Trojan horse strategy in Agrobacterium transformation: abusing MAPK defense signaling.

Nuclear import of transfer DNA (T-DNA) is a central event in Agrobacterium transformation of plant cells and is thought to occur by the hijacking of certain host cell proteins. The T-DNA-associated virulence protein VirE2 mediates this process by binding to the nuclear import machinery via the host cell factor VIP1, whose role in plants has been so far unknown. Here we show that VIP1 is a transcription factor that is a direct target of the Agrobacterium-induced mitogen-activated protein kinase (MAPK) MPK3. Upon phosphorylation by MPK3, VIP1 relocalizes from the cytoplasm to the nucleus and regulates the expression of the PR1 pathogenesis-related gene. MAPK-dependent phosphorylation of VIP1 is necessary for VIP1-mediated Agrobacterium T-DNA transfer, indicating that Agrobacterium abuses the MAPK-targeted VIP1 defense signaling pathway for nuclear delivery of the T-DNA complex as a Trojan horse.