Agrobacterium induces expression of a host F-box protein required for tumorigenicity

Abstract

Agrobacterium exports DNA into plant cells, eliciting neoplastic growths on many plant species. During this process, a Skp1-Cdc53-cullin-F-box (SCF) complex that contains the bacterial virulence F-box protein VirF facilitates genetic transformation by targeting for proteolysis proteins, the Agrobacterium protein VirE2 and the host protein VIP1, that coat the transferred DNA. However, some plant species do not require VirF for transformation. Here, we show that Agrobacterium induces expression of a plant F-box protein, which we designated VBF for VIP1-binding F-box protein, that can functionally replace VirF, regulating levels of the VirE2 and VIP1 proteins via a VBF-containing SCF complex. When expressed in Agrobacterium and exported into the plant cell, VBF functionally complements tumor formation by a strain lacking VirF. VBF expression is known to be induced by diverse pathogens, suggesting that Agrobacterium has co-opted a plant defense response and that bacterial VirF and plant VBF both contribute to targeted proteolysis that promotes plant genetic transformation.

Figure 1. VBF interacts with VIP1 and is upregulated by Agrobacterium infection

(A) BiFC assay for the VBF-VIP1 interaction in microbombarded Arabidopsis leaves. (B) BiFC assay for the VBF-VIP1 interaction in agroinfiltrated N. benthamiana leaves. FBX is encoded by the At1g31350 gene. Free DsRed2 labels the cell cytoplasm and the nucleus and identifies the transformed cells. All images are projections of several confocal sections. (C) Yeast two-hybrid assay for the VBF-VIP1 interaction. The indicated dilutions of cell cultures were grown either in the absence (left panel) or in the presence of histidine (right panel). Lane 1, Gal4AD-VBF+LexA-VIP1; lane 2, Gal4AD-VBF+LexA-FBX; lane 3, Gal4AD+LexA-VIP1, lane 4, Gal4AD-VirE2+LexA-VIP1; lane 5, Gal4AD-VBF+LexA-lamin C; lane 6, Gal4AD-VBF+LexA-ASK1; lane 7, Gal4AD+LexA-ASK1. (D) RT-PCR analysis of the VBF gene expression following inoculation by Agrobacterium. Constitutively-expressed TUBULIN was used as internal control. (E) Q-PCR analysis of the VBF gene expression following inoculation by Agrobacterium. The data represent average values of three independent experiments with indicated standard deviations. (F) Expression of GUS reporter from the VBF promoter in Arabidopsis roots following inoculation by Agrobacterium. (-), (+) indicate mock-inoculated or Agrobacterium-inoculated plants, respectively.

Figure 1. VBF interacts with VIP1 and is upregulated by Agrobacterium infection

(A) BiFC assay for the VBF-VIP1 interaction in microbombarded Arabidopsis leaves. (B) BiFC assay for the VBF-VIP1 interaction in agroinfiltrated N. benthamiana leaves. FBX is encoded by the At1g31350 gene. Free DsRed2 labels the cell cytoplasm and the nucleus and identifies the transformed cells. All images are projections of several confocal sections. (C) Yeast two-hybrid assay for the VBF-VIP1 interaction. The indicated dilutions of cell cultures were grown either in the absence (left panel) or in the presence of histidine (right panel). Lane 1, Gal4AD-VBF+LexA-VIP1; lane 2, Gal4AD-VBF+LexA-FBX; lane 3, Gal4AD+LexA-VIP1, lane 4, Gal4AD-VirE2+LexA-VIP1; lane 5, Gal4AD-VBF+LexA-lamin C; lane 6, Gal4AD-VBF+LexA-ASK1; lane 7, Gal4AD+LexA-ASK1. (D) RT-PCR analysis of the VBF gene expression following inoculation by Agrobacterium. Constitutively-expressed TUBULIN was used as internal control. (E) Q-PCR analysis of the VBF gene expression following inoculation by Agrobacterium. The data represent average values of three independent experiments with indicated standard deviations. (F) Expression of GUS reporter from the VBF promoter in Arabidopsis roots following inoculation by Agrobacterium. (-), (+) indicate mock-inoculated or Agrobacterium-inoculated plants, respectively.

Figure 2. Formation of ternary VirE2-VIP1-VBF complexes in microbombarded N. benthamiana leaves

(A) Bridge-BiFC assay. (B) Multi-color bridge-BiFC assay. nCerulean/cCFP and nVenus/cCFP signals are indicated in blue and green, respectively; merged image represents overlay of both BiFC signals and DsRed2. Free DsRed2 labels the cytoplasm and the nucleus and identifies the transformed cells. (C) Coprecipitation. Left panel: Lane 1, GFP-VIP1+His-VBF+HA-VirE2; lane 2, GFP-VIP1+HA-VirE2. Input, the GFP-VIP1+His-VBF+HA-VirE2 sample processed without precipitation. Right panel: Lane 1, GFP-GFP+His-VBF. Input, the GFP-GFP+His-VBF sample processed without precipitation.

Figure 3. Formation of ternary VIP1-VBF-ASK1 complexes in microbombarded N. benthamiana leaves

(A) Multi-color bridge-BiFC assay. nCerulean/cCFP and nVenus/cCFP signals are indicated in blue and green, respectively; merged image represents overlay of both BiFC signals and DsRed2. Free DsRed2 labels the cytoplasm and the nucleus and identifies the transformed cells. (B) Coprecipitation. Lane 1, GFP-VIP1+His-VBF+HA-ASK1; lane 2, GFP-VIP1+HA-ASK1. Input, the GFP-VIP1+His-VBF+HA-ASK1 sample processed without precipitation.

