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, 9 (2), 125-36

Specific Threonine Phosphorylation of a Host Target by Two Unrelated Type III Effectors Activates a Host Innate Immune Receptor in Plants

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Specific Threonine Phosphorylation of a Host Target by Two Unrelated Type III Effectors Activates a Host Innate Immune Receptor in Plants

Eui-Hwan Chung et al. Cell Host Microbe.

Abstract

The Arabidopsis NB-LRR immune receptor RPM1 recognizes the Pseudomonas syringae type III effectors AvrB or AvrRpm1 to mount an immune response. Although neither effector is itself a kinase, AvrRpm1 and AvrB are known to target Arabidopsis RIN4, a negative regulator of basal plant defense, for phosphorylation. We show that RIN4 phosphorylation activates RPM1. RIN4(142-176) is necessary and, with appropriate localization sequences, sufficient to support effector-triggered RPM1 activation, with the threonine residue at position 166 being critical. Phosphomimic substitutions at T166 cause effector-independent RPM1 activation. RIN4 T166 is phosphorylated in vivo in the presence of AvrB or AvrRpm1. RIN4 mutants that lose interaction with AvrB cannot be coimmunoprecipitated with RPM1. This defines a common interaction platform required for RPM1 activation by phosphorylated RIN4 in response to pathogenic effectors. Conservation of an analogous threonine across all RIN4-like proteins suggests a key function for this residue beyond the regulation of RPM1.

