Ancient horizontal gene transfer from Rhizobium rhizogenes to European genera of the Figwort family (Scrophulariaceae)

Rhizobium rhizogenes exploits horizontal gene transfer as part of its mechanism of pathogenesis. In this respect, genetic material, transfer DNA (T-DNA) from the bacterium, is transferred transiently to the host plant genome. However, plant species within Nicotiana, Linaria and Ipomea genera contain genomic traces of ancient bacterial T-DNA. To determine if ancient bacterial T-DNA is present in uncultivated plants of European genera within Scrophulariaceae, seeds of Linaria, Antirrhinum, Digitalis, and Veronica were analysed for the presence of one of the root oncogenic loci genes; rolC of R. rhizogenes. This study discloses remnants of ancient rolC haplotypes in several species of Linaria, Antirrhinum, Digitalis and Veronica. The distribution of plant species harbouring rolC sequences within each genus was not uniform. In total, 7 of the 16 investigated species were found to be positive for at least one of the rolC haplotypes, where 6 of these are new additions to the group of naturally transformed plants. Sequence alignment showed high interspecies homology of rolC. Five unique rolC haplotypes (rolCa-rolCe) were found in several plant species. The most abundant, rolCa, was identified in all the plant species holding ancient bacterial DNA. Transcripts of rolC were not detected in leaves, which indicates that rolC may not have a function in leaves under non-stress conditions. This study adds Antirrhinum, Digitalis and Veronica to the list of plant genera within Scrophulariaceae which have been subjected to ancient transformation events via horizontal gene transfer from bacteria to plants.