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, 17 (6), 752-62

Pseudomonas Syringae Pv. Tomato DC3000 Type III Secretion Effector Polymutants Reveal an Interplay Between HopAD1 and AvrPtoB

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Pseudomonas Syringae Pv. Tomato DC3000 Type III Secretion Effector Polymutants Reveal an Interplay Between HopAD1 and AvrPtoB

Hai-Lei Wei et al. Cell Host Microbe.

Abstract

The bacterial pathogen Pseudomonas syringae pv. tomato DC3000 suppresses the two-tiered plant innate immune system by injecting a complex repertoire of type III secretion effector (T3E) proteins. Beyond redundancy and interplay, individual T3Es may interact with multiple immunity-associated proteins, rendering their analysis challenging. We constructed a Pst DC3000 polymutant lacking all 36 T3Es and restored individual T3Es or their mutants to explore the interplay among T3Es. The weakly expressed T3E HopAD1 was sufficient to elicit immunity-associated cell death in Nicotiana benthamiana. HopAD1-induced cell death was suppressed partially by native AvrPtoB and completely by AvrPtoBM3, which has mutations disrupting its E3 ubiquitin ligase domain and two known domains for interacting with immunity-associated kinases. AvrPtoBM3 also gained the ability to interact with the immunity-kinase MKK2, which is required for HopAD1-dependent cell death. Thus, AvrPtoB has alternative, competing mechanisms for suppressing effector-triggered plant immunity. This approach allows the deconvolution of individual T3E activities.

