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, 9 (2), 137-46

A Receptor-Like Cytoplasmic Kinase Phosphorylates the Host Target RIN4, Leading to the Activation of a Plant Innate Immune Receptor

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A Receptor-Like Cytoplasmic Kinase Phosphorylates the Host Target RIN4, Leading to the Activation of a Plant Innate Immune Receptor

Jun Liu et al. Cell Host Microbe.

Abstract

Plants have evolved sophisticated surveillance systems to recognize pathogen effectors delivered into host cells. RPM1 is an NB-LRR immune receptor that recognizes the Pseudomonas syringae effectors AvrB and AvrRpm1. Both effectors associate with and affect the phosphorylation of RIN4, an immune regulator. Although the kinase and the specific mechanisms involved are unclear, it has been hypothesized that RPM1 recognizes phosphorylated RIN4. Here, we identify RIPK as a RIN4-interacting receptor-like protein kinase that phosphorylates RIN4. In response to bacterial effectors, RIPK phosphorylates RIN4 at amino acid residues T21, S160, and T166. RIN4 phosphomimetic mutants display constitutive activation of RPM1-mediated defense responses and RIN4 phosphorylation is induced by AvrB and AvrRpm1 during P. syringae infection. RIPK knockout lines exhibit reduced RIN4 phosphorylation and blunted RPM1-mediated defense responses. Taken together, our results demonstrate that the RIPK kinase associates with and modifies an effector-targeted protein complex to initiate host immunity.

