Trojan Horse Strategy in Agrobacterium Transformation: Abusing MAPK Defense Signaling

Abstract

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.

The Arabidopsis G-Protein β-Subunit Is Required for Defense Response againstAgrobacterium tumefaciens

“…32) MPK3-dependent phophorylation of VIP1 is necessary for nuclear delivery of the T-DNA complex, which leads to enhanced A. tumefaciens transformation. 32) Since EFR-AGB1 signaling tends to limit A. tumefaciens transformation, there might be an antagonistic interaction between MPK3-VIP1 signaling and EFR-AGB1 signaling. However, we cannot rule out the possibility that AGB1 is involved in EFR signaling indirectly.…”

Interaction of the Agrobacterium tumefaciens virulence protein VirD2 with histones

“…Interactions between virulence proteins associated with the Tcomplex and chromatin-associated proteins may play a role in T-DNA targeting. For example, VIP1, a plantencoded transcription factor, interacts with VirE2 outside the nucleus and helps to target the complex into the nucleus (Tzfira et al, 2001;Djamei et al, 2007). VIP1 was also shown to interact with histone proteins, suggesting that VIP1 may also serve to target the T-DNA to the plant chromatin as an early step in T-DNA integration (Loyter et al, 2005).…”

The Role of the Ubiquitin-Proteasome System in Agrobacterium tumefaciens-Mediated Genetic Transformation of Plants

“…A fascinating hypothesis is that this F-box protein might also be necessary to subvert the host defense system against A. tumefaciens. A recent study has demonstrated that, upon A. tumefaciens infection, VIP1 becomes phosphorylated via the mitogenactivated protein kinase defense signaling (Djamei et al, 2007). The phosphorylated form of VIP1 then enters the cell nucleus and serves as a transcription factor that activates the expression of stress-dependent genes, including PATHOGENESIS-RELATED GENE1 (PR1; Djamei et al, 2007;Pitzschke et al, 2009).…”

“…A recent study has demonstrated that, upon A. tumefaciens infection, VIP1 becomes phosphorylated via the mitogenactivated protein kinase defense signaling (Djamei et al, 2007). The phosphorylated form of VIP1 then enters the cell nucleus and serves as a transcription factor that activates the expression of stress-dependent genes, including PATHOGENESIS-RELATED GENE1 (PR1; Djamei et al, 2007;Pitzschke et al, 2009). These observations suggest that the role of VirF is not restricted to the T-complex uncoating but, rather, that this bacterial F-box effector also acts to down-regulate the VIP1-mediated host defense responses through the degradation of VIP1.…”

“…Among the exported proteins, VirE2 serves to package the mobile single-stranded T-DNA molecule, covalently associated with a single molecule of the bacterial endonuclease VirD2 (Dürrenberger et al, 1989), into a nucleoprotein complex (T-complex), in which numerous VirE2 molecules cover the entire length of the T-DNA molecule (Citovsky et al, 1997;Abu-Arish et al, 2004). Moreover, A. tumefaciens also utilizes the plant factor VIP1 (for VirE2-interacting protein1), which directly binds to VirE2 and facilitates the nuclear import and chromatin targeting of the entire T-complex (Tzfira et al, 2001;Li et al, 2005;Djamei et al, 2007;Lacroix et al, 2008). In the current model, the T-complex is most likely uncoated of its protein components by the VirF-mediated proteasomal degradation before the T-DNA becomes integrated into the host genome .…”