The Agrobacterium F-Box Protein Effector VirF Destabilizes the Arabidopsis GLABROUS1 Enhancer/Binding Protein-Like Transcription Factor VFP4, a Transcriptional Activator of Defense Response Genes

Abstract

Agrobacterium-mediated genetic transformation not only represents a technology of choice to genetically manipulate plants, but it also serves as a model system to study mechanisms employed by invading pathogens to counter the myriad defenses mounted against them by the host cell. Here, we uncover a new layer of plant defenses that is targeted by A. tumefaciens to facilitate infection. We show that the Agrobacterium F-box effector VirF, which is exported into the host cell, recognizes an Arabidopsis transcription factor VFP4 and targets it for proteasomal degradation. We hypothesize that VFP4 resists Agrobacterium infection and that the bacterium utilizes its VirF effector to degrade VFP4 and thereby mitigate the VFP4-based defense. Indeed, loss-of-function mutations in VFP4 resulted in differential expression of numerous biotic stress–response genes, suggesting that one of the functions of VFP4 is to control a spectrum of plant defenses, including those against Agrobacterium tumefaciens. We identified one such gene, ATL31, known to mediate resistance to bacterial pathogens. ATL31 was transcriptionally repressed in VFP4 loss-of-function plants and activated in VFP4 gain-of-function plants. Gain-of-function lines of VFP4 and ATL31 exhibited recalcitrance to Agrobacterium tumorigenicity, suggesting that A. tumefaciens may utilize the host ubiquitin/proteasome system to destabilize transcriptional regulators of the host disease response machinery.

Genome-Wide Identification, Expansion Mechanism and Expression Profiling Analysis of GLABROUS1 Enhancer-Binding Protein (GeBP) Gene Family in Gramineae Crops

“…The transcript levels of the pathogen response marker genes PR1 and PR5 are increased in the mutant, indicating that the GeBP/GPLs are repressors of PR genes. A recent study revealed that VirF and its plant functional homolog VBF of the Agrobacterium F-box effector interact with the Arabidopsis GeBP-like transcription factor VFP4 [27]. Loss-of-function mutation of VFP4 results in the differential expression of numerous biotic stress response genes, suggesting that one of the functions of VFP4 is to control a spectrum of plant defenses, including against Agrobacterium tumefaciens.…”

Application of phiLOV2.1 as a fluorescent marker for visualization of Agrobacterium effector protein translocation

“…This complex may enable T-DNA integration by proteasomal degradation of VIP1 and VirE2 (Tzfira et al, 2004). In addition, it may destabilize VFP4, a transcriptional activator of defense response genes (Garc ıa-Cano et al, 2018). VBF, a plant F-box protein, might functionally replace VirF in plant species that do not require VirF for successful transformation (Zaltsman et al, 2010).…”

Pathways of DNA Transfer to Plants from Agrobacterium tumefaciens and Related Bacterial Species

“…The F-box protein activity of A6-VirF was demonstrated in yeast and in plant cells, and one of its potential substrates and interacting partners was identified; specifically, VirF was shown to bind VIP1 and destabilize via the ubiquitin–proteasome system (UPS) both VIP1 and its associated VirE2 (128). More recently, several other Arabidopsis interactors of VirF were identified (42, 43), one of which, VFP4, a transcriptional regulator of genes involved in stress or defense response, was targeted by VirF for proteasomal degradation (42). In addition, VirF itself is destabilized via UPS, and this destabilization is prevented by VirF interaction with another effector, VirD5 (82).…”