Virulence genes (vir genes)
Virulence genes aid in the transfer of T-DNA into the host plant cell. Ti plasmid contains 35 vir genes arranged in 8 operons.

5-4.3. DNA transfer into the plant genome
The schematic representation of Agrobacterium-mediated transformation method is shown in figure 5-4.3. T-DNA transfer and integration into the plant genome involves following steps-

Recognition and vir gene induction

• Agrobacterium perceives signaling molecules (phenolic compounds and sugars) released by the wounded plant cells. These signaling molecules act as chemotactic attractants for Agrobacterium. This is followed by recognition by the bacterial VirA/VirG2-component signal transduction system.
  
  
• Vir A, a kinase present in the inner bacterial membrane senses these phenolics (e.g. acetosyringone), gets autophosphorylated and then activates Vir G. VirG protein is a transcriptional activator of other vir genes and is inactive in non-phosphorylated form. The activation of Vir G thus induces the expression of the other vir genes.

Formation of T-DNA complex
Vir D1/D2 border-specific endonucleases recognize the left and right borders of T-DNA. Vir D2 induces single stranded nicks in Ti plasmid causing the release of the ss-T DNA. Vir D2 then attaches to the 5'-end of the displaced ss-T DNA forming an immature T-complex.

Transfer of T-DNA and integration into the plant cell

• The transfer of T-DNA to the plant cell is mediated by Type-IV secretion system composed of proteins encoded by Vir B and Vir D4that form a conjugative pilus (T-pilus). VirD4 serves as a “linker” that helps in the interaction of the processed T-DNA/VirD2 complex with the VirB- encoded pilus. Other vir genes (Vir E2, Vir E3, VirF, Vir D5) also pass through this T-pilus to aid in the assembly of T-DNA/vir protein complex in the plant cytoplasm forming a mature T-complex.