Figures

Figure 1
Figure 1. Deletion of 8 additional T3E genes from DC3000D28E produces effectorless mutant DC3000D36E and reveals that HopAD1 is responsible for the residual ability of DC300028E to elicit cell death and to suppress a ROS burst and PTI in N. benthamiana
(A) Overview of the Pst DC3000 T3E gene repertoire and deletions in polymutant strains. (B) ETI-like rapid, confluent, plant cell death is triggered in N. benthamiana by Pst DC3000 polymutants that still carry hopAD1. Bacteria suspensions at the high inoculum level of 5 ×108 CFU/ml were infiltrated into the marked zones of N. benthamiana leaves. The leaf response was photographed 2 days later. The fraction under each image indicates the number of times that tissue death was observed relative to the number of test inoculations. (C) ROS production is triggered in N. benthamiana by Pst DC3000 polymutants that lack hopAD1. Bacterial suspensions at 5 ×108 CFU/ml were infiltrated into leaves, and ROS production was determined 15 h post-inoculation. Means marked with the same letter were not significantly different at the 5% confidence level on the basis of Duncan’s multiple-range test. (D) PTI elicitation activity is restored to DC3000D28E derivatives by deletion of hopAD1. The fraction under each image indicates the number of times cell death was inhibited (indicating PTI elicitation by the first inoculum) relative to the number of test inoculations. All experiments were repeated three times with similar results (see also Table S1, Table S2 and Figure S1).
Figure 2
Figure 2. Evidence that HopAD1 is an avirulence determinant in N. benthamiana based on heterologous expression of hopAD1 in P. syringae pv. tabaci (Pta)11528 and deletion of hopAD1 from Pst DC3000
(A) Pta 11528 expressing HopAD1 is avirulent in N. benthamiana. Leaves were infiltrated with the indicated strains at the low level of 3×104 CFU/ml and photographed 8 days later. (B) Pst DC3000ΔhopAD1 mutants cause disease lesions in N. benthamiana. The leaves were infiltrated with the indicated strains at 3×104 CFU/ml using a blunt syringe and photographed 8 days after inoculation. (C) Deletion of hopAD1 from Pst DC3000 results in the same high level of growth as observed with Pst DC3000ΔhopQ1-1 in N. benthamiana. Bacteria were infiltrated at 3×104 CFU/ml and populations were measured from three 0.6-cm-diameter leaf discs 6 days after inoculation. Error bars indicate the standard deviation of populations measured from thee leaf discs from each of three plants. Means marked with the same letter were not significantly different at the 5% confidence level on the basis of Duncan’s multiple-range test. All experiments were repeated three times with similar results (see also Figures S2 and S3).
Figure 3
Figure 3. Deletion of hopAD1 has no effect on the growth or symptom production in N. benthamiana of DC3000D28E derivatives carrying T3E gene subsets that include avrPtoB, and AvrPtoB is sufficient to reduce cell death elicitation by HopAD1
(A) Bacterial growth assay by DC3000D28E derivatives with different subsets of effectors (Table S1). White fill in the genotype grid indicates that the locus was present or restored in DC3000D28E; gray fill indicates that it was missing. Bacteria were infiltrated at the low inoculum level of 3×104 CFU/ml, and populations in three 0.6-cm-diameter leaf discs were determined at 6 days after inoculation. Error bars indicate the standard deviation of populations measured from three leaf discs from each of three plants. Means with the same letter were not significantly different at the 5% confidence level based on Duncan’s multiple range test. (B) Symptom production by DC3000D28E derivatives with different subsets of effectors. The indicated bacteria were inoculated as above, and leaves were photographed at 8 days post inoculation. Asterisks indicate necrosis, plus signs indicate chlorosis, and minus signs indicate no symptoms. (C) AvrPtoB but not AvrPto suppresses cell death elicitation by HopAD1 in N. benthamiana. Leaves were infiltrated with indicated bacteria in the marked circles at the high inoculum level of 5×108 CFU/ml, and 2 days later they were photographed and scored for tissue collapse (unstained) or stained with trypan blue to assay cell death (stained). The fraction under each image indicates the number of times that confluent tissue collapse or trypan blue staining was observed relative to the number of test inoculations. All experiments were repeated three times with similar results (see also Figure S4).
Figure 4
Figure 4. AvrPtoB homologs from diverse P. syringae pathovars, expressed in DC3000D28E, suppress HopAD1-dependent cell death elicitation in N. benthamiana, and their suppression efficacy correlates with having an inactive or missing E3 ligase domain
(A) Bacterial cell suspensions in MgCl2 buffer, adjusted to 5×108 CFU/ml, were infiltrated into leaves, and the plant response was photographed 2 days later. The fraction under each image indicates the number of times tissue collapse occurred relative to the number of test inoculations. All avrPtoB homologs (hopAB family) were expressed from the avrPto promoter in pCPP5372. The experiment was repeated three times with similar results (see also text and Figure S5). (B) Interaction of HopAB family members with Pto and Fen based on a yeast two-hybrid system. Blue patches indicate positive interactions.
Figure 5
Figure 5. AvrPtoB mutants show that suppression of cell death elicitation by HopAD1 is enhanced by mutation of the AvrPtoB E3 ligase domain and correlates with AvrPtoB interaction with MKK2 but not Pto or Fen, and expression of AvrPtoBM3 in wild-type Pst DC3000 promotes bacterial growth and symptom production in N. benthamiana
(A) Diagram of AvrPtoB from Pst DC3000 showing the type III secretion system targeting region (T3S) and mutations ablating function of the Pto interaction domain (PID), Fen interaction domain (FID), and E3 ubiquitin ligase domain (E3). (B) Effect of mutations in AvrPtoB domains on relative ability to suppress cell death elicitation in N. benthamiana by HopAD1 in DC3000D28E. Bacteria at the high inoculum level of 5×108 CFU/ml were infiltrated into leaves, and the plant response was photographed 4 days later. The fraction under each image indicates the frequency of tissue collapse relative to the number of test inoculations. Similar results were found in three independent tests. (C) Interaction of AvrPtoB mutants with Pto, Fen, and MKK2 based on a yeast two-hybrid system. Blue patches indicate positive interactions. (D) Bacteria were inoculated at the low level of 3×104 CFU/ml in N. benthamiana, and populations were assayed 6 days later. Error bars indicate the standard deviation of populations measured from thee leaf discs from each of three plants. Means with the same letter were not significantly different at the 5% confidence level based on Duncan’s multiple-range test. (E) N. benthamiana leaves were inoculated as above and photographed 15 days later. Typical symptoms are shown, and all the experiments were repeated three times with similar results.
Figure 6
Figure 6. Suppression of ETI by AvrPtoBM3 action on MKK2 is supported by the ability of AvrPtoBM3 to inhibit AvrPto and AvrPtoB1-387 cell death elicitation in Pto+ N. benthamiana (despite AvrPtoBM3 lacking a functional Pto interaction domain) and by the MKK2-dependence of HopAD1 cell death elicitation
(A) AvrPtoBM3 suppresses cell death elicitation by avrPto+ or AvrPtoB1-387+ DC3000D29E in N. benthamiana that is transgenically producing the tomato Pto kinase. Leaves were infiltrated with indicated bacteria in the marked circles at the high inoculum level of 5×108 CFU/ml and stained with trypan blue at 2 days post inoculation. The fraction under each image indicates the number of times cell death was observed relative to the number of test inoculations. (B) Cell death elicited by HopAD1 in DC300028E is compromised by silencing MKK2 but not Prf in N. benthamiana, but cell death elicited by HopQ1-1 in DC3000D29E is not suppressed by MKK2 or Prf. Bacteria at 5×108 CFU/ml were infiltrated into leaves, and the plant response was photographed 2 days later. The fraction under each image indicates the number of times that tissue collapse was observed relative to the number of test inoculations. Similar results were found in three independent experiments.
Figure 7
Figure 7. Model of the competing virulence activities of AvrPtoB and AvrPtoBM3 in the context of plant immune signaling, relevant activities of HopAD1 and AvrPto, and the domain structure of AvrPtoB
(A) HopAD1 is proposed to disrupt PTI-associated kinase signaling but be detectable by an unknown resistance protein (R1) that signals through MKK2 to ETI. AvrPtoB suppresses PTI through interaction of PID and FID domains with kinase domains of pattern recognition co-receptors (PRRs), FLS2, BAK1, and Bti9, but recognition by Fen/Pto decoy kinases leads to ETI unless blocked by E3 ligase action at the FID domain. The M3 mutant of AvrPtoB, lacking kinase-interaction and E3 ligase domains, has enhanced ability to interact with MKK2 and suppress ETI elicited by HopAD1, AvrPto, and AvrPtoB1-387. High polymorphism in the E3 ligase domain of HopAB family members suggests that these effectors may rapidly evolve between states favoring E3 ligase activity (for suppression of FID-Fen-triggered ETI) or loss of such activity (for suppression of ETI triggered by HopAD1 and other effectors that signal through MKK2) depending on the host R gene composition. (B) Depiction of the AvrPtoB activities described above in terms of AvrPtoB domains and interacting host proteins highlights the conflicting roles of the M3 activity (within gray), the PID/FID domains, and the E3 ubiquitin ligase activity. Outcomes involving different AvrPtoB variants and R proteins have been previously reported (black font)(Mathieu et al., 2014; Rosebrock et al., 2007) or shown here (blue font). Variants a-c depict the conflicting activity of the E3 ubiquitin ligase domain in suppression of intramolecular (self domain) ETI versus ETI elicited by one or more other effectors signaling through MKK2. Variants c-e address the ability of AvrPtoBM3 (e) to suppress Pto-dependent ETI elicitation by AvrPtoB1-387 (d), which demonstrates that PID/FID-dependent interaction with Pto (but not with PRRs) conflicts with the ability of AvrPtoB variants to suppress MKK2-dependent ETI elicitation (See also Figures S6 and S7).

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