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Abstract
Agrobacterium species transfer DNA (T−DNA) to plant cells where it may integrate into plant chromosomes. The process of integration is thought to involve invasion and ligation of T-DNA, or its copying, into nicks or breaks in the host genome. Integrated T−DNA often contains, at its junctions with plant DNA, deletions of T−DNA or plant DNA, filler DNA, and/or microhomology between T-DNA and plant DNA pre-integration sites. T−DNA integration is also often associated with major plant genome rearrangements, including inversions and translocations. These characteristics are similar to those often found after repair of DNA breaks, and thus DNA repair mechanisms have frequently been invoked to explain the mechanism of T−DNA integration. However, the involvement of specific plant DNA repair proteins and Agrobacterium proteins in integration remains controversial, with numerous contradictory results reported in the literature. In this review I discuss this literature and comment on many of these studies. I conclude that either multiple known DNA repair pathways can be used for integration, or that some yet unknown pathway must exist to facilitate T−DNA integration into the plant genome.
Highlights
The process of Agrobacterium T-DNA integration into the genomes of infected plants has important implications both for understanding plant DNA break repair processes and for the use of Agrobacterium as a tool for manipulating the plant genome
I summarize and analyze some of these conclusions, and relate what we know about plant DNA repair processes to possible mechanisms of T-DNA integration
During T-DNA border nicking, VirD2 covalently links to the 5 end of T-DNA, resulting in a single-strand form of T-DNA, the T-strand, that on its 3 end contains nucleotides 4–25 of the left border (LB), and on its 5 end contains nucleotides 1–3 of the right border (RB) [18,19,20,21,22]
Summary
The process of Agrobacterium T-DNA integration into the genomes of infected plants has important implications both for understanding plant DNA break repair processes and for the use of Agrobacterium as a tool for manipulating the plant genome. Scientists have studied T-DNA integration for decades, we still have a very incomplete picture of how integration occurs, and numerous contradictory conclusions abound in the literature In this short article, I summarize and analyze some of these conclusions, and relate what we know about plant DNA repair processes to possible mechanisms of T-DNA integration. Stable transformation of plants using this VirD2 ω mutant was decreased, the precision of the integration of sequences near the RB was similar to that observed when using wild-type VirD2 [29]. The VirD2 ω mutant Agrobacterium strain identified by Shurvinton et al [27] showed moderately lower transient transformation activity but greatly reduced stable transformation, two other ω domain mutants (a precise deletion of the DGRGG ω amino acids, or their replacement with five glycine residues) resulted in both greatly decreased transient and stable transformation frequency [31]. The different relative transformation activities conferred by the various VirD2 ω mutants may result from altered protein structure conferred by the serine residue substitutions
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