Figures

Figure 1
Figure 1. RIPK interacts with RIN4 and RIPK expression is induced by avrB and avrRpm1
(A) Maltose Binding Protein (MBP) pulldown between purified recombinant His-RIN4 and MBP-RIPK in vitro. MBP alone is used as a negative control. Proteins were subjected to immunoblot analyses with antibodies recognizing RIN4 and MBP. (B) Quantitative Real Time PCR (qRT-PCR) illustrating RIPK induction upon bacterial inoculation. Four-week-old Arabidopsis leaves were syringe infiltrated with 2.5×107 cfu/ml of Pst DC3000 with the broad host range vector pVSP61carrying empty vector (EV), avrRpm1, or avrB. Y-axis indicates fold change. Error bars represent means ± standard deviation for qRT-PCR, n = 3. See also Figure S1 and Table S1.
Figure 2
Figure 2. The ripk knockout line is more resistant, while RIPK overexpression lines are more susceptible to Pst DC3000
(A) and (B) Four-week-old Ler and ripk plants were spray-inoculated with 1×109 cfu/ml of Pst DC3000. Four days post-inoculation, plants were subjected to growth curve analysis and photographed. (C) Complementation analyses of the ripk knockout with npro∷RIPK-myc. Two homozygous T3 complementation lines and controls were subjected to spray inoculation as described above. (D) 35S∷RIPK-HA overexpression lines are more susceptible to Pst DC3000. Two homozygous T3 overexpression lines were subjected to spray inoculation as described above. Statistical differences were detected by a two-tailed t-test for (A) and (D), alpha = 0.01, and by Fisher’s LSD for (C), alpha = 0.05. Error bars represent means ± standard deviation, n = 6. The data shown are representative of three independent experiments with similar results. See also Figure S2.
Figure 3
Figure 3. RIPK phosphorylates RIN4 in vitro
(A) Kinase assays using recombinant MBP-RIPK, His-RIN4, MyBP (myelin basic protein), and MBP (maltose binding protein). The kinase assay was initiated by adding γ-32P-ATP to the reaction mixture and phosphorylated proteins were visualized by autoradiography (upper panel). SDS-PAGE gel stained with coomassie blue (lower panel). (B) RIN4 phosphorylation sites detected by LC-MS/MS. The vertical bars represent the observed fragmentation sites of the precursor ion in the MS2 spectrum. The observed y and b ions are numbered. (C) RIPK cannot efficiently phosphorylate a RIN4 dephosphorylation mimic (dpRIN4, RIN4(T21A/S160A/T166A)). Recombinant His-dpRIN4, His-RIN4, and MBP-RIPK were incubated in a radiolabeled kinase assay as described in (A). (D) The F/YTxxFxK motif surrounding RIN4 T21 and T166. ClustalW alignment of RIN4 and other NOI proteins in Arabidopsis. RIN4 phosphorylated residues are indicated. See also Figure S3.
Figure 4
Figure 4. RPM1 recognizes phosphorylated RIN4
(A) RIN4 phosphorylation mimics induce RPM1-dependent dwarfism. npro∷pRIN4 (T21E/S160E/T166E) and npro∷dpRIN4 (T21A/S160A/T166A) were transformed into rpm1/rps2 (r1/r2), rps2/rin4 (r2/r4) and rpm1/rps2/rin4 (r1/r2/r4). Representative pictures were taken from 4 week old T1 plants. (B) and (C) Four-week-old Arabidopsis plants were spray inoculated with 1×109 cfu/ml of Pst DC3000. Growth curve analysis was performed 4 days post inoculation. Statistical differences were detected by Fisher’s LSD, alpha = 0.05. Experiments were performed on homozygous T3 lines in the rps2/rin4 background and T2 lines in the rpm1/rps2/rin4 background. Transgenic lines originate from the T1 plants as indicated in (A). Error bars represent means ± standard deviation, n = 6. (D) Homozygous T3 transgenic lines expressing npro∷dpRIN4 in an rps2/rin4 background are no longer able to elicit HR in response to avrB or avrRpm1. Plants were syringe infiltrated with 2.5×107 cfu/ml of Pst DC3000 carrying empty vector, avrRpm1, avrB or avrPphB. Plants were photographed at 8h and 24h post-inoculation. (E) RPM1 recognizes RIN4 phosphorylation at T166. RPM1, T7-RIN4, T7-pRIN4(T21E/S160E/T166E), T7-RIN4(T21E), T7-RIN4(S160E), and T7-RIN4(T166E) were co-expressed in N. benthamiana using Agrobacterium-mediated transient expression. Leaf disks were sampled 40h post-infiltration for anti-T7 immunoblotting. Leaves were photographed for RPM1-induced HR 72h post-infiltration. The data shown are representative of three independent experiments with similar results.
Figure 5
Figure 5. RIN4 phosphorylation is induced by AvrB and AvrRpm1 in vivo
(A) An antibody raised against a phosphorylated T166 RIN4 peptide (CGADGYpTHIFNK) specifically recognizes phosphorylated RIN4. Lane 1 (RIN4) = recombinant His-RIN4 and MBP-RIPK proteins incubated in the absence of ATP, Lane 2 (pRIN4) = recombinant His-RIN4 and MBP-RIPK proteins incubated in the presence of ATP. Immunoblot analyses with antibodies recognizing phosphorylated RIN4 (anti-pRIN4, upper panel) and RIN4 (anti-RIN4, lower panel). (B) T7-RIN4 phosphorylation in N. benthamiana after co-expression with AvrB or AvrRpm1 using Agrobacterium-mediated transient expression. Protein was extracted from leaf disks 40h post-infiltration and subjected to immunoblot analyses. (C) RIN4 phosphorylation in Arabidopsis Ler and ripk after delivery of AvrB and AvrRpm1. Arabidopsis leaves were infiltrated with 5×107cfu/ml of Pst DC3000 and Pst DC3000(avrB) or (avrRpm1). Immunoblot analysis was performed 6h post-inoculation. Upper panel = phosphorylated RIN4 immunoblot, lower panel = anti-RIN4 immunoblot, control = 0h time point, EV = Pst DC3000 control. (D) AvrB-HA is expressed at similar levels in Ler and ripk plants. Ler and ripk plants were syringe infiltrated with 5×107cfu/ml of Pst DC3000(avrB-HA). Immunoblot analysis was performed 6h post-inoculation. The data shown are representative of three independent experiments with similar results. See also Figure S4.
Figure 6
Figure 6. RIPK interacts with and phosphorylates AvrB in vitro
(A) The ripk knockout is more susceptible to Pst DC3000(avrB). Four-week-old Ler and ripk plants were syringe infiltrated with 0.5×105 cfu/ml, and growth curve analyses were performed 4 days post inoculation. Statistical differences were detected by a two-tailed t-test, alpha = 0.01. Error bars represent means ± standard deviation, n = 6. . The experiments were repeated three times with similar results. (B) The ripk knockout exhibits decreased single-cell HR compared to Ler after infiltration with Pst DC3000(avrB) or (avrRpm1). Leaves were infiltrated with 2.5×105 cfu/ml of bacteria, stained with trypan blue 16h post-inoculation, and photographed to visualize cell death. Lower panel = number of dead cells detected on 12 leaf images. Error bars represent means ± standard deviation, n = 12 (leaves/genotype). Experiments were repeated two times with similar results. (C) AvrB interacts with RIPK by yeast two-hybrid. Mutations in AvrB’s RIN4 (T125, R209) and ADP binding sites (Y65, R266) impaired AvrB and RIPK interactions in yeast. Lower panels: immunoblot analyses demonstrating HA-AvrB and myc-RIPK expression in yeast. (D) Purified His-AvrB-HA and His-RIN4 recombinant proteins were incubated with MBP-RIPK and the kinase reaction was initiated by adding γ-32P-ATP. Phosphorylated proteins were visualized by autoradiography. Increasing the amount of AvrB in the reaction, denoted by the triangle, increased the amount of AvrB phosphorylation by RIPK. SDS-PAGE gel stained with coomassie blue demonstrates recombinant protein purity (lower panel). BSA (bovine serum albumin) served as a negative control. See also Figure S5.
Figure 7
Figure 7. RIN4 phosphorylation status alters protein associations
(A) RIN4 and RIN4 phosphorylation mimic (pRIN4, RIN4(T21E/S160E/T166E)) interaction with AvrB by yeast-two hybrid. Lower panel: immunoblot analyses demonstrating myc-RIN4 and HA-AvrB expression in yeast. (B) AvrB disrupts the RIN4 and RIPK complex in vivo. GFP, T7-RIN4, RIPK-HA, and AvrB-FLAG were transiently expressed in N. benthamiana by Agrobacterium-mediated protein expression. T7-RIN4 was immunoprecipitated with T7 antisera and associated proteins were detected by immunoblot analyses. Bottom panel: Anti-FLAG immunoprecipitation. AvrB-FLAG expression could only be detected by immunoprecipitation in the input due to low-level expression.

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