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. 2014 Jan;77(2):235-45.
doi: 10.1111/tpj.12381. Epub 2013 Dec 9.

The Pseudomonas syringae effector HopF2 suppresses Arabidopsis immunity by targeting BAK1

Affiliations

The Pseudomonas syringae effector HopF2 suppresses Arabidopsis immunity by targeting BAK1

Jinggeng Zhou et al. Plant J. 2014 Jan.

Abstract

Pseudomonas syringae delivers a plethora of effector proteins into host cells to sabotage immune responses and modulate physiology to favor infection. The P. syringae pv. tomato DC3000 effector HopF2 suppresses Arabidopsis innate immunity triggered by multiple microbe-associated molecular patterns (MAMP) at the plasma membrane. We show here that HopF2 possesses distinct mechanisms for suppression of two branches of MAMP-activated MAP kinase (MAPK) cascades. In addition to blocking MKK5 (MAPK kinase 5) activation in the MEKK1 (MAPK kinase kinase 1)/MEKKs-MKK4/5-MPK3/6 cascade, HopF2 targets additional component(s) upstream of MEKK1 in the MEKK1-MKK1/2-MPK4 cascade and the plasma membrane-localized receptor-like cytoplasmic kinase BIK1 and its homologs. We further show that HopF2 directly targets BAK1, a plasma membrane-localized receptor-like kinase that is involved in multiple MAMP signaling. The interaction between BAK1 and HopF2 and between two other P. syringae effectors, AvrPto and AvrPtoB, was confirmed in vivo and in vitro. Consistent with BAK1 as a physiological target of AvrPto, AvrPtoB and HopF2, the strong growth defects or lethality associated with ectopic expression of these effectors in wild-type Arabidopsis transgenic plants were largely alleviated in bak1 mutant plants. Thus, our results provide genetic evidence to show that BAK1 is a physiological target of AvrPto, AvrPtoB and HopF2. Identification of BAK1 as an additional target of HopF2 virulence not only explains HopF2 suppression of multiple MAMP signaling at the plasma membrane, but also supports the notion that pathogen virulence effectors act through multiple targets in host cells.

Keywords: Arabidopsis thaliana; BAK1; BIK1; MAPK cascade; Pseudomonas syringae; bacterial effector; pattern-triggered immunity.

