The bacterial type III-secreted protein AvrRps4 is a bipartite effector

Abstract

Bacterial effector proteins secreted into host plant cells manipulate those cells to the benefit of the pathogen, but effector-triggered immunity (ETI) occurs when effectors are recognized by host resistance proteins. The RPS4/RRS1 pair recognizes the Pseudomonas syringae pv. pisi effector AvrRps4. AvrRps4 is processed in planta into AvrRps4N (133 amino acids), homologous to the N-termini of other effectors including the native P. syringae pv. tomato strain DC3000 effector HopK1, and AvrRps4C (88 amino acids). Previous data suggested that AvrRps4C alone is necessary and sufficient for resistance when overexpressed in heterologous systems. We show that delivering AvrRps4C from DC3000, but not from a DC3000 hopK1- strain, triggers resistance in the Arabidopsis accession Col-0. Delivering AvrRps4C in tandem with AvrRps4N, or as a chimera with HopK1N, fully complements AvrRps4-triggered immunity. AvrRps4N in the absence of AvrRps4C enhances virulence in Col-0. In addition, AvrRps4N triggers a hypersensitive response in lettuce that is attenuated by coexpression of AvrRps4C, further supporting the role of AvrRps4N as a bona fide effector domain. Based on these results we propose that evolutionarily, fusion of AvrRps4C to AvrRps4N may have counteracted recognition of AvrRps4N, and that the plant RPS4/RRS1 resistance gene pair was selected as a countermeasure. We conclude that AvrRps4 represents an unusual chimeric effector, with recognition in Arabidopsis by RPS4/RRS1 requiring the presence of both processed effector moieties.

The Conserved Arginine Required for AvrRps4 Processing Is Also Required for Recognition of Its N-Terminal Fragment in Lettuce

“…In the first layer, plasma membrane localized receptors detect the presence of conserved microbial molecular patterns, known as Microbial/Pathogen Associated Molecular Patterns (MAMPs/PAMPs), to initiate MAMP/PAMP-triggered immunity (MTI/PTI). In order to overcome host MTI/PTI, a sophisticated pathogen delivers a repertoire of virulence effectors into plant cells to interfere with plant functions at multiple levels (Jones and Dangl, 2006;Block and Alfano, 2011;Feng and Zhou, 2012;Xin and He, 2013;Su et al, 2018). A second layer of the plant immune system utilizes resistance (R) proteins to interact with effectors directly or to monitor effector-induced modifications.…”

“…A second layer of the plant immune system utilizes resistance (R) proteins to interact with effectors directly or to monitor effector-induced modifications. Recognition of effectors by intracellular R proteins initiates effector-triggered immunity (ETI) which is often associated with rapid cell death known as the hypersensitive response (HR) (Jones and Dangl, 2006;Spoel and Dong, 2012;Cui et al, 2015; Kourelis and van der Hoorn, 2018;Su et al, 2018;Lolle et al, 2020). In Arabidopsis, it is established that two R protein pairs, RRS1-RPS4 and RRS1B-RPS4B, function redundantly to recognize the Pseudomonas type III effector AvrRps4 (Hinsch and Staskawicz, 1996;Gassmann et al, 1999;Narusaka et al, 2009;Saucet et al, 2015).…”

“…Estradiol-inducible expression of AvrRps4-C alone in the Arabidopsis accession Columbia-0 (Col-0) triggers HR to a similar extent as full-length AvrRps4 (Li et al, 2014), which correlates well with the finding that transient overexpression of AvrRps4-C is sufficient to trigger HR in turnip (Sohn et al, 2009). In contrast, dexamethasone (DEX)-inducible expression of AvrRps4-N in Col-0 alone does not trigger HR (Halane et al, 2018). It is worth noting that the Pseudomonas type III secretion system-mediated delivery of AvrRps4 at natural levels does not trigger HR in Col-0 though it triggers strong resistance to Pseudomonas syringae pv.…”

“…Nonetheless, transient overexpression mediated-HR has proven to be a rapid and useful tool to study RPS4-RRS1-and RPS4B-RRS1B-mediated recognition of AvrRps4 (Sarris et al, 2015;Saucet et al, 2015;Ma et al, 2018). We recently demonstrated that AvrRps4-N and AvrRps4-C are mutually required to trigger resistance in Arabidopsis when delivered by the Pseudomonas type III secretion system either as a full-length protein or when delivered by separate constructs (Halane et al, 2018).…”

Post‐translational modifications in effectors and plant proteins involved in host–pathogen conflicts

“…For example, cleavage of AvrRps4, which targets EDS1 (Bhattacharjee et al ., ), results in the release of an N‐terminal AvrRps4 N (133 amino acids) and a C‐terminal AvrRps4 C (88 amino acids) peptide. AvrRps4 N activates an HR in lettuce that can be suppressed by AvrRps4 C (Halane et al ., ).…”