding sequences. The results obtained give a proof-of-concept for transposon tagging in a tree system. Given the long generation cycles in tree species, gene tagging strategies are practical only to obtain dominant gain-of-function mutants that do not require selfing or test crossing. In order to obtain recessive loss-of-function mutants, we have regenerated haploid lines from immature pollen. These lines were successfully transformed with a construct containing the rolC transgene from Agrobacterium rhizogenes and Ac element from maize. The results indicate that Ac is also active in haploid aspen and hence can be used in general for gene tagging in trees.