Agrobacterium virulence factors induce the expression of host DNA repair-related genes without promoting major genomic damage

This study aimed to investigate whether the plant DNA damage levels and DNA damage response (DDR) are regulated during Agrobacterium infection and potentially manipulated by Agrobacterium to facilitate T-DNA integration. We investigated the plant genomic response to Agrobacterium infection by measuring gamma H2AX levels, which reflect the levels of double-strand DNA breaks (DSBs), and by characterizing transcription of three major DNA repair marker genes NAC82, KU70, and AGO2. These experiments revealed that, globally, Agrobacterium infection did not result in a major increase in DSB content in the host genome. The transcription of the DNA damage repair genes, on the other hand, was elevated upon the wild-type Agrobacterium infection. This transcriptional outcome was largely negated by a mutation in the bacterial virB5 gene which encodes the virulence (Vir) protein B5, a minor component of Agrobacterium pilus necessary for the translocation of Vir effector proteins into the host cell, suggesting that the transcriptional activation of the cellular DNA damage repair machinery requires the transport into the host cell of the Agrobacterium effectors, i.e., the VirD2, VirD5, VirE2, VirE3, and VirF proteins. Most likely, a combination of several of these Vir effectors is required to activate the host DNA repair as their individual loss- or gain-of-function mutants did not significantly affect this process.