Figures

Figure 1
Figure 1. The C-terminal NOI of RIN4 is required for RPM1 function
(A) Schematic diagram of RIN4 derivatives. Gray boxes are N- and C-terminal NOI domains, the black bar is the AvrB binding site (BBS), the arrows indicate the two AvrRpt2 cleavage sites, and the “3C” represents the C-terminal palmitoylation/prenylation site (Kim et al., 2005). Within the derivatives, the amino acids flanking the breakpoints are indicated. Each derivative has an N-terminal T7 epitope-tag. (B) α-T7 immunoblot of microsomal membrane protein fractions from transgenic lines expressing the indicated RIN4 derivatives from (A) under control of the native RIN4 promoter in RPM1-myc rpm1 rps2 rin4 plants. The background pattern differs in the right hand panel because this is a higher percentage gel used to resolve 1Δ141 (9 kDa). Line numbers designate plant families homozygous for a single insertion locus that were derived from independent T-DNA insertion events. (C) HR phenotypes of the indicated plants after infiltration with 5×107 cfu/mL of Pto DC3000 expressing AvrRpm1 or AvrB, as noted at right. Representative leaves were photographed 20 hours after infiltration and the numbers below indicate the occurrence of macroscopic HR per number of tested leaves. (D) Conductivity measurements after infiltration of the indicated plants with 5×107 cfu/mL of Pto DC3000 expressing AvrRpm1 (left) or AvrB (right). Eight leaf discs that received the same infiltration were floated in a single tube and the conductivity of the solution was measured over time. Standard errors are from data combined from three separate experiments. (E) Growth analysis 3 days after infiltration of 105 cfu/mL of Pto DC3000 expressing AvrRpm1 or AvrB into the indicated plants. The day 0 measurements show the number of bacteria in Col-0 plants immediately following infiltration. The results are from one of four representative experiments. Standard errors are from three separate experiments. (F) RPM1 expression in microsomal fractions from the indicated lines. The strong signal in the line RPM1:myc rpm1–3 shows the high level of RPM1:myc accumulation in the presence of native RIN4.
Figure 2
Figure 2. RIN4 T166 is required for AvrB-mediated RPM1-dependent HR in Nicotiana benthamiana and a phosphomimic of this residue confers effector independent RPM1 activation
(A) Conductivity measurements after agro-infiltration with strains expressing the indicated proteins. N. benthamiana leaves were hand-infiltrated with Agrobacterium C58C1 strains expressing AvrB / AvrB G2A, RIN4, H167A, I168A, F169A, HIF-AAA mutant and RPM1 as described in Figure S2A. 30µM of Est was applied two days after co-infiltration. Some error bars are smaller than the symbols. (B) Co-infiltration of AvrB and RPM1 with RIN4 T166A and T166D mutants. This result was obtained from the same experiments in (A). Error bars in (A) and (B) represent 2x SE. These results were confirmed four times. (C) Visible phenotypes of infiltrated N. benthamiana leaves. Two independent leaves were infiltrated with the indicated constructs. One leaf was used to take the picture for phenotypes and the second leaf was used to extract proteins for immunoblot in (D). Pictures were taken 12 hours post Est-treatment. The result is one of four replicates. (D) Immunobots with α-HA, α-T7 and α-myc for AvrB / AvrB G2A, RIN4 / BBS mutants and RPM1, respectively, following Agrobactrium-mediated transient expression. Protein samples were harvested 6 hours post Est-treatment.
Figure 3
Figure 3. RIN4 T166D activity is dependent on RPM1 P-loop function in Nicotiana benthamiana
(A) The phosphomimic RIN4 T166D mutant drives effector-independent RPM1-mediated HR. Conductivity measurements were performed with N. benthamiana leaves infiltrated with Agrobacterium C58C1 strains expressing RIN4 BBS mutants (OD600=0.4) and RPM1:myc (OD600=0.4). The measurements began two days post infiltration. Repeated three times with similar result. The error bars represent 2x SE. (B) Phenotypes of RIN4 T166 derivatives. RIN4 T166D, RIN4 T166E and RIN4 T166K driven by the RIN4 native promoter were co-infiltrated as in (A). Photo was taken 3 days after co-infiltration. (C) Expression of RPM1 and RIN4 T166 derivatives. RPM1 and RIN4 T166 derivatives were expressed, and variation does not account for the observed phenotypes. Immunoblots with α-myc and α-T7 were performed with 2 day-old-samples post infiltration. (D) The RIN4 phosphomimic T166D is localized to a microsomal fraction. N. benthamiana leaves were co-infiltrated as in (A). Proteins were extracted from leaf tissues at the onset of HR from T166D/RPM1 co-infiltration, which corresponded to 8 hours in the conductivity experiment in (A). Repeated twice. Total (T), soluble (S) and microsomal (M) fractions were loaded at a 1:1:5 ratio, followed by immunoblotting with α-T7 and α-myc to detect RIN4 and RPM1, respectively. (E) Two-phase partitioning of RIN4 and RPM1. RIN4 and RIN4 T166D mutant were co-infiltrated with RPM1 as described in (D). The microsomal extraction was used as input for two-phase partitioning. The upper fraction, for plasma membrane (PM), and the lower fraction for endomembranes (EM) were loaded at equal yield, followed by immunoblotting with α-myc and α-T7 to detect RPM1(*) and RIN4, respectively. Plasma membrane-localized (PM) ATPase and ER-localized BIP proteins represented the efficiency of fractioning for PM and EM. (F) Conductivity measurements and HR phenotype after co-infiltration of RIN4 T166D with either RPM1 or an RPM1 G205E. N. benthamiana leaves were hand-infiltrated with Agrobacterium C58C1 strains expressing T166D mutant (OD600=0.4) and either pRPM1:RPM1:myc (OD600=0.4) or RPM1:myc G205E (OD600=0.8). C58C1 was used as filler to make up the difference in OD between RPM1:myc and RPM1:myc G205E with OD600=0.4. The measurements started two days post infiltration. This result was one of two repeats. Trypan Blue staining with leaf discs covering half of the infiltrated zone was performed 2.5 days after infiltration indicated 12 hr in conductivity measurement. (G) Expression of RPM1:myc and RPM1:myc G205E. Protein samples from (F) were prepared 2 days post infiltration. The immunoblot was performed with α-myc.