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Figures

Figure 1
Figure 1. HopF2 suppresses flg22-induced phosphorylation of BIK1 and homologs
(A) HopF2 blocks flg22-induced mobility shift of BIK1 and homologs. Arabidopsis protoplasts were co-transfected with HA-tagged BIK1, PBL1 or PBS1 and GFP-tagged HopF2 for 10 hr and treated with 1 μM flg22 for 10 min. (B) Conserved surface residues of HopF2 are required for its suppression of flg22-induced BIK1 phosphorylation. Protoplasts were co-transfected with FLAG-tagged BIK1 and HA-tagged HopF2 or its mutants, and treated with flg22 as in (A).
Figure 2
Figure 2. HopF2 suppresses two branches of flg22-induced MAPK cascades
(A) Scheme of two branches of MAPK cascades in Arabidopsis flagellin signaling. (B) HopF2 does not suppress MKK1/2-mediated MPK4 activation. Arabidopsis protoplasts were co-transfected with myc-tagged constitutively active form of MKK1/2 (MKK1ac-myc/MKK2ac-myc), HA-tagged MPK4 (MPK4-HA) and GFP-tagged HopF2 (HopF2-GFP). MPK4-HA was immunoprecipitated by an α-HA antibody and subjected to an immunocomplex kinase assay using myelin basic protein as a substrate. (C) HopF2 does not suppress MEKK1-mediated MPK4 activation. HA-tagged MEKK1 (MEKK1-HA) was coexpressed with MPK4-HA and HopF2-GFP, and MPK4-HA kinase activity was detected in an immunocomplex kinase assay as in (B). (D) HopF2 suppresses MKK5-mediated MPK3 activation. MKK5ac-myc was coexpressed with MPK3-HA and HopF2-GFP, and MPK3-HA kinase activity was detected in an immunocomplex kinase assay as in (B).
Figure 3
Figure 3. HopF2, AvrPto and AvrPtoB interact with BAK1
(A) HopF2 and AvrPto/B interact with BAK1 in Arabidopsis protoplasts. The α-HA co-IP was performed with protoplasts co-expressing FLAG-tagged BAK1 and HA-tagged AvrPto, AvrPtoB or HopF2, and the immunoprecipitated proteins were analyzed in Western blot with an α-FLAG antibody. HopF2 (B) and AvrPto (C) interact with BAK1 in Arabidopsis plants. pBAK1::BAK1-GFP transgenic seedlings with DEX-inducible effector transgene were treated with 5 μM DEX for 12 hr and subjected to an α-GFP co-IP assay, and the immunoprecipitated proteins were analyzed in Western blot with an α-HA antibody. (D) The BiFC assays for HopF2-BAK1 or AvrPto-BAK1 interactions in Arabidopsis protoplasts. The various BiFC constructs were transfected into protoplasts and the fluorescence were visualized under a confocal microscope. Bar=50 μm. (E) The pull-down assays for HopF2-BAK1 or AvrPto-BAK1 interactions. GST, GST-AvrPto and GST-HopF2 were expressed individually in E. coli, purified with glutathione agarose, and used to pull-down the proteins from protoplasts expressing FLAG-tagged BAK1. The pull-downed proteins were analyzed in Western blot with an α-FLAG antibody.
Figure 4
Figure 4. Transmembrane, juxtamembrane and kinase domains of BAK1(BAK1TJK) are sufficient for BAK1-HopF2 or BAK1-AvrPto interaction
(A) The α-HA co-IP was performed with protoplasts co-expressing FLAG-tagged BAK1TJK with AvrPto-HA or HopF2-HA, and the immunoprecipitated proteins were analyzed in Western blot with an α-FLAG antibody. (B) The pull-down assay was performed by using GST, GST-AvrPto and GST-HopF2 proteins expressed in E. coli, purified with glutathione agarose to bind the total proteins from protoplasts expressing BAK1TJK-FLAG.
Figure 5
Figure 5. Ectopic expression of AvrPto in bak1-4 mutant plants
(A) Phenotype of 4-week-old 35S::AvrPto-HA/Col-0 and 35S::AvrPto-HA/bak1-4 transgenic plants. The expression of AvrPto protein was shown with an α-HA Western blot. (B) Phenotype of 10-week-old 35S::AvrPto-HA/Col-0 and 35S::AvrPto-HA/bak1-4 transgenic plants.
Figure 6
Figure 6. Ectopic expression of AvrPto, AvrPtoB or HopF2 in bak1-1 mutant plants
Phenotype of 4-week-old WS, bak1-1, 35S::AvrPto-HA/bak1-1 (A), 35S::AvrPtoB-HA/bak1-1 (B) and 35S::HopF2-HA/bak1-1 (C) plants. The expression of corresponding effector proteins is shown with an α-HA Western blot.
Figure 7
Figure 7. A model of multiple host targets of HopF2
Plant innate immunity includes pattern-triggered immunity (PTI) and effector-triggered immunity (ETI). Perception of bacterial flagellin by FLS2 activates FLS2/BAK1/BIK1 complex phosphorylation and two branches of MAPK cascades, MEKK1-MKK1/2-MPK4 and MEKK1/MEKKs-MKK4/5-MPK3/6 in PTI signaling. Bacterial type III secretion system (TTSS) effectors AvrB, AvrRpm1 and AvrRpt2 modify host RIN4 protein, which is sensed by corresponding RPM1 and RPS2 proteins to activate ETI signaling. Bacterial effector proteins have the ability to suppress both PTI and ETI signaling. AvrPto and AvrPtoB target BAK1 to suppress PTI signaling. HopF2 suppresses PTI signaling by targeting BAK1 and MKK5, and suppresses ETI signaling by targeting RIN4.

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