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Agrobacterium rhizogenes and A. tumefaciens transfer DNA and effector proteins into plant cells, where transferred DNA (T-DNA) is inherited and expressed. Most transgenic plants are created using A. tumefaciens, but transformation by A. rhizogenes yields desirable single-copy transgenes more frequently than A. tumefaciens does. DNA transfer from both species resembles plasmid conjugation, but later events differ between species. Efficient transformation by A. tumefaciens requires single-strand DNA-binding protein VirE2, which A. rhizogenes lacks, so substrates for T-DNA integration differ greatly. In A. rhizogenes, the GALLS proteins substitute for (but do not resemble) VirE2. GALLS proteins occur in two forms: full-length (FL) and a more abundant C-terminal domain (CT). Both have protein-binding domains and type IV secretion signals. GALLS-FL has ATPase/strand transferase and nuclear localization (NLS) domains, allowing it to enter the nucleus and bind VirD2, a pilot protein attached to single-stranded T-DNA (T-strands). GALLS-FL ATPase may pull T-strands into the nucleus. GALLS-CT stimulates an early step in gene transfer to plants; this effector protein alters host gene expression and stimulates T-DNA transfer, apparently by suppressing host defenses. These observations challenge the assumption that A. rhizogenes and A. tumefaciens transform plants and mitigate host defenses by the same mechanisms. |
