Article snapshot taken from Wikipedia with creative commons attribution-sharealike license.
Give it a read and then ask your questions in the chat.
We can research this topic together.
VirA is a protein histidine kinase which senses certain sugars and phenolic compounds. These compounds are typically found from wounded plants, and as a result VirA is used by Agrobacterium tumefaciens to locate potential host organisms for infection. This detection is the first stage in the Ti plasmid transfer.
References
Agrobacterium and plant genes involved in T-DNA transfer and integration - Annu. Rev. Plant Physiol. Plant Mol. Biol. 2000. 51:223–56