Figure 4
Figure 4. RIN4 T166 contributes to AvrRpm1-dependent RPM1-mediated HR in N. benthamiana
(A) Conductivity measurements after agro-infiltration with strains expressing the indicated proteins. N. benthamiana leaves were hand-infiltrated with Agrobacterium C58C1 strains as in Figure 2A except Est:AvrRpm1-HA (OD600=0.1) instead of Est:AvrB:HA. Co-infiltration of RIN4 and RPM1:myc was used as a negative control with C58C1 cells (OD600=0.1). The result was repeated three times. Measurement started 2 hours post induction with 30µM Estradiol. Error bars represent 2x SE. (B) HR Phenotypes of infiltrated N. benthamiana leaves. Trypan blue staining was performed with leaf discs which covered half of an infiltrated zone at 8 hours after Est-treatment. Data represent one of three independent experiments with consistent result. (C) Immunoblots with α-HA, α-T7 and α-myc to detect AvrRpm1, RIN4 and RPM1, respectively. Protein samples were extracted from leave tissues harvested 6 hours post Est-treatment.
Figure 5
Figure 5. RIN4 T166 is required for AvrB-, and contributes to AvrRpm1-dependent, RPM1-mediated HR in Arabidopsis
(A) HR determined by Trypan Blue staining. 20 independent leaves from transgenic lines expressing each RIN4 BBS mutant were inoculated with 5×107 cfu/ml (OD600=0.1) of Pto DC3000(avrB) or (avrRpm1) in half of each leaf (dotted area). Leaves were harvested 6 hours after inoculation. The numbers are leaves which displayed the HR phenotype shown over the total. The result was repeated with two independent homozygous transgenic lines for each BBS mutant with similar results. (B) Conductivity measurements. Two independent homozygous T166A mutant lines and controls shown at right were used to monitor the loss-of-function phenotype with 5×107cfu/mL of Pto DC3000 (avrB) or (avrRpm1). Error bar represents 2X SE for RIN4 T166A mutant. Four leaf discs were used to measure the conductivity of Col-0 and RPM1:myc rpm1rps2rin4. (C) Bacteria growth analysis of Pto DC3000 (avrB) or (avrRpm1). Bacteria recovered from infiltrated leaves of each transgenic line indicated or controls at bottom were counted after hand-inoculation with 105 cfu/mL for each strain on day 0 and day 3. The result was repeated twice with two independent T3 homozygous transgenic Arabidopsis lines from each RIN4 mutant. Error bars represent 2X SE. Pair-wise comparisons for all means from the day 3 data were performed with One-Way ANOVA test followed by Tukey-Kramer HSD at 95% confidence limits.
Figure 6
Figure 6. RIN4 T166 residue is phosphorylated by AvrB and AvrRpm1 in planta
(A) T166-dependent RIN4 phosphorylation in N. benthamiana. Immunoprecipitation with α-T7 conjugated agarose beads was used to enrich RIN4 or RIN4 T166A from leaves co-infiltrated with Est:AvrB:HA or AvrRpm1:HA and T7:RIN4 or T7-RIN4 T166A, followed by immunoblotting with α-pRIN4 (phophopeptide-specific polyclonal antibody) and α-T7. Samples 18 hours post 30µM Est-induction were prepared and input levels established by immunoblot with appropriate antibodies (top). α-T7 immunoprecipitates (bottom) were used for immunoblots with α-pRIN4. An immunoblot with α-T7 demonstrated equal expression levels of RIN4 and RIN4 T166A in these immunoprecipitates. The experiment was repeated three times. (B) RIN4 T166 is phosphorylated in Arabidopsis following AvrB or AvrRpm1 delivery from P. syringae. Transgenic Arabidopsis RIN4 or T166A mutant were inoculated with Pto DC3000(avrB:HA) or (avrRpm1:HA) as described in figure 5B. Samples were collected 18 hours after infection. Immunoblots and immunoprecipitations were performed as in (A). Asterisk indicates an Arabidopsis background band mobility similar to that of AvrB. The data represent one of three experiments with similar results. (C) The α-pRIN4 antiserum detects phosphorylated RIN4-pT166 in N. benthamiana and transgenic Arabidopsis. α-T7 immunoprecipitates from either N. benthamiana transiently expressing RIN4 and RIN4 with AvrB or AvrRpm1 (left), and transgenic Arabidopsis uninfected or infected with Pto DC3000 (avrB:HA) or (avrRpm1:HA) (right) were divided a half to treat calf intestinal phosphatase (CIP). Tissue samples were prepared as in (A) for N. benthamiana and (B) for Arabidopsis.
Figure 7
Figure 7. Differential co-immunoprecipitation of RPM1 with RIN4 BBS mutants identifies residues required for interaction and RPM1 accumulation
(A) Co-immunoprecipitation of RIN4 BBS mutants with RPM1. The microsomal fraction was enriched in extracts from each RIN4 BBS mutant transgenic Arabidopsis, followed by immunoprecipitation with α-myc. The overall level of RPM1 is displayed in the input (left top). RIN4 expression in each mutant was confirmed by immunoblotting with α-RIN4. Immunoprecipitated RPM1 was shown by immunoblotting with α-myc. Co-immunoprecipitated RIN4 with RPM1 was confirmed with α-RIN4 immunoblot Two week-old seedlings from each line were used to collect the microsomal fraction. (B) Loss of co-immunoprecipitation of RIN4 T166D F169A with RPM1. Agrobacterium transient assays were performed as in Figure 3. Loading controls, immunoprecipitation with α-myc and subsequent immunoblots were performed as in Figure 7A, with the use of α-T7 to detect RIN4 and RIN4 BBS mutants. (C) Loss of effector-independent RPM1 activation in RIN4 T166D F169A. Agrobacterium transient assays, conductivity measurements and trypan blue staining were performed as in Figure 3.

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