Pathways of DNA Transfer to Plants from Agrobacterium tumefaciens and Related Bacterial Species

Abstract

Genetic transformation of host plants by Agrobacterium tumefaciens and related species represents a unique model for natural horizontal gene transfer. Almost five decades of studying the molecular interactions between Agrobacterium and its host cells have yielded countless fundamental insights into bacterial and plant biology, even though several steps of the DNA transfer process remain poorly understood. Agrobacterium spp. may utilize different pathways for transferring DNA, which likely reflects the very wide host range of Agrobacterium. Furthermore, closely related bacterial species, such as rhizobia, are able to transfer DNA to host plant cells when they are provided with Agrobacterium DNA transfer machinery and T-DNA. Homologs of Agrobacterium virulence genes are found in many bacterial genomes, but only one non-Agrobacterium bacterial strain, Rhizobium etli CFN42, harbors a complete set of virulence genes and can mediate plant genetic transformation when carrying a T-DNA-containing plasmid.

Keywords: bacterium–plant interactions; horizontal gene transfer; macromolecular transport.

Figure 1

Schematic representation of the main steps of T-DNA transfer from Agrobacterium to the plant cell genome. Circled numbers represent the major steps in the pathway. ❶ Plant-derived and environmental signals activate the bacterial virulence system, resulting in the induction of vir (virulence) gene expression and the generation of the single-stranded T-DNA. ❷ The T-DNA covalently attaches to VirD2, and several vir-encoded effector proteins (VirD5, VirE2, VirE3, and VirF) are exported out of the bacterial cell via the VirB/VirD4 T4SS. ❸ T-DNA and effector proteins enter in the plant cell and are targeted into the nucleus. ❹ The T-DNA is processed in the nucleus and integrated into the plant cell chromosomal DNA. Abbreviations: AS, acetosyringone; Chv, chromosomal virulence protein; DSB, double-strand break; dsT-DNA, double-stranded transferred DNA; Exo, exocellular; ssT-DNA, single-stranded transferred DNA; T-DNA, transferred DNA; T4SS, type IV secretion system; Ti plasmid, tumor-inducing plasmid; vir, virulence gene region; Vir, virulence protein; VBP, VirD2 binding protein; VIP1, VirE2 interacting protein 1; VIP2, VirE2 interacting protein 2.