Agrobacterium Induces Expression of a Host F-Box Protein Required for Tumorigenicity

Abstract

In plant-pathogen interactions, the host defends against the invading pathogen and the pathogen aims to suppress or subvert this defense. Whereas the defense suppression strategy is relatively well understood for many pathogens, the mechanisms by which pathogens can actively utilize the defense machinery of the host remain obscure. We report that Agrobacterium, a microorganism that elicits neoplastic growths on many plant species, induces expression of a plant defense-related F-box protein, VBF, which it incorporates into its own pathway for genetic transformation. Our data suggest that VBF may function to uncoat the bacterial transferred DNA from its associated virulence VirE2 and host VIP1 proteins via the SCFVBF pathway. Suppression of VBF elevates the intracellular content of VIP1, but renders the plant largely resistant to Agrobacterium, indicating that, in the infection pathway, VBF is functionally epistatic to VIP1. When expressed in Agrobacterium and exported into the plant cell, VBF facilitates tumor formation.

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

“…In such plants, A. tumefaciens may exploit a host VBF-like protein during infection as an alternative to VirF. Consistent with this model, expression of VBF in the VirF-lacking strain and its export into a plant cell functionally complemented tumor formation on tomato (Solanum lycopersicum), which is usually recalcitrant to the VirF-lacking strain (Zaltsman et al, 2010).…”

“…However, at least one of the induced genes, VIP1-BINDING F-BOX PROTEIN (VBF) of Arabidopsis, was suggested to play a crucial role in A. tumefaciens infection (Ditt et al, 2006;Zaltsman et al, 2010). Like the bacterial F-box protein VirF, the plant F-box protein VBF interacts with VIP1 and targets it for proteasomal degradation via the SCF VBF complex (Zaltsman et al, 2010). The VBF transcripts are up-regulated in Arabidopsis upon inoculation with not only A. tumefaciens (Ditt et al, 2006) but also fungal pathogens (Li et al, 2006).…”

“…Consistent with this notion, the host UPS plays a critical role in the A. tumefaciens-plant interaction. Recent studies have shown that, upon A. tumefaciens infection, the host plants up-regulate or down-regulate several UPS-associated genes and proteins (Ditt et al, 2006;Anand et al, 2007Anand et al, , 2012Zhao et al, 2011;Tie et al, 2012), some of which likely affect the efficiency of the A. tumefaciens infection (Zaltsman et al, 2010;Anand et al, 2012). In addition, A. tumefaciens is known to export into the host cell an F-box protein, a component of the SCF (for S-PHASE KINASE-ASSOCIATED PROTEIN1 (SKP1)-CULLIN1 (CUL1)-F-box protein) ubiquitin ligase complex, and facilitate infection via the UPSmediated protein degradation .…”

Agrobacterium May Delay Plant Nonhomologous End-Joining DNA Repair via XRCC4 to Favor T-DNA Integration

“…Vir proteins associated with the proposed T-complex interact with various plant factors to help direct the import of the T-complex from the cytoplasm into the plant nucleus. These proteins may eventually be stripped from the T-DNA by the SCF virF complex (Zaltsman et al, 2010;Anand et al, 2012). The T-DNA then presumably is converted into a double-stranded moiety and integrates into the host genome (Tzfira et al, 2004).…”

Disassembly of synthetic Agrobacterium T-DNA–protein complexes via the host SCF VBF ubiquitin–ligase complex pathway

“…Thus, once the T-complex reaches the host chromatin, its proteins must be removed. This process has been proposed to involve the host ubiquitin/proteasome system (UPS) (23-25) based on the observations that challenge of plants by bacteria, including Agrobacterium, induces expression of a defense-related F-box protein, VBF, that recognizes and targets VIP1 and its bound VirE2 for degradation (25). F-box proteins-a large protein family with almost 700 predicted members in the Arabidopsis genome (26)-represent a component of the Skp1/ Cullin/F-box protein (SCF) complex (27,28) that acts as a E3 ubiquitin ligase to polyubiquitinate target proteins and tag them for subsequent degradation by the 26S proteasome.…”