Abstract
Plants recognize microbes via specific pattern recognition receptors that are activated by microbe-associated molecular patterns (MAMPs), resulting in MAMP-triggered immunity (MTI). Successful pathogens bypass MTI in genetically diverse hosts via deployment of effectors (virulence factors) that inhibit MTI responses, leading to pathogen proliferation. Plant pathogenic bacteria like Pseudomonas syringae utilize a type III secretion system to deliver effectors into cells. These effectors can contribute to pathogen virulence or elicit disease resistance, depending upon the host plant genotype. In disease resistant genotypes, intracellular immune receptors, typically belonging to the nucleotide binding leucine-rich repeat family of proteins, perceive bacterial effector(s) and initiate downstream defense responses (effector triggered immunity) that include the hypersensitive response, and transcriptional re-programming leading to various cellular outputs that collectively halt pathogen growth. Nucleotide binding leucine-rich repeat sensors can be indirectly activated via perturbation of a host protein acting as an effector target. AvrRpm1 is a P. syringae type III effector. Upon secretion into the host cell, AvrRpm1 is acylated by host enzymes and directed to the plasma membrane, where it contributes to virulence. This is correlated with phosphorylation of Arabidopsis RIN4 in vivo. RIN4 is a negative regulator of MAMP-triggered immunity, and its modification in the presence of four diverse type III effectors, including AvrRpm1, likely enhances this RIN4 regulatory function. The RPM1 nucleotide binding leucine-rich repeat sensor perceives RIN4 perturbation in disease resistant plants, leading to a successful immune response. Here, demonstrate that AvrRpm1 has a fold homologous to the catalytic domain of poly(ADP-ribosyl) polymerase. Site-directed mutagenesis of each residue in the putative catalytic triad, His63-Tyr122-Asp185 of AvrRpm1, results in loss of both AvrRpm1-dependent virulence and AvrRpm1-mediated activation of RPM1, but, surprisingly, causes a gain of function: the ability to activate the RPS2 nucleotide binding leucine-rich repeat sensor.
Highlights
Pseudomonas syringae is a Gram-negative phytopathogen that utilizes various biochemical means, including analogous enzymatic activity or molecular mimicry of host proteins, to block or bypass the plant immune system
We found that AvrRpm1 consists of the fold from the catalytic domain of poly(ADPribosyl)polymerase-1 (PARP-1)
Each of the AvrRpm1 missense mutations used in this assay accumulated normally in P. syringae (Figure 3D) and, as noted above, was translocated (Figure 2B). These results demonstrate that the ability of the AvrRpm1D185A to restrict growth of an otherwise virulent pathogen is dependent on its myristoylation and localization at the plasma membrane, and its ‘off-target’ perception there by RPS2
Summary
Pseudomonas syringae is a Gram-negative phytopathogen that utilizes various biochemical means, including analogous enzymatic activity or molecular mimicry of host proteins, to block or bypass the plant immune system. The type III secretion system is shared by many Gram-negative pathogens of plants and animals that use effector proteins to subvert host cell physiology and bypass defenses [1,2,3]. Several well-studied nucleotide binding leucine-rich repeat (NB-LRR)-dependent responses to effectors are mediated by indirect recognition of effector action on a host target, as described by the Guard Hypothesis [4,5]. In this model effector targets functions as a molecular lure or ‘guardee’, and a specific NB-LRR protein functions as a ‘guard’ [6,7,8,9]. In the absence of the corresponding NB-LRR, manipulation of the guardee can contribute to the virulence activity of the effector [4,7]
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