Widespread occurrence of natural genetic transformation of plants by Agrobacterium

Naturally transgenic plant species occur on an unexpectedly large scale. Agrobacterium-mediated gene transfer leads to the formation of crown galls or hairy roots, due to expression of transferred T-DNA genes. Spontaneous regeneration of transformed cells can produce natural transformants carrying cellular T-DNA (cT-DNA) sequences of bacterial origin. This particular type of horizontal gene transfer (HGT) could play a role in plant evolution. However, the material available today is not enough for generalizations concerning the role of Agrobacterium in HGT from bacteria to plants. In this study, we searched for T-DNA-like genes in the sequenced genomes of dicots and monocots. We demonstrate the presence of cT-DNAs in 23 out of 275 dicot species, within genera Eutrema, Arachis, Nissolia, Quillaja, Euphorbia, Parasponia, Trema, Humulus, Psidium, Eugenia, Juglans, Azadirachta, Silene, Dianthus, Vaccinium, Camellia, and Cuscuta. Analysis of transcriptome data of 356 dicot species yielded 16 additional naturally transgenic species. Thus, HGT from Agrobacterium to dicots is remarkably widespread. Opine synthesis genes are most frequent, followed by plast genes. Species in the genera Parasponia, Trema, Camellia, Azadirachta, Quillaja, and Diospyros contain a combination of plast and opine genes. Some are intact and expressed, but the majority have internal stop codons. Among the sequenced monocot species, Dioscorea alata (greater yam) and Musa acuminata (banana) also contain T-DNA-like sequences. The identified examples are valuable material for future research on the role of Agrobacterium-derived genes in plant evolution, for investigations on Agrobacterium strain diversity, and for studies on the function and evolution of cT-DNA genes in natural transformants.