Figure 4. VBF destabilizes VIP1 and VirE2

(A) VBF-mediated and Skp1-dependent destabilization of GFP-VIP1 in yeast. GFP signal in the presence of VBF was calculated as percent of the signal measured in the absence of VBF expression, which was defined as 100% signal. Standard deviations are indicated. (B) VBF-mediated destabilization of CFP-VIP1 in agroinfiltrated N. benthamiana leaves. (C) VBF-mediated destabilization of CFP-VIP1 and YFP-VirE2 in agroinfiltrated N. benthamiana leaves. FBX is encoded by the At1g31350 gene. Arrows indicate cell nuclei identified by the presence of free DsRed2, which also identifies the transformed cells. (D) The effect of VBF expression on the number of transformed plant cells that accumulate CFP-VIP1 or CFP-VIP1 and YFP-VirE2 and on the amounts of the CFP-VIP1 transcript in these cells. The amounts of the CFP-VIP1 transcript were estimated by Q-PCR and expressed as percent of the amount of the coexpressed DSRED2 transcript in the same sample. The data represent three independent experiments (n=3) with indicated standard deviations (SD). NA, not applicable. (E) Quantification of VIP1 and VirE2 destabilization in N. benthamiana leaves. Amounts of each protein are represented by the intensities of the corresponding western bands and calculated as percent of those observed when VBF was replaced with FBX and defined as 100% signal. Standard deviations are indicated.

Figure 4. VBF destabilizes VIP1 and VirE2

(A) VBF-mediated and Skp1-dependent destabilization of GFP-VIP1 in yeast. GFP signal in the presence of VBF was calculated as percent of the signal measured in the absence of VBF expression, which was defined as 100% signal. Standard deviations are indicated. (B) VBF-mediated destabilization of CFP-VIP1 in agroinfiltrated N. benthamiana leaves. (C) VBF-mediated destabilization of CFP-VIP1 and YFP-VirE2 in agroinfiltrated N. benthamiana leaves. FBX is encoded by the At1g31350 gene. Arrows indicate cell nuclei identified by the presence of free DsRed2, which also identifies the transformed cells. (D) The effect of VBF expression on the number of transformed plant cells that accumulate CFP-VIP1 or CFP-VIP1 and YFP-VirE2 and on the amounts of the CFP-VIP1 transcript in these cells. The amounts of the CFP-VIP1 transcript were estimated by Q-PCR and expressed as percent of the amount of the coexpressed DSRED2 transcript in the same sample. The data represent three independent experiments (n=3) with indicated standard deviations (SD). NA, not applicable. (E) Quantification of VIP1 and VirE2 destabilization in N. benthamiana leaves. Amounts of each protein are represented by the intensities of the corresponding western bands and calculated as percent of those observed when VBF was replaced with FBX and defined as 100% signal. Standard deviations are indicated.

Figure 5. Expression of the VBF and VIP1 genes and accumulation of the VIP1 and PR-1 proteins in wild-type and VBF antisense Arabidopsis plants

(A) Detection of the VBF antisense transgene by PCR using primers specific for the 35S promoter and terminator sequences of pSAT4-35SP-MCS-35ST. (B) Detection of the VBF transcripts by PCR. VBF cod, PCR products obtained with primers specific for the VBF coding sequence; VBF utr, PCR products obtained with primers specific for the VBF 5’ and 3’ UTR sequences. (-), (+) indicate mock-inoculated or Agrobacterium-inoculated plants, respectively. (C) Detection of the VBF transcripts by Q-PCR. Bars 1–4, VBF cod; bars 5–8, VBF utr; bars 1, 2 and 5, 6, wild-type plants; bars 3, 4 and 7, 8, VBF antisense plants; bars 1, 3, 5, 7, and 2, 4, 6, 8, mock-inoculated or Agrobacterium-inoculated plants, respectively. The data represent average values of three independent experiments with indicated standard deviations. (D) Detection of the VIP1 transcripts. (E) Detection of the VIP1 and PR-1 proteins.

Figure 6. Reduced tumor formation in Agrobacterium-infected VBF antisense Arabidopsis plants

(A, C) Tumor formation in roots from the wild-type and VBF antisense plants infected with VirF(-) or VirF(+) Agrobacterium strains LBA1517 or LBA1010, respectively. (B, D) Quantification of LBA1517 and LBA1010 tumorigenicity, respectively, in the wild-type (black bars) and VBF antisense plants (gray bars). Standard deviations are indicated. (E) RT-PCR analysis of the PR-1 gene expression following SA treatment of the wild-type plants. (F) Tumor formation in roots from untreated or SA-treated wild-type plants infected with VirF(+) Agrobacterium strain LBA1010. (G) Quantification of LBA1010 tumorigenicity in untreated (black bars) and SA-treated wild-type plants (gray bars). Standard deviations are indicated.

Figure 7. VBF enhances tumorigenesis in tomato infected by a VirF(-) strain of Agrobacterium

(A) Schematic structure of the VBF expression cassette in pEX-VBF. VBF expression in Agrobacterium is directed by the virF promoter, and VBF export from the bacterial cell is promoted by the VirE3 export signal (EX) fused to the C-terminus of VBF. (B) Representative images of tumors developed on tomato stems following infection by VirF(-) Agrobacterium strains LBA1517, LBA1517 that expresses VBF from pEX-VBF, or LBA1517 that expresses VirF from pEX-VirF. (C) Quantitation of tumorigenicity of LBA1517 (black bar), VBF-expressing LBA1517 (gray bar), or VirF-expressing LBA1517 (positive control, white bar) Agrobacterium strains in tomato plants. Standard deviations